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Review

Lodgepole Pine and White Spruce Thinning in Alberta―A Review of North American and European Best Practices

by
Mark Baah-Acheamfour
1,*,
Amanda Schoonmaker
1,
Mark Dewey
2 and
Brian Roth
3
1
Centre for Boreal Research, Northern Alberta Institute of Technology, Peace River, AB T8S 1R2, Canada
2
School of Applied Sciences and Technology, Northern Alberta Institute of Technology, Edmonton, AB T5G 2R1, Canada
3
Forest Growth Organization of Western Canada, fRI Research, Hinton, AB T7V 1V3, Canada
*
Author to whom correspondence should be addressed.
Land 2023, 12(6), 1261; https://doi.org/10.3390/land12061261
Submission received: 4 May 2023 / Revised: 9 June 2023 / Accepted: 12 June 2023 / Published: 20 June 2023
(This article belongs to the Special Issue Diversifying Forest Landscape Management Approaches)
Browse Figures
Review Reports Versions Notes

Abstract

:
A significant portion of the harvested land base in western Canada is becoming old enough or entering a phase where thinning is a legitimate forest management option. A comprehensive review of the existing knowledge of commercial thinning (CT) treatments applied to pine and spruce-dominated stands in Alberta was conducted, with particular regard to the intensity, timing of interventions, method, and impacts on crop tree growth responses. Although the geographical focus of this review is Alberta, information on this topic is more complete in other areas of North America and Europe, where there is a long history of density management. In areas of eastern North America, our review revealed that CT from below, with tree removal levels from 27 to 43% of the basal area, could increase total merchantable wood produced from 11 to 60 m3 ha−1 over a rotation, depending on stand age and intensity of thinning. For Alberta conditions, and considering the risks, we conclude that commercial thinning basal area removal should be in the range of 25 to 40%, depending on a variety of factors such as species, wind firmness, and insect or disease incidence and risk. Thinning too aggressively and/or too late will increase the blowdown risk but the literature is fairly consistent in suggesting that live crown ratios should be >40% to maximize the chance of growth response and minimize the blowdown risk. In cases where stands are also threatened by stressors such as drought, wind, and insect or disease outbreaks, CT treatments likely offer the potential at limiting the overall risk, but localized knowledge and experience are critical. It is intended that the information presented may support ongoing and future research trials and growth and yield (G&Y) model development about potential CT treatments to apply and the likely results of practical application to commercial forestry.

1. Introduction

The forest industry is central to Alberta’s economy, providing well-paying jobs for thousands of people and making significant contributions to many Alberta communities for multiple generations. In 2019–2020, a total of 14.5 million cubic metres of coniferous timber and 8.2 million cubic metres of deciduous timber were harvested, representing approximately $13.6 billion in economic output, $2.7 billion in labour income, $5.8 billion in the provincial GDP, and more than 31,500 jobs in Alberta [1]. Despite the Government of Alberta’s attempts to increase the annual allowable cut (AAC) by up to 13% for Alberta’s forestry companies, industry observers forecast that total Alberta tree harvesting, including regions regulated by the annual allowable, will decline in the years ahead. The causes of this expected decline have their roots in the devastating impact of the mountain pine beetle (Dendroctonus ponderosae; MPB) from 1999 to 2006, wildfires, land withdrawals (largely due to oil and gas development and resource mining such as sand and gravel), and climate change [2,3,4]. The cumulative effect of these factors has resulted in a continually shrinking operational land base for forest harvesting activities. Currently, there is interest from the industry in finding alternate ways to mitigate the projected timber supply shortage as demand for wood products stages a comeback [5].
Meeting these timber supply goals will require the establishment of more intensively managed plantations [6] and the application of density management treatments on existing stands to shorten rotation lengths and close harvesting gaps [7,8,9]. Density management treatments are currently regulated through the Alberta Forest Management Planning Standard as defined in the Partial Harvest (non-clearcut) Planning and Monitoring Guidelines [10]. This review focuses largely on stand density management through thinning in natural and managed stands of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and white spruce (Picea glauca (Moench) Voss). Thinning treatments can facilitate a stable wood supply by filling age gaps in the timber supply through the capture of natural mortality and focusing growth on fewer trees which reach merchantable size sooner, thereby reducing rotation ages [11,12,13,14]. Pre-commercial thinning treatments are typically implemented within natural disturbance-origin or artificially regenerated (e.g., seeded) density-stressed stands during the sapling stage or early post-crown-closure stem exclusion phase of development [15]. This involves the removal of the smallest non-commercial-sized individuals in a manner that attempts to redistribute the newly available growing space equitably among the residual crop trees. Commercial thinning treatments are prescribed during the semi-mature stage of stand development typically within managed or natural-origin stands that may or may not have been subjected to pre-commercial thinning in the past. Thinning can also be classified in terms of the crown class of tree removed or concerning the financial benefits of the operation. The obvious benefit of increasing the growth of merchantable trees may be the primary objective of thinning. Secondary to this stand-level objective in thinning includes reducing the time to merchantability and the final harvest cost, securing stable wood supply, enhancing stand stability and vigour, and reducing fuel loading [16,17,18].
While thinning may improve the growth efficiency of residual trees, planning and implementation needs to consider the existence of trade-offs between maximizing stand growth and volume risk. For example, gross production will be reduced if the stand is thinned too heavily (Figure 1). However, a positive trade-off will occur if merchantability standards are low and the thinning is light, and the total yield may be higher as demonstrated in lodgepole pine in central Alberta where 50% of the mortality was captured [19]. It is therefore important to be able to establish when it is ideal to thin and if so, how intensively and to understand the nature of post-thinning responses. The post-thinning responses of merchantable volume may be described as parallel, convergent, or divergent to the volume yield of similar-size trees in unthinned stands [20]. A parallel response occurs when the volume of residual trees plus the volume of thinned trees (total volume) did not vary from the total volume present in an unthinned stand. A convergent response could result when one-stage or multiple-entry captures natural mortality, causing an increase in total volume recovered compared with unthinned stands [21]. As for the divergent response, if the stand is thinned too heavily or if the treatment is performed too late, the total volume produced may be lowered in the thinned stand when compared to the unthinned stand [22].
Alberta has large areas of lodgepole pine and white spruce cut blocks that have entered or are entering a phase where commercial thinning is an option. There is an opportunity to develop adaptive thinning treatments, which could form the basis for reducing rotation ages, accessing merchantable volume before final harvest, and adding value to end products (i.e., increasing sawlog to pulp proportion, appearance, and quality). However, to carry out such a treatment effectively, a review of existing literature is of value to inform those processes. This paper reviews the existing knowledge of the thinning treatments, especially as it may apply to lodgepole and white spruce-dominated stands in Alberta. Extra emphasis is placed on thinning intensity, the timing of interventions, methods and impacts on crop tree growth responses, and the associated risk factors. Although the geographical focus of this review is Alberta, information on this topic is more complete in other areas of North America, Scandinavia, and northern Europe, where there is a long history of density management. It is intended that the information presented may support operations, ongoing and future research trials, and growth and yield (G&Y) model development.

2. The Evolving Shifts in Alberta’s Stand-Tending Practices

Forest conservation and management in Alberta have a long history that dates back to the early 20th century [23]. Until the early 1950s, reforestation in Alberta was generally left to nature as many of the cleared or degraded areas were regenerated naturally. Yet it became clear as early by the late 1940s that natural regeneration was less successful for most principal commercial species. This proved true for lodgepole pine and white spruce on most sites. The success story of forest regeneration in Alberta began with the period of adaptive regeneration promoted by Crossley [24]. Crossley advocated systematic clear-cutting with the idea of promoting long-term sustainable harvesting by creating stands with an even age–class distribution. Moreover, in Canada’s northwestern boreal region, the annual amount of forested area burned by wildfires had risen steadily over the second half of the 20th century. Since lodgepole pine regenerates rapidly after a fire event, stands can quickly become overstocked [25,26], to the point where competition results in reduced tree growth and a decline in vigour, and mortality—including mortality from infestations of the mountain pine beetle (Dendroctonus ponderosae Hopkins) [27]. The scale and impact of the mountain pine beetle outbreaks challenged the status quo approaches to tending even-aged pure stands. Between 2005 and 2010, the years after the peak of the most recent mountain pine beetle epidemic, conifers other than Douglas fir (Pseudotsuga menziesii var. glauca Franco), white spruce and some broadleaf trees (trembling aspen (Populus tremuloides Michx.), and balsam poplar (Populus balsamifera L.) have emerged as dominants, with lodgepole pine nearly absent in some regions [28]. Trembling aspen regenerates in the stand after a fire and hastens density-dependent mortality [28].
By the early 2000s, the concept of thinning, which had been advocated by Quaite [29] and Crossley [24], was further promoted by the newly established Foothills Growth and Yield Association [30] following the presentation of the Provincial Enhanced Forest Management report and recommendations in January 1997. Before that time, a series of pre-commercial and commercial thinning trials had already been established in lodgepole pine stands by the Canadian Forestry Service and the Alberta Forest Service (Table 1). The trials were initiated in fire-origin stands throughout the Alberta foothills between 1941 and 1984.
The average timing of the data collected from these old trials was 34 years post-thinning treatment with the range of response data ranging from 19 to 58 years post-treatment. The thresholds defined as light, moderate, and heavy thinning vary among studies regarding the thinning intensities. The average ranges of the basal area percentage removed in the thinning treatments were light (<20%), moderate (20–35%), and heavy (>35).
Unfortunately, many of the comparisons in the old thinning experiments were limited by inappropriate statistical designs, such as low replication or high site variability. Most of these trials involved one density management intervention or thinning prescriptions that were quite different from likely operational scenarios, lacked pre-treatment observations or sufficient documentation of the thinning prescription, and an absence of data on the attributes of removed trees (volumes, piece sizes, etc.) which are essential for assessing the operational feasibility of thinning. Dewey (pers comm, 19 September 2022) suggested that it would be necessary to exercise great care in interpreting data from trials with only one density management intervention over time. Without multiple interventions on at least some of the treatments, the single thinning may only defer stagnation into the future, and results are likely to be misleading. Furthermore, research should attempt to reflect operational responses to actual conditions, depending on stand development. For example, flexibility is needed where density management prescriptions can be adjusted with parts or all of a stand with variable initial conditions dropped from treatment. Research designs should attempt to mimic this operational flexibility. Limitations of the historic trials described above indicate a need for new thinning trials in Alberta that capture a range of conditions around the operational optimum that can be used to characterize tree and stand responses to thinning, while also providing data for future growth and yield modelling efforts.
To this end, several pre-commercial [32,33] and commercial [19] thinning research trials in lodgepole pole pine have been recently initiated, largely focused on the foothill subregions in Alberta and following a more robust statistical procedure. However, the information on thinning of lodgepole pine stands gathered from current experiments in Alberta does not provide a sufficient baseline for developing a thinning practice for the species. While new studies with well-balanced designs can provide the data needed to develop practical thinning guidelines and make better thinning decisions, they will take some time and delay before obtaining results (Michel Huot, pers comm, 13 November 2022). Michel provides some recommendations, suggesting a work-in-progress situation where existing data are considered first while waiting for more robust data from new studies. Examples of such surveys exist in Nova Scotia for red spruce (Picea rubens Sar.) and balsam poplar, where knowledge and experience in the existing operational handbooks are utilized for their estimates of growth and yield [34].

3. Approaches to Thinning Decisions

3.1. How Much to Remove

The decision whether to thin and if so, when and how heavily, must ultimately be guided by the intended volumetric yield, end-product quality, and value and ecosystem service outcomes [35,36]. Strong common threads run through most of the reviewed literature. Particularly where dense, even-aged, and homogenous (single-story) stands are concerned, the indications are that individual residual crop trees may benefit from heavier thinning. A thinning experiment in naturally regenerated jack pine (Pinus banksiana Lamb.) in New Brunswick with three pre-commercial thinning intensities showed 75% and 20% differences in tree diameter and merchantable stem volume between light (>20%) or moderate (40%) thinning and heavy (70%) thinning [16] after 34 years (stand age 59). Soucy et al. [37] investigated a long-term thinning experiment in upland black spruce in Quebec’s north shore region. The stands were established with three thinning intensities: 0%, 25%, and 50% of the total basal area removed. After 15 years, heavily thinned plots showed a net stand merchantable volume increment 33% greater than that of the unthinned plots. When a spruce budworm (Choristoneura fumiferana Clemens) outbreak affected the site, Soucy et al. [37] found that the heavily thinned plots maintained a superior tree growth rate and did not show senescence mortality that was common for balsam fir [38,39], allowing stand volume to catch up to that of the unthinned plots after 33 years. However, some research from Scandinavia [40,41] has shown that a series of light thinning treatments repeated at frequent intervals of thinning (e.g., every 5 years) can limit density-induced mortality and yield higher quality end-products than heavy, single-entry thinning (i.e., removing more than 50% of basal area). These positive outcomes were generally linked to the long-term responses of crown architecture to repeated light thinning, resulting in higher leaf/sapwood ratios for trees consistently released from the competition [42].
Five-year multiple entry timings with commercial thinning appear to be a classical example of a selection treatment, which is often aimed at young uneven-aged management and irregular forests. Since Alberta’s forests are primarily even-aged and selection harvest is not appropriate, these frequent commercial thinnings are not likely to be applicable. In general, we noted that the most effective commercial thinning method in the northeastern portion of North America in terms of large tree response, sawlog volume, and stand value is thinning mainly from below [8,21,34,43] or crown [44], with tree removal level up to 50% (30 to 40% of basal area) for spruce or fir stands. This intensity range is somewhat heavier than what Gauthier and Tremblay [15] recommend (27%) for jack pine stands in eastern Canada, but it corresponds to what current guidelines suggest (43%) for lodgepole pine in the interior of British Columbia (BC) [20,45] and for spruce–fir stands (32 to 47%) in Nova Scotia. Moulinier et al. [46] also demonstrated there is an upper limit to thinning intensity of jack pine in eastern Canada (64% basal area removal) above, where thinning would no longer benefit residual stand volume. Taken together, the results of these trials would appear to indicate that a high thinning intensity produces larger trees on average, while a lower intensity maximizes volume production per hectare. The results do, however, provide other useful insights into the overall effects of thinning conifers and therefore may be useful in providing some directions for further thinning intensity trials in different geographic regions. For instance, the 30 to 40% minimum basal area removal proposed by Gauthier et al. [8,21] may correspond to greater than four competitors removed per residual tree crop (in a stand with an initial stocking density of 2500 stems ha−1). However, this would very much depend on the vegetation management that took place during the reforestation phase.

3.2. When to Thin

The question of when to thin and the form is also important, as poor timing of thinning can have serious consequences for the stand. Indeed, McGrath et al. [47], Bjelanovic et al. [48], and Hawe and Short [49] point out certain positive silvicultural and economic aspects of avoiding late-entry pre-commercial or commercial thinning. Late-entry commercial thinning in general has been shown to produce windthrow problems that result in a clearcut instead of a partial cut. A typical example is the damage caused by Hurricane Juan on commercially thinned stands in central Nova Scotia in September 2003. In an attempt to determine whether stand conditions affected damage levels in commercially thinned stands, both positive and negative relationships were discovered to exist between the thinning intensity, stand age at thinning, the slenderness of the trees, and wind damage [47].
While one proposed benefit of early intervention is to maintain growth on high-quality residual stems [50], a further benefit may be that the residual trees are less susceptible to attack by disease and insect pests [51]. Traugott and Dicke [51] go on to recommend the following basic commercial thinning guidelines in pine stands intended for sawlog timber production in the southeastern region of the United States: (1) natural pruning is at least 5.5 m in height, (2) the mean stand diameter at breast height (DBH) (1.3 m aboveground) is at least 15 cm, (3) the last 3 years’ annual growth rates are less than 10 percent, and (4) the average total tree height is at least 12.2 m. The need for a first thinning can occur sooner on land with a high site index compared with land that has a low site index [35,52]. It is thought that the significance of the site index’s effects in influencing the timing of the first thinning is better understood when considered with stocking density [53]. Even on the best sites, the timing of the first thinning can be delayed in stands with poor survival and/or low initial planting density. Site occupancy and stand differentiation are often delayed under these conditions, and maintenance of high crown ratios sustains better diameter growth than can be attained in more dense stands [54]. While the influence of rotation length and the use of additional thinnings on the timing and form of the first thinning has seen limited empirical research. However, yield simulation work by Nebeker et al. [55] has shed some light on the relationships. These simulations verified earlier observations about the time of thinning and site index and stand density relationships.

3.3. Which Trees Should Be Removed in Thinning?

The overall principles that determine which should be removed during thinning have been well documented [55,56,57]. Pukkala and Miina [56] optimized a tree selection rule which was based on the effect of tree removal on the competitive positions of remaining trees. In their work, it was found that it was optimal to thin from above (i.e., remove large trees). Theoretically, it may be thought that the removal of the largest trees in a stand equates, to some extent, to the removal of the best producers. Concerns about decreasing growth arising from repeated removal of the largest trees in the stand were raised in the early 20th century and intensively discussed. Even later there has been discussion about the risk of growth losses due to negative genetic selection after repeated thinning from above [40]. In addition to potential risks associated with genetic selection, there is some evidence from the Scandinavian [58] and eastern North America [55] that thinning from above will increase the residual stands risk for windthrow. It was reported that a retained cubic metre of trees in a small diameter class has higher stem volume production than a retained cubic metre of a large diameter class [59]. Nebeker et al. [55] and Pukkala et al. [57] proposed a more straightforward approach to optimizing tree selection for thinning; the criteria of the cutting rule were the tree’s health condition, stumpage value, value increment, and the effect of removal on the growth of surrounding trees.
As stand densities increase, the method used to remove trees becomes more important due to lasting effects on the residual growing stock. While mechanical thinning (e.g., row or corridor thinning, where trees are removed strictly based on spacing with little or no regard to the crown position) is accepted, thinning should primarily target the removal of future crop tree competitors [49,55]. Most comparisons (e.g., [60]) have shown that mechanical thinning plus selective thinning (i.e., trees are removed individually based primarily on spacing and stem quality) results in higher growth rates and better stem quality compared to pure mechanical-type thinning. There is, however, a downside to this practice, because mechanical thinning can potentially expose the stands to damaging agents such as wind and ice. In keeping with the above, Nebeker et al. [55] go on to advise that in stands with a high incidence rate of diseased or malformed trees, mechanical thinning in the form of rows or corridors would be inappropriate as the treatment often leaves too many defective trees at the expense of better ones. In contrast, trees removed in the selective thinning are those in the lower crown classes and poorly formed or diseased trees. For that reason, post-thinning mortality (with a combination of mechanical and selective thinning) should be less than that for row or corridor thinning alone and comparable to that for selective thinning alone.

4. Commercial Thinning of Individual Species

4.1. Lodgepole Pine

Little debate surrounds the thinning needs of overly stocked lodgepole pine regeneration, where early and heavy thinning are commonly recommended for rapid diameter growth [61]. Early, light, and frequent thinnings (two or three entries) have also been recommended for maximum pulpwood plus saw timber production in Scots pine in northern Europe [40]. The guidelines established by Dennis et al. [62], on maximizing the resistance of both young and mature lodgepole pine before mountain pine beetle attacks, are highly practical in their application. The strategy is to remove no more than 25 percent of the stand’s basal area in commercial thinning during each cut and to carefully monitor stands to ensure the proper timing of the necessary re-entries. Maintaining average stem diameters of >20 cm and stand densities >18 square meters of basal area per hectare is recommended (Figure 2). There may also be good reasons for starting to thin young lodgepole pine later than the standard pre-commercial thinning age (e.g., after age 15), notably where the standing density of the trees in such a thinning is low. If lodgepole pine thinning is delayed at ages >40 years, a heavier removal will be needed to return the stands to the correct growing stock level, consistent with that of the standard pre-commercial thinning age [40,55].
Thinning handbooks, such as Thinning Southern Pines [63] and Southern Pine Density Management [55], provide intensity recommendations based on the number of stems per ha, basal area, and/or volume removal. Demers et al. [63] outlined the methodology to reduce stocking, including the systematic removal of entire rows of trees (trails) to facilitate access, combined with the selection of future crop trees and related competitor removals. Only British Columbia provides an interim operational manual or guidance for commercially thinning lodgepole pine in Canada [64]. This thinning type is based on current practices in BC’s interior where timber is removed on 5 m wide trails, measured bole to bole, and established every 20 m. The criterion for selecting stands suitable for thinning applications involve stand age, site index, stand density, basal area, and merchantable volume per hectare. The operational manual for commercial thinning in interior British Columbia by Pavel [64] provides the optimal thinning ages by site index for the most common species. For lodgepole pine, there seems to be a general agreement that stands with a site index of 16 to 26 m are good candidates for commercial thinning [45]. The principal thinning method is ‘systematic’, with 5 m wide access tracks and a 15 m selection zone. The 15 m strips between trails are then thinned from below to enhance the volume and value of the residual crop trees at the time of final harvest. With thinning from below, the lower crown classes are removed to favour the most vigorous trees in the stand. Trees ‘thinned from below’ can also include ‘quality thinning’ where dead, damaged, and diseased trees are removed to utilize tree mortality losses while improving the health, resilience, and growth of the remaining trees.

4.2. White Spruce

Since it is slower growing and more shade-tolerant than lodgepole pine, white spruce is generally thought to require a later first thinning than lodgepole pine [65]. While the white spruce thinning response projects from Alberta are in their infancy (e.g., Alberta Thinning Network; Figure 3), there is information available from studies in similar species. The inception of programs such as the Green River thinning trials in naturally regenerating balsam fir- (Abies balsamea [L.] Mill.) dominated stands in northwestern New Brunswick [18,66,67], the University of Maine’s Commercial Thinning Research Network (CTRN), and Austin Pond studies in spruce-fir (Picea-abies) stands in the northeastern US [44,68,69], as well as studies in Norway spruce (Picea abies L.) in northern Europe [40,59,70], have provided further insights regarding thinning need. Based on long-term data generated in the Green River thinning trials, Pitt et al. [18] demonstrated that thinning balsam fir to a nominal spacing of 1.8 m (2990 stems ha−1) offered the best balance between individual tree growth and adequate density for maximizing per-hectare production. Another study in New Brunswick has shown that early single-entry thinning (age 19) increased quadratic mean diameter and mean merchantable volume per stem at the end of the observation period by 10% and 24%, respectively, over unthinned stands; a second thinning removed an additional 48–64 m3 ha−1, increased diameter and resulted in volume gains of 25% and 71%, respectively [65]. In some cases, delaying the first thinning to 34 years resulted in marginal differences with early thinning at age 19 or 24, but the risk of windthrow increased [71]. One note of caution, however, is that the study by Achim et al. [71] was conducted on a red spruce (Picea rubens Sarg.) plantation, making the responses more relative to the site conditions. Wagle et al. [43] report the results of a replicated spruce-fir thinning trial in northern Maine, USA. The stand used was derived from natural regeneration either after shelterwood removal cutting or salvage clearcutting following the eastern spruce budworm (Choristoneura fumiferana) outbreak in the 1970s–1980s. After 16–18 years from treatment, they found that delaying commercial thinning in young spruce-fir stands did not enhance total yield and stand value. Thinning from below enhanced average tree size and sawlog production, while thinning from above had detrimental effects on tree size and total yield [72,73]. The White Spruce Management Handbook for DNR Forestry-Administered Lands by Schnell et al. [74] recommended commercial thinning from below to a basal area of 25–27 m2 ha−1 for around 20–25 years. It went on to suggest a commercial thinning between 30 and 50 years if the live crown ratio of both the dominant and co-dominant trees is more than or equal to 40%. Stands allowed to grow beyond this stage may be too constricted to fully respond to commercial thinning.
In some experiments, the difference in volume production between conventional thinning from below and thinning from above has been small and insignificant. Given the same basal area removal in each cutting, Eriksson and Karlsson [75] found a slight growth reduction (6%) in Norway spruce stands after repeated thinning from above compared with thinning from below. In general, there is limited literature regarding the thinning of white spruce in Alberta, though some examples include the studies in [48,76]. While white spruce commercial thinning trials in Alberta are scarce with incomplete information for developing province-specific guidelines (especially in managed stands), there is growing evidence from eastern Canada that thinning managed stands from below to basal areas in the range of 22 to 35 m2 ha−1 could increase the merchantable wood volume from 11 to 60 m3 ha−1, depending on stand age and intensity of thinning [65,77,78,79].

5. Risk Factors

Besides the typical trade-offs between wood yield and wood quality, thinning can involve other trade-offs such as those between wood quality and environmental factors or damaging agents. Accounting for the risks of windthrow, snow, fire, drought, and the possibility of increased incidence of insects and disease in management will affect the optimal choice of the thinning regime in terms of financial returns. It should be recognized that most stands will face at least one or more of these risks at some point through their rotation and therefore these risks must be considered thoughtfully when developing density management programs. Consideration of the risks will become increasingly useful, as additional information can help improve the thinning decision-making process. This consideration will require a fundamental shift in how current thinning decisions are viewed; to help mitigate risk and uncertainties, either plans should be revised as new information becomes available or the possibility of adaptation should be accounted for while preparing the plans. In this section, we conduct a brief review of some of the risks and stresses in relation to thinning regimes and highlight how risk management implies trade-offs.

5.1. Wind and Snow Damage

The risk of tree damage by wind (i.e., stem breakage or tree uprooting) following thinning is usually associated with stand age, tree height, and the timing of thinning and its intensity (Table 2). By removing a part of the initial stock, thinning immediately lowers stand stability as remaining trees are not accustomed to increasing wind loads, thereby increasing susceptibility to wind and storm damage. The most severe wind damage appears to occur in larger-diameter trees regardless of thinning intensity [55], and the duration of increased susceptibility is estimated to last from 2 to 10 years after thinning [80]. Detailed scrutiny of broad-scale experiments suggests the risk of wind damage following thinning increases with stand age and tree height [81,82,83]. According to Gardiner et al. [82], the damage is more extensive in heavy thinning performed in the late stages of the rotation. However, light to moderate thinning may even increase the risk of wind and storms in overly stocked mature stands [84]. Trees in such stands are typically under greater stress with high height-to-diameter ratios and/or low stem taper [59,85].
However, there is ample evidence to suggest that well-timed thinning performed at appropriate ages promptly can reduce vulnerability to wind and storm damage. Achim et al. [71] recommended early thinning, particularly on highly fertile sites, to increase wood quality and minimize the risk of wind and storm by promoting the development of structural roots and a more tapered stem. Tarita and Musaka [86] concurred with the recommendation for a first early thinning followed by an ongoing selection of thinning aimed at the continual release of the crown. According to information from Northern Europe, both Norway spruce and Scots pine stands with a mean height of <10 m are associated with a low incidence of wind damage because of improved development of the root system and increased soil anchorage of the remaining trees [58,87]. Several studies on Norway spruce or Scots pine in Europe (e.g., [82,88]) estimated the period of higher probability of wind damage to range between 2 to 10 years, with heavy thinnings prescribed in the late stages of a rotation (i.e., commercial thinning) leading to the highest increase in risk.
Experiments conducted in recent years reveal sufficient evidence that light thinning performed at an early stand age can increase stand resistance to the risk of wind and storm damage [13,82,83,86,89]. Beyond the thinning intensity, the method and form of thinning strongly influence stand stability to snow or wind. Thinning from above or below increases the risk of wind damage in Norway spruce, Scott pine, and European silver fir (Abies alba Mill.) [83,84]; the risk reduces over time with faster recovery when thinned from below than from above [90]. To gain a broader understanding of the underlying processes involved in wind damage following thinning, research has focussed on the development of processed-based models of the interactions between wind and/or snow damage and trees. Duperat et al. [91] described the method of generating balsam fir-specific values of parameters to integrate into the wind risk model Forest GALES, to simulate the impact of different types of commercial thinning on wind damage risk and to determine which practice potentially minimizes the risk in a naturally regenerated stand. The method is based primarily on measuring the wind-induced bending moments experienced in a sample of balsam fir trees by placing an anemometer at canopy height and attaching strain gauges to the trunks. Wind climate parameters for prediction of the probability of damage were calculated using the PC- based airflow model―Wind Atlas Analysis and Application program [92,93]. The early result of this exercise indicates that thinning from below has a reduced risk of wind damage compared with thinning from above. This and other related concepts [89,94,95] may, however, be explored through further trials and is potentially of interest to the silviculture of the most widely regenerated and commercially planted conifer species in Alberta. When thinning from below is performed, the tree with the highest height-to-diameter ratios is removed, and the residual trees reduce their slenderness coefficient, leading to a gain in stability. The subsequent gain in stability may ultimately lead to a reduction of stand vulnerability over the full lifetime of the stand [16,58,96].
Table
Table 2. Summary of the effect of thinning treatments on wind damage risk.
Table 2. Summary of the effect of thinning treatments on wind damage risk.
Author(s)Biome (Country)SpeciesThinning Trmt
(Entries)		Basal Area
Removed		Risk of Wind DamageRemark
Short-TermLong-Term
Achim et al. Boreal Balsam firPre-commercial (1) +30% −The wind speed increased with
[71](Canada) Commercial (1) a reduced stand density
Gardiner et al.Europe *Several conifersPre-commercial; +/−−Late heavy thinning increased wind vulnerability
[82] Commercial Pre-commercial thinning increased wind resistance
Pukkala et al. Boreal Norway sprucePre-commercial +Variable+−Thinning from above increased
[83](Finland) Commercial–below and tree vulnerability to wind damage;
Albrecht et al.TemperateNorway spruceCommercial–above50%+ Heavy thinning from above-destabilized stand
[84](Germany)Douglas firand below
Hanewinkel et al.TemperateNorway spruceCommercialVariable0/− No effect of thinning intensity; wind
[88](Switzerland) damage decreased with time since thinning
[90](Czech Republic) and above (2) after treatment
Duperat et al.Boreal Balsam firPre-commercial (1) +
[92](Canada) Commercial–below 0/+ Thinning from above decreased
Bigelow et al. TemperatePine-broadleafCommercial − Only marginal effect
[97](USA)mix
Ruel et al.BorealBalsam firCommercial–above35.64%0/− No effect of thinning method; risk decreased
[98](Canada)Black spruce with time since thinning (measured up to 9 years)
White spruce
Valinger and BorealNorway spruceCommercial–below20%, 70%+ Thinning from above or below
Pettersson(Sweden)Scott pineand above destabilized stands; higher wind mortality in
[99] thinned stands than unthinned stand
* Positive and negative effects are indicated by + and −, respectively; 0 indicates no effect; empty cells represent non-available information.
In general, the published evidence collectively suggests that in areas where stands are threatened by wind and/or snow loads, it may be necessary to avoid delayed thinning and methods such as thinning from above (dominant thinning). The early result of this exercise indicates that thinning from below has a reduced risk of wind damage compared with thinning from above. This and other related concepts [89,94,95] may, however, be explored through further trials and is potential of interest to the silviculture of the most widely regenerated and commercially planted conifer species in Alberta. When thinning from below is performed, the tree with the highest height-to-diameter ratios is removed, and the residual trees reduce their slenderness coefficient, leading to a gain in stability. The subsequent gain in stability may ultimately lead to a reduction of stand vulnerability over the full lifetime of the stand [16,58,96]. In general, the published evidence collectively suggests that in areas where stands are threatened by wind and/or snow loads, it may be necessary to avoid delayed thinning and methods such as thinning from above (dominant thinning).

5.2. Insects and Pathogens Outbreaks

Thinning may also subject the residual stand to indirect damage from biotic factors such as insects and diseases (Table 3). Pure stands of lodgepole pine are susceptible to infection by stem canker (Atropellis piniphila (Weir) Lohman and Cash), blister rusts (Cronartium spp.), western gall rust (Peridermium harknessii), and Armillaria root disease (Armillaria spp.). Although cankered stems often render the wood useless for lumber or posts and poles, western gall rust is especially damaging [100,101]. Although historically restricted by climatic conditions [102,103], the mountain pine beetle is the most severe insect pest of lodgepole pine. In white spruce, the spruce beetle (Dendroctonus rufipennis (Kirby) has been associated with sites with blowdown, logging slash, or damaged standing timber. Young regenerating sites are very susceptible to Armillaria root rot (Armillaria mellea), which causes scattered mortality and windthrow [104].
Results from the Green River study provide evidence that pre-commercial thinning may increase the incidence of butt rot in balsam fir-dominated stands, with incidence proportional to the thinning intensity [67]. They also observed the incidence and volume of butt rot to increase with stem diameter. Cruickshank et al. [104] and Morrison et al. [105] found that the percentage of Douglas-fir trees with Armillaria root lesions and subsequent mortality was significantly greater in thinned than unthinned stands in interior British Columbia, but when the same intervention was carried out in ponderosa pine (Pinus ponderosa) stands in Central Oregon [106,107], thinning increased tree diameter growth and decreased the incidence of crop-tree mortality after 30 years. Filip and Ganio [108] later reported results of early thinning of a mixed-species forest of Douglas-fir, Hemlock, and True-fir; from a root-disease perspective, pre-commercial thinning does not affect the incidence of crop-tree mortality after 20 years. Filip et al. [106,107] explained that thinning boosts vigour by reducing competition stress, thereby conceivably enhancing disease resistance among residual host trees. Given this suggestion, the discrepancy between these studies may be in part due to interacting effects (and stresses) associated with climate, such as drought. By further considering the importance of root rot disease in southern pine, it would be of practical value to know whether the overall risk for spreading root rot, given the same periodic mean basal area, is highest with one early heavy thinning or with a more frequent thinning schedule in which smaller amounts of basal area are removed in each thinning [55]. Wang et al. [109] concluded with recommendations for reducing the spread of butt rot in Norway spruce, which supports the view that thinning should occur in winter combined with stump treatment. However, this recommendation from Europe requires some caution as in western North America, Armillaria spore production is at its highest level in January and February. Consequently, thinning during the winter months increases the likelihood of infection because cutting exposes stump surfaces to infection [55].
Experiments conducted regarding the effects of thinning on gull rust formation are rare in Alberta’s forests and any case, are still at too early a stage to draw any general conclusions, although a direct relationship between stand age at thinning and stem gall incidences (post-thinning infection) has been identified in western Canada [110]. Blenis and Duncan [108] found the incidence of stem gull rust increases as stand density decreases, which supports the view by Anderson and Mistretta [111] that thinning should be delayed in stands where gull rust is common. One note of caution is that delaying first thinning to such an extent in naturally regenerating lodgepole pine or white spruce stands may compromise the competitive status of future residual trees as described by Le Goff and Ottorini [112], i.e., reducing the crown ratio as a proportion of overall tree height and therefore the tree’s ability to respond to thinning. Interestingly, when considering both gall rust and root diseases together, thinning has been effective in mitigating the spread of western gall rust and improving the overall forest growth of pine affected by Armillaria root disease [113]. In the case of root rot infections of lodgepole pine, the use of chemicals such as borax (sodium borate) or direct stump removal may provide the most positive control of pathogen incidence [114].
Mason [115] found that thinning attracted large numbers of southern pine engraver (Ips avulsus Eichh.) and eastern five-spined engraver (Ips grandicollis Eichh.) to infested logging slash in experimental areas.
Table
Table 3. Overview of published evidence on the effects of thinning treatments on insect and pathogens infestation (assessed through a percentage of trees attacked following thinning and mortality).
Table 3. Overview of published evidence on the effects of thinning treatments on insect and pathogens infestation (assessed through a percentage of trees attacked following thinning and mortality).
Author (s)Biome (Country)Species Insect/PathogenThinning trmt. (Entries)% Basal Area RemovedEffect/
Attacked		Mortality Remark
Filip et al.TemperateDouglas firArmillaria rootPre-commercial–10 −Positive effect on reducing
[105](USA) diseasebelow root disease
Hood and Sala [116]TemperateDouglas firBark beetleCommercial–0, 50–60 0/−Heavy thinning was
(USA)Ponderosa below (1) was not effective
pine
Scheller et al. [117]Temperate White firBark beetleCommercial–below + 0/−Positive effect on reducing
(USA) prescribed burning (1) tree mortality; ineffective
at the landscape scale
Stadelmann et al.
[118]		TemperateNorway Bark beetleSanitation cut (1) +/−Windstorms increased
Switzerland)spruce bark beetle infestation
Negrón et al. [119]TemperatePonderosa Mountain pineCommercial–below (1) −Thinning from below was
(USA)pinebeetle most effective
Steel et al. [120] TemperateWhite fir Commercial–variable+/−0/−Thinning + burning
(USA) below and above + increased beetle
burning (1) infestation probability
Regolini et al. [121]TemperateMaritime Root and but rotCommercial —0, 33, 500–Complementary stump chemical
(France)pine below + stump showed great potential
to reduce root rot infection
Six et al. [122]Temperate Mountain pineCommercial 0/−Commercial thinning performed at an
(USA) beetle early stand age generally reduces
mortality
Morris et al. [123]TemperateLodgepole Mountain pineCommercialvariable Only marginal effect on resistance
(USA)pinebeetle
Positive and negative effects are indicated by + and − respectively; 0 indicates no effect; empty cells represent non-available information.
However, the beetles did not attack residual trees and, upon emergence, dispersed to new sources of attraction. Nebeker et al. [124] made similar observations of experimental efforts near Starkville, Mississippi. However, during the following 2 years, some mortality of residual stems occurred when thinning slash was left around the base of residual trees. Despite the negative findings reported by Mason [115] and Nebeker et al. [124], there is growing evidence that thinning directly reduces the negative effect of different insect outbreaks, such as mountain pine beetles (e.g., Stadelmann et al. [118]; Negrón et al. [119], sirex woodwasp (Sirex noctilio) [125], and spongy moths (Lymantria dispar L.) [126].
To summarize, the interactions between thinning treatments and the spread of disease and insects are mixed with thinning, often resulting in negative outcomes, though some positive outcomes have been noted across studies. In some instances, mitigation strategies can be used to offset this potential risk. Given the mixed results, the importance of a localized understanding of pathogens is imperative when conducting thinning. Research programs involving PCT and CT should continue to report on outcomes associated with increased or decreased risk of diseases and insects to provide a more comprehensive body of knowledge in this respect.

5.3. Drought Resistance

Several studies suggest that a wide range of species and ecosystems are already suffering from drought-related mortality in several parts of their distribution area due to recent warming [127,128,129]. Although drought impacts on forest systems are most severe in water-limited regions, recent trends in elevated tree mortality during changes in the hydrological balance within temperate and boreal ecosystems; for example, Conly and Van der Kamp [130] and Vardy et al. [131] highlight the importance of future drought impacts in northern forests. Lodgepole pine is a ubiquitous species that can grow under a wide variety of climatic and soil conditions (tolerance to minimum temperatures ranges from −7 °C on the Pacific Coast to −57 °C in the northern Rocky Mountains), but it is already suffering from a drought-related decline in several parts of its distribution; see Monserud et al. [132] and Liepe et al. [133]. White spruce is less tolerant of drought than lodgepole pine [134] and there is also evidence suggesting a decline in the growth of white spruce in western Canada due to recent climatic dryings [135].
A key question related to managing forests within the context of climate change is how to minimize the impacts of increasing drought frequency and intensity. Even if recent work provides incomplete information, there is considerable evidence that thinning treatments have the potential to improve growth and reduce mortality under drought conditions. Water availability in the soil is likely to increase after thinning due to lower interception and consumption through transpiration and thus ameliorating, albeit temporarily, the negative effects of increased drought [136,137]. This positive effect increases with thinning intensity, with heavy thinning that removes more than 40% of the basal area of conifers or broadleaved trees being the most effective [138,139]. Carbon et al. [140] and Bello et al. [141], in an attempt to understand the role of thinning in mitigating climate change, removed 80% of the basal area of Scots pine (Pinus sylvestris) and oak (Quercus spp.) in mixtures and monoculture. They found that such thinning intensity significantly improved radial growth recovery after drought events, but it scarcely affected the resistance to drought-induced mortality. In the case of >40% basal area removal, if the crown size is allowed to increase in the case of a single heavy thinning intervention or because of very late and light multiple thinnings, then the initial positive effects on drought can even be reversed as the stand matures [42,142,143]. In the forestry context of Alberta, removing more than 40% of a stand’s basal area during treatment is not recommended because of increased blowdown risk. Such a reversal of positive effect could commence within 1–40 years post-thinning as the leaf/sapwood area ratio increases [42]. A higher leaf/sapwood area ratio in mature large trees often results in increased water demand, which can result in higher vulnerability for thinned stands. Timely intervention may therefore be critical in improving the response and resilience of thinned stands to drought.

6. Biodiversity, Climate Change and Forest Resilience

As thinning opens the overstory canopy, changes in understory vegetation abundance and composition have been documented [144,145,146]. These increases can be short-lived; plant communities may shift toward their pre-treatment character within 10 years, though in some situations it may take several decades [147]. Thinned trees that fall to the ground increase understory plant composition and diversity because fallen trees modulate ground temperature, provide cover, and increase habitat complexity [148]. Fallen trees ultimately decompose and provide a humic-rich forest floor with high moisture-holding capacity and biologically complex and diverse soil taxa [149]. In some cases, there can be initial severe damage to the understory vegetation, reduced growth in residual trees, and increased susceptibility to pest attack if a considerable disturbance is caused by density management [150]. While these studies illustrate that the vegetation community changes, the implications for forest values are unclear as these changes may be illustrative of a vegetation community that is associated with older forests—if this is the case, there may be an opportunity to utilize density management treatments to shift understory structure to favour a greater degree of heterogeneity in the landscape and/or utilize this knowledge to enhance habitat for species conservation.
An additional but largely unexplored benefit of thinning practices in managed and unmanaged forests can be the acceleration of carbon sequestration. Thinning operations tend to provoke a loss in soil carbon due to alterations in soil temperature and moisture conditions [16]. The choice of tree species, the intensity of the thinning regime, and the length of the rotation cycle are some of the decisions that can directly influence biomass and soil carbon stock. Tree species affect biomass carbon and influence inputs in soils due to differential growth rates and crown development. In the case of the thinning intensity, it is debated whether heavy thinning with a short rotation length or light thinning with a longer rotation be given priority to meet carbon sequestration goals. Further, given the changing climate expected for the boreal forest bringing more frequent and severe droughts, density management practices such as thinning have the potential to increase tree and forest resilience. Fewer trees in a stand mean that each remaining tree has access to more resources and specifically in this context, water, thereby enabling the biodiversity across the entire landscape to better withstand drought [151].

7. Concluding Remarks

In general, the literature compiled in this review has revealed sufficient evidence to support the view that stands density management through thinning is likely to have positive effects on tree growth, although the magnitude depends on the thinning regime, the region, site, and stand age. Heavy thinning (removing more than 40% of basal area) can be effective at maximizing the diameter increment of future crop trees and may provide short-term mitigation of drought conditions. However, if not performed at an early stage, moderate to heavy thinning can destabilize stands and cause wood volume losses by increasing vulnerability to wind and storm damage. This higher vulnerability can last from 2 to 10 years following thinning. Numerous studies recommend thinning lodgepole pine stands to a basal area of approximately 18 square meters of basal area per hectare to reduce both the frequency and intensity of mountain pine beetle infestations, though heavy thinning may not be universally appropriate because of other more certain risks and losses. Light thinning (removing no more than 25% of the stand’s basal area) can be an effective tool for resisting insect and pathogen outbreaks. In case of a significant insect outbreak, light and repeated thinning treatments could have a positive legacy effect on shaping post-outbreak successional trajectories. Regarding pathogen infections such as Armillaria root disease, contemporary stump treatment may be necessary to avoid the spread of infection in stands where this disease is prevalent. Thinning part of the stand temporarily increases the risk of wind damage to residual trees. Since the risk of wind damage through thinning increases with stand age and height, it is recommended that delayed heavy thinning should be avoided in mature stands where the risk of wind damage is high.
Forest managers cannot be expected to maximize all ecosystem services and functions at the same time, so it is crucial to have a solid understanding of the key motivations and objectives behind a thinning program and to identify trade-offs or limitations that may arise as a consequence. Besides the typical trade-offs between wood yield and value, thinning can involve other trade-offs such as those between wood yield or quality and factors causing indirect thinning damage including wind, ice, and the possibility of increased incidence of insects and disease, which need to be accounted for. Trade-offs between adaptation to and mitigation of climate change impacts such as drought are especially important for thinning interventions. To adapt lodgepole pine or white spruce to the severity of drought (which is predicted for many regions in Alberta), some of the research identified suggests that heavy thinning may provide some respite to remaining trees; however, the upper limit for effective high-intensity thinning as well as the timeframe that this benefit may be realized remains uncertain.
While this review mainly investigated the potential of thinning in these contexts from North American and European perspectives, studies from Alberta’s boreal forests are drastically underrepresented, with few rigorous experiments highlighting what influences density-independent mortality following thinning. It is therefore not surprising that forest managers in the province currently lack strong evidence to identify interventions that maximize volumetric yields, end-product quality, and value without compromising the forest’s resilience against multifaceted and unexpected risks in the future. For thinning to become part of normal forestry operations in Alberta, the first step should be to revisit the existing trials and studies, to link key stand attributes, such as density, structure, and composition to thinning prescriptions and stressors and their interactions. In parallel, there is an imperative for new thinning trials that include a variety of thinning treatments, to identify key stand attributes that can be linked with resistance and resilience to future forest stressors. Future thinning treatment trials in Alberta need to be located across a broad range of stand types in natural or managed stands, representing a continuum of homogenous (single species) to heterogeneous (mixed-species and strata) blocks. This would facilitate the full replication of a broader range of treatments; future focus on heterogeneous stands and potentially integrating reforestation practices to encourage more of these stand types is likely a wise step forward from a risk-mitigation standpoint. Multi-species stands may include multiple coniferous species plantings as well as mixed wood stands.

Funding

Funding for this work was provided by the Forest Growth Organization of Western Canada (FGrOW) and a grant from the Forest Resource Improvement Association of Alberta (FRIAA) project FOOMOD-01-040.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

We also acknowledge with genuine thanks and appreciation the support received from Michel Huot, as well as the very helpful editing, and the provision of relevant thinning materials, particularly from eastern Canada and the US. We would like to thank two anonymous reviewers for commenting on earlier versions of this review.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. AFE (Alberta’s Forest Economy). A Handbook of Public Economic and Socioeconomic Accounts | Alberta Agriculture and Forestry. 2021. Available online: https://open.alberta.ca/dataset/bcec0091-cac0-4257-a8fd-bee57b8a0e6a/resource/001d198d-f495-4cf7-a10c-729f9e7e4887/download/af-albertas-forest-economy-2021.pdf (accessed on 20 July 2022).
  2. Perez-Garcia, J.; Joyce, L.A.; McGuire, A.D.; Xiao, X. Impacts of climate change on the global forest sector. Clim. Chang. 2002, 54, 439–461. [Google Scholar] [CrossRef]
  3. Lusebrink, I.; Erbilgin, N.; Evenden, M.L. The lodgepole× jack pine hybrid zone in Alberta, Canada: A stepping stone for the mountain pine beetle on its journey east across the boreal forest? J. Chem. Ecol. 2013, 39, 1209–1220. [Google Scholar] [CrossRef]
  4. Shegelski, V.A.; Campbell, E.O.; Thompson, K.M.; Whitehouse, C.M.; Sperling, F.A. Source and spread dynamics of mountain pine beetle in central Alberta, Canada. Can. Entomol. 2021, 153, 314–326. [Google Scholar] [CrossRef]
  5. Stanturf, J.A.; Mansuy, N. COVID-19 and forests in Canada and the United States: Initial assessment and beyond. Front. For. Glob. Chang. 2021, 4, 666960. [Google Scholar] [CrossRef]
  6. Pinno, B.D.; Thomas, B.R.; Lieffers, V.J. Wood supply challenges in Alberta–Growing more timber is the only sustainable solution. For. Chron. 2021, 97, 106–108. [Google Scholar] [CrossRef]
  7. White, J. Evaluating the Opportunities for Nutrition and Density Management of Fire Origin Lodgepole Pine in Alberta: An Opinion Paper; Alberta Research Council Inc.: Vegreville, AB, Canada, 2002. [Google Scholar]
  8. Gauthier, M.M.; Barrette, M.; Tremblay, S. Commercial thinning to meet wood production objectives and develop structural heterogeneity: A case study in the spruce-fir forest, Quebec, Canada. Forests 2015, 6, 510–532. [Google Scholar] [CrossRef]
  9. Hossain, K.L.; Lieffers, V.J.; Pinno, B.D. Thinning to meet sawlog objectives at shorter rotation in lodgepole pine stands. Can. J. For. Res. 2022, 52, 940–950. [Google Scholar] [CrossRef]
  10. Greenway, K.; Klappstein, G.; White, B. Partial Harvest (Non-Clearcut) Planning and Monitoring Guidelines. A Supplement to the Alberta Forest Management Planning Standard. CAT89231716/PDF. 2006. Available online: https://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/formain15749/$FILE/Partial_Harvest_Guidelines_July_2006_Final.pdf (accessed on 20 July 2022).
  11. Oliver, C.D.; Larson, B.C. Forest Stand Dynamics; Updated Edition; John Wiley and Sons: New York, NY, USA, 1996; Available online: https://elischolar.library.yale.edu/fes_pubs/1/ (accessed on 3 March 2021).
  12. Cameron, A.D. Importance of early selective thinning in the development of long-term stand stability and improved log quality: A review. Forestry 2002, 75, 25–35. [Google Scholar] [CrossRef]
  13. Novak, J.; Dusek, D.; Slodicak, M.; Kacalek, D. Importance of the first thinning in young mixed Norway spruce and European beech stands. J. For. Res. 2017, 63, 254–262. [Google Scholar] [CrossRef] [Green Version]
  14. Newton, P.F. Quantifying growth responses of black spruce and jack pine to thinning within the context of density management decision-support systems. Open J. For. 2015, 5, 409. [Google Scholar] [CrossRef] [Green Version]
  15. Zhang, J.; Oliver, W.W. Stand structure and growth of Abies magnifica responded to five thinning levels in northeastern California, USA. For. Ecol. Manag. 2006, 223, 275–283. [Google Scholar] [CrossRef]
  16. Cao, T.; Valsta, L.; Härkönen, S.; Saranpää, P.; Mäkelä, A. Effects of thinning and fertilization on wood properties and economic returns for Norway spruce. For. Ecol. Manag. 2008, 256, 1280–1289. [Google Scholar] [CrossRef]
  17. Pitt, D.; Lanteigne, L.; Hoepting, M.K.; Plamondon, J.; Duchesne, I.; Bicho, P.; Warren, G. Effects of precommercial thinning on the forest value chain in northwestern New Brunswick: Part 6–Estimating the economic benefits. For. Chron. 2013, 89, 502–511. [Google Scholar] [CrossRef] [Green Version]
  18. Das Gupta, S.; Pinno, B.D.; McCready, T. Commercial thinning effects on growth, yield and mortality in natural lodgepole pine stands in Alberta. For. Chron. 2020, 96, 111–120. [Google Scholar] [CrossRef]
  19. BC Ministry of Forests. Guidelines for Commercial Thinning; British Columbia Ministry of Forests: Victoria, BC, Canada, 1999; 71p. Available online: http://www.for.gov.bc.ca/hfp/publications/00007/ct0726.pdf (accessed on 3 March 2021).
  20. Gauthier, M.M.; Tremblay, S. Late-entry commercial thinning effects on Pinus banksiana: Growth, yield, and stand dynamics in Québec. Can. J. For. Res. 2019, 30, 95–106. [Google Scholar] [CrossRef] [Green Version]
  21. Cochran, P.; Dahms, W.G. Growth of Lodgepole Pine Thinned to Various Densities on Two Sites with Differing Productivities in Central Oregon; Pacific Northwest Research Station Research Paper: PNW-RP-520; USDA Forest Service: St. Paul, MN, USA, 2000. [Google Scholar]
  22. Bott, R.D.; Murphy, P.J.; Udell, R. Growing the new forest–The evolution of applied silviculture in Alberta. For. Chron. 2014, 90, 324–329. [Google Scholar] [CrossRef] [Green Version]
  23. Crossley, D.I. Lodgepole Pine Studies at the Strachan Experimental Block in Alberta; Forest Resesrch Division Technical Note No. 19; Department of Northern Affairs and National Resources Forestry Branch: Ottawa, ON, Canada, 1955; Available online: https://d1ied5g1xfgpx8.cloudfront.net/pdfs/30490.pdf (accessed on 3 March 2021).
  24. Smithers, L. Lodgepole Pine in Alberta; Department of Forestry, Government of Canada Bulletin 127; Government of Canada: Ottawa, ON, Canada, 1961; pp. 1–152. Available online: https://d1ied5g1xfgpx8.cloudfront.net/pdfs/23870.pdf (accessed on 20 August 2022).
  25. Amoroso, M.M.; Daniels, L.D.; Bataineh, M.; Andison, D.W. Evidence of mixed-severity fires in the foothills of the Rocky Mountains of west-central Alberta, Canada. For. Ecol. Manag. 2011, 262, 2240–2249. [Google Scholar] [CrossRef]
  26. McIntosh, A.C.; Macdonald, S.E. Potential for lodgepole pine regeneration after mountain pine beetle attack in newly invaded Alberta stands. For. Ecol. Manag. 2013, 295, 11–19. [Google Scholar] [CrossRef]
  27. Oboite, F.O.; Comeau, P.G. Release response of black spruce and white spruce following overstory lodgepole pine mortality due to mountain pine beetle attack. For. Ecol. Manag. 2019, 432, 446–454. [Google Scholar] [CrossRef]
  28. Reyes-Hernandez, V.; Comeau, P.G.; Bokalo, M. Static and dynamic maximum size–density relationships for mixed trembling aspen and white spruce stands in western Canada. For. Ecol. Manag. 2013, 289, 300–311. [Google Scholar] [CrossRef]
  29. Quaite, J. Severe thinning in an overstocked lodgepole pine stand. Can. Dep. Resour. Dev., For. Branch, For. Res. Div., Ottawa, ON. Silvicult. Leafl. 1950, 47, 2. [Google Scholar]
  30. Stewart, J.D.; Jones, T.N.; Noble, R.C. Long-Term Lodgepole Pine Silviculture Trials in Alberta: History and Current Results. 2006. Available online: https://d1ied5g1xfgpx8.cloudfront.net/pdfs/26202.pdf (accessed on 4 June 2021).
  31. Bella, I.E. Thinning Lodgepole Pine by brute force: Three implements in a decade’s perspective. For. Chron. 1990, 66, 611–615. [Google Scholar] [CrossRef] [Green Version]
  32. Pinno, B.D.; Lieffers, V.J.; Landhäusser, S.M. Inconsistent growth response to fertilization and thinning of lodgepole pine in the Rocky Mountain Foothills is linked to site index. Int. J. For. Res. 2012, 2012, 193975. [Google Scholar] [CrossRef] [Green Version]
  33. Dempster, W.R. Effects of Planting, Vegetation Management, and Pre-Commercial Thinning on the Growth and Yield of Lodgepole Pine Regenerated after Harvesting in Alberta, Canada. Forests 2022, 13, 929. [Google Scholar] [CrossRef]
  34. Kent, J.; McGrath, T.; Murray, B.; Rushton, T. Commercial Thinning Survey: 5-Year Results; Report FOR 2012-6 No. 93; Forest Management Planning Section, Nova Scotia Department of Natural Resources: Halifax, NS, Canada, 2012; pp. 1–22. [Google Scholar]
  35. Smith, R.G.B.; Brennan, P. First thinning in sub-tropical eucalypt plantations grown for high-value solid-wood products: A review. Aust. For. 2006, 69, 305–312. [Google Scholar] [CrossRef]
  36. Newton, P.F. Stand density management diagrams: Modelling approaches, variants, and exemplification of their potential utility in crop planning. Can. J. For. Res. 2021, 51, 236–256. [Google Scholar] [CrossRef]
  37. Soucy, M.; Lussier, J.M.; Lavoie, L. Long-term effects of thinning on growth and yield of an upland black spruce stand. Can. J. For. Res. 2012, 42, 1669–1677. [Google Scholar] [CrossRef]
  38. Morin, H. Dynamics of balsam fir forests in relation to spruce budworm outbreaks in the boreal zone of Quebec. Can. J. For. Res. 1994, 24, 730–741. [Google Scholar] [CrossRef]
  39. Pothier, D.; Elie, J.G.; Auger, I.; Mailly, D.; Gaudreault, M. Spruce budworm-caused mortality to balsam fir and black spruce in pure and mixed conifer stands. For. Sci. 2012, 58, 24–33. [Google Scholar] [CrossRef] [Green Version]
  40. Nilsson, U.; Agestam, E.; Ekö, P.M.; Elfving, B.; Fahlvik, N.; Johansson, U.; Karlsson, K.; Lundmark, T.; Wallentin, C. Thinning of Scots Pine and Norway Spruce Monocultures in Sweden No. 219; Swedish University of Agricultural Sciences Faculty of Forest Sciences: Umeå, Sweden, 2010. [Google Scholar]
  41. Bergh, J.; Nilsson, U.; Allen, H.L.; Johansson, U.; Fahlvik, N. Long-term responses of Scots pine and Norway spruce stands in Sweden to repeated fertilization and thinning. For. Ecol. Manag. 2014, 320, 118–128. [Google Scholar] [CrossRef]
  42. D’Amato, A.W.; Bradford, J.B.; Fraver, S.; Palik, B.J. Effects of thinning on drought vulnerability and climate response in north temperate forest ecosystems. Ecol. Appl. 2013, 23, 1735–1742. [Google Scholar] [CrossRef] [Green Version]
  43. Wagle, B.H.; Weiskittel, A.R.; Kizha, A.R.; Berrill, J.P.; D’Amato, A.W.; Marshall, D. Long-term influence of commercial thinning on stand structure and yield with/without pre-commercial thinning of spruce-fir in northern Maine, USA. For. Ecol. Manag. 2022, 522, 120453. [Google Scholar] [CrossRef]
  44. Olson, M.G.; Meyer, S.R.; Wagner, R.G.; Seymour, R.S. Commercial thinning stimulates natural regeneration in spruce–fir stands. Can. J. For. Res. 2014, 44, 173–181. [Google Scholar] [CrossRef]
  45. BC Ministry of Forests. Interim Guidance for Commercial Thinning—Interior British Columbia; Ministry of Forests, Lands, Natural Resource Operations and Rural Development: Victoria, BC, Canada, 2021. Available online: https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/forestry/stand-tending/interim_guidance_for_commercial_thinning.pdf (accessed on 3 March 2021).
  46. Moulinier, J.; Brais, S.; Harvey, B.D.; Koubaa, A. Response of boreal jack pine (Pinus banksiana Lamb.) stands to a gradient of commercial thinning intensities, with and without N fertilization. Forests 2015, 6, 2678–2702. [Google Scholar] [CrossRef] [Green Version]
  47. McGrath, T.; Ellingsen, J. The Effects of Hurricane Juan on Managed Stands Commercially in Central Nova Scotia; Report FOR 2009-4 No. 89; Forest Management Planning Section, Nova Scotia Department of Natural Resources: Halifax, NS, Canada, 2009; pp. 1–9. [Google Scholar]
  48. Bjelanovic, I.; Comeau, P.; Meredith, S.; Roth, B. Precommercial thinning increases spruce yields in boreal mixed woods in Alberta, Canada. Forests 2021, 12, 412. [Google Scholar] [CrossRef]
  49. Hawe, J.; Short, I. Broadleaf thinning in Ireland—A review of European silvicultural best practice. Irish. For. 2016, 73, 25–64. [Google Scholar]
  50. Kanninen, M.; Pérez, D.; Montero, M.; Víquez, E. Intensity and timing of the first thinning of Tectona grandis plantations in Costa Rica: Results of a thinning trial. For. Ecol. Manag. 2004, 203, 89–99. [Google Scholar] [CrossRef]
  51. Traugott, T.A.; Dicke, S.G. Are My Pine Trees Ready to Thin? Publication Number 2260; Mississippi State University Extension Service: Starkville, MS, USA, 2006; pp. 1–11. [Google Scholar]
  52. Wright, J.P. First thinning options row thinning vs selection thinning. NZJ For. Sci. 1976, 6, 308–317. [Google Scholar]
  53. Varmola, M.; Salminen, H. Timing and intensity of precommercial thinning in Pinus sylvestris stands. Scand. J. For. Res. 2004, 19, 142–151. [Google Scholar] [CrossRef]
  54. Choi, J.; HEBurkhart, R.L. Amateis. Modelling trends in stem quality characteristics of loblolly pine trees in unthinned loblolly pine plantations. Can. J. For. Res. 2008, 38, 1446–1457. [Google Scholar] [CrossRef]
  55. Nebeker, T.E.; Riggins, J.; Auel, J.; Henderson, J.; Ezell, A.; ASelf, A.B.; Matney, T.; London, A. Thinning Practices in Southern Pines—With Pest Management Recommendations; Publication 2893 (1M-10-15); United States Department of Agriculture, Forest Service: Washington, DC, USA, 2015. [Google Scholar]
  56. Pukkala, T.; Miina, J.; Kellomäki, S. Response to different thinning intensities in young Pinus sylvestris. Scand. J. For. Res. 1998, 13, 141–150. [Google Scholar] [CrossRef]
  57. Pukkala, T.; Lähde, E.; Laiho, O. Which trees should be removed in thinning treatments? For. Ecosyst. 2015, 2, 32. [Google Scholar] [CrossRef] [Green Version]
  58. Wallentin, C.; Nilsson, U. Storm and snow damage in a Norway spruce thinning experiment in southern Sweden. Forestry 2014, 87, 229–238. [Google Scholar] [CrossRef] [Green Version]
  59. Mäkinen, H.; Isomäki, A. Thinning intensity and long-term changes in increment and stem form of Scots pine trees. For. Ecol. Manag. 2004, 203, 21–34. [Google Scholar] [CrossRef]
  60. Volkova, L.; Bi, H.; Hilton, J.; Weston, C.J. Impact of mechanical thinning on forest carbon, fuel hazard and simulated fire behaviour in Eucalyptus delegatensis forest of south-eastern Australia. For. Ecol. Manag. 2017, 405, 92–100. [Google Scholar] [CrossRef]
  61. Ballard, L.A.; Long, J.N. Influence of stand density on log quality of lodgepole pine. Can. J. For. Res. 1988, 18, 911–916. [Google Scholar] [CrossRef]
  62. Dennis, F.C.; Burke, J.; Duda, J.; Green, C.; Hessel, D.; Kaufmann, M.; Lange, D.; Lee, B.; Rinke, H.; Sheppard, W.; et al. Lodgepole Pine Management Guidelines for Land Managers in the Wildland-Urban Interface. 2010. Available online: https://static.colostate.edu/client-files/csfs/pdfs/lpp-guide-LS-www.pdf (accessed on 3 March 2021).
  63. Demers, C.; Andreu, M.; McGowan, B.; Long, A.; Nowak, J. Thinning Southern Pines—A Key to Greater Returns; SSFOR24; Ask IFAS—Powered by EDIS: Gainesville, FL, USA, 2013. [Google Scholar]
  64. Pavel, M. Operational Manual for Commercial Thinning in British Columbia; Technical Report: TR 2021 No.93; BC Ministry of Forests: Victoria, BC, Canada, 2021. [Google Scholar]
  65. Pelletier, G.; Pitt, D.G. Silvicultural responses of two spruce plantations to mid-rotation commercial thinning in New Brunswick. Can. J. For. Res. 2008, 38, 851–867. [Google Scholar] [CrossRef]
  66. Plamondon, J.; Pitt, D.G. Effects of precommercial thinning on the forest value chain in northwestern New Brunswick: Part 2–efficiency gains in cut-to-length harvesting. For. Chron. 2013, 89, 458–463. [Google Scholar] [CrossRef]
  67. Warren, G.; Baines, P.; Plamondon, J.; Pitt, D.G. Effects of precommercial thinning on the forest value chain in northwestern New Brunswick: Part 3–Incidence of root and butt decay. For. Chron. 2013, 89, 464–473. [Google Scholar] [CrossRef]
  68. Bataineh, M.M.; Wagner, R.G.; Weiskittel, A.R. Long-term response of spruce-fir stands to herbicide and pre-commercial thinning: Observed and projected growth, yield, and financial returns in central Maine, USA. Can. J. For. Res. 2013, 43, 385–395. [Google Scholar] [CrossRef]
  69. Kuehne, C.; Weiskittel, A.R.; Wagner, R.G.; Roth, B.E. Development and evaluation of individual tree-and stand-level approaches for predicting spruce-fir response to commercial thinning in Maine, USA. For. Ecol. Manag. 2016, 376, 84–95. [Google Scholar] [CrossRef] [Green Version]
  70. Krajnc, L.; Farrelly, N.; Harte, A.M. The effect of thinning on mechanical properties of Douglas fir, Norway spruce, and Sitka spruce. Ann. For. Sci. 2019, 76, 3. [Google Scholar] [CrossRef] [Green Version]
  71. Achim, A.; Ruel, J.C.; Gardiner, B.A. Evaluating the effect of precommercial thinning on the resistance of balsam fir to windthrow through experimentation, modelling, and development of simple indices. Can. J. For. Res. 2005, 35, 1844–1853. [Google Scholar] [CrossRef]
  72. Zeide, B. Thinning and growth: A full turnaround. J. For. 2001, 99, 20–25. [Google Scholar]
  73. Clune, P.M. Growth and Development of Maine Spruce-Fir Forests Following Commercial Thinning Electronic Theses and Dissertations 1983. Master’s Thesis, The University of Maine, Orono, ME, USA, 2013. [Google Scholar]
  74. Schnell, B.; Albers, M.; Dubuque, P.; Liedholm, J.; Klevorn, R. White Spruce Management Guidance for DNR Forestry-Administered Lands. Version 1. 2012. Available online: https://files.dnr.state.mn.us/forestry/ecssilviculture/covertype/manageguidance_whiteSpruce.pdf (accessed on 20 August 2022).
  75. Eriksson, H.; Karlsson, K. Effects of Different Thinning and Fertilization Regimes on the Development of Scots Pine (Pinus sylvestris (L.)) and Norway Spruce (Picea abies (L.) Karst.) Stands in Long-Term Silvicultural Trials in Sweden; Report No. 42 1135; Swedish University of Agricultural Sciences, Department of Forest Yield Research: Uppsala, Sweden, 1997; ISSN 0348-7636. (In Swedish with English Summary). [Google Scholar]
  76. Comeau, P.G. Effects of thinning on dynamics and drought resistance of aspen-white spruce mixtures: Results from two study sites in Saskatchewan. Front. For. Glob. Chang. 2021, 3, 621752. [Google Scholar] [CrossRef]
  77. Stiell, W.M. Thinning 35-Year-Old White Spruce Plantations from Below: 10-Year Results; Publications No. 1258; Forest Service: Burnaby, BC, Canada, 1970. [Google Scholar]
  78. Stiell, W.M. Response of white spruce plantation to three levels of thinning from below 1958–1978. For. Chron. 1980, 56, 21–27. [Google Scholar] [CrossRef] [Green Version]
  79. Dupont-Leduc, L.; Schneider, R.; Sirois, L. Preliminary results from a structural conversion thinning trial in Eastern Canada. J. For. 2020, 118, 515–533. [Google Scholar] [CrossRef]
  80. Houtmeyers, S.; Brunner, A. Thinning responses of individual trees in mixed stands of Norway spruce and Scots pine. Scand. J. For. Res. 2020, 35, 351–366. [Google Scholar] [CrossRef]
  81. Teste, F.P.; Lieffers, V.J. Snow damage in lodgepole pine stands brought into thinning and fertilization regimes. For. Ecol. Manag. 2011, 261, 2096–2104. [Google Scholar] [CrossRef]
  82. Gardiner, B.; Schuck, A.; Schelhaas, M.-J.; Orazio, C.; Blennow, K.; Nicoll, B. Living with Storm Damage to Forests In What Science Can Tell Us, European Forest Institute. 2013, p. 129. Available online: https://efi.int/publications-bank/living-storm-damage-forests (accessed on 19 December 2020).
  83. Pukkala, T.; Laiho, O.; Lähde, E. Continuous cover management reduces wind damage. For. Ecol. Manag. 2016, 372, 120–127. [Google Scholar] [CrossRef]
  84. Albrecht, A.; Hanewinkel, M.; Bauhus, J.; Kohnle, U. How does silviculture affect storm damage in forests of south-western Germany? Results from empirical modelling based on long-term observations. Eur. J. For. Res. 2012, 131, 229–247. [Google Scholar] [CrossRef]
  85. Saarinen, N.; Kankare, V.; Yrttimaa, T.; Viljanen, N.; Honkavaara, E.; Holopainen, M.; Hyyppä, J.; Huuskonen, S.; Hynynen, J.; Vastaranta, M. Assessing the effects of thinning on stem growth allocation of individual Scots pine trees. For. Ecol. Manag. 2020, 474, 118344. [Google Scholar] [CrossRef]
  86. Torita, H.; Masaka, K. Influence of planting density and thinning on timber productivity and resistance to wind damage in Japanese larch (Larix kaempferi) forests. J. Environ. Manag. 2020, 268, 110298. [Google Scholar] [CrossRef]
  87. Peltola, H.; Kellomäki, S.; Väisänen, H.; Ikonen, V.P. A mechanistic model for assessing the risk of wind and snow damage to single trees and stands of Scots pine, Norway spruce, and birch. Can. J. For. Res. 1999, 29, 647–661. [Google Scholar] [CrossRef]
  88. Hanewinkel, M.; Kuhn, T.; Bugmann, H.; Lanz, A.; Brang, P. Vulnerability of uneven-aged forests to storm damage. Forestry 2014, 87, 525–534. [Google Scholar] [CrossRef] [Green Version]
  89. Kamimura, K.; Gardiner, B.A.; Koga, S. Observations and predictions of wind damage to Larix kaempferi trees following thinning at an early growth stage. Int. J. For. Res. 2017, 90, 350–540. [Google Scholar] [CrossRef] [Green Version]
  90. Moreau, G.; Chagnon, C.; Achim, A.; Caspersen, J.; D’Orangeville, L.; Sánchez-Pinillos, M.; Thiffault, N. Opportunities and limitations of thinning to increase resistance and resilience of trees and forests to global change. Forestry 2022, 95, 595–615. [Google Scholar] [CrossRef]
  91. Duperat, M.; Gardiner, B.; Ruel, J.C. Testing an individual tree wind damage risk model in a naturally regenerated balsam fir stand: Potential impact of thinning on the level of risk. Forestry 2021, 94, 141–150. [Google Scholar] [CrossRef]
  92. Troen, I.; Petersen, E.L. European Wind Atlas; Risø National Laboratory: Roskilde, Denmark, 1989; 656p, Available online: https://orbit.dtu.dk/files/112135732/European_Wind_Atlas.pdf (accessed on 31 August 2020).
  93. Mortensen, N.G.; Heathfield, D.N.; Landberg, L.; Rathmann, O.; Troen, I.; Petersen, E.L. Wind Atlas Analysis and Application Program: WAsP 7 Help Facility; Risø National Laboratory: Roskilde, Denmark, 2002; Available online: https://core.ac.uk/download/pdf/13802337.pdf (accessed on 3 March 2021).
  94. Blennow, K.; Sallnäs, O. WINDA—A system of models for assessing the probability of wind damage to forest stands within a landscape. Ecol. Modell. 2004, 175, 87–99. [Google Scholar] [CrossRef]
  95. Hart, E.; Sim, K.; Kamimura, K.; Meredieu, C.; Guyon, D.; Gardiner, B. Use of machine learning techniques to model wind damage to forests. Agric. For. Meteorol. 2019, 265, 16–29. [Google Scholar] [CrossRef] [Green Version]
  96. Lundqvist, L.; Chrimes, D.; Elfving, B.; Mörling, T.; Valinger, E. Stand development after different thinnings in two uneven-aged Picea abies forests in Sweden. For. Ecol. Manag. 2007, 238, 141–146. [Google Scholar] [CrossRef]
  97. Bigelow, S.W.; Looney, C.E.; Cannon, J.B. Hurricane effects on climate-adaptive silviculture treatments to longleaf pine woodland in southwestern Georgia, USA. For. Int. J. For. Res. 2021, 94, 395–406. [Google Scholar] [CrossRef]
  98. Ruel, J.C.; Pin, D.; Cooper, K. Windthrow in riparian buffer strips: Effect of wind exposure, thinning and strip width. For. Ecol. Manag. 2001, 143, 105–113. [Google Scholar] [CrossRef]
  99. Valinger, E.; Pettersson, N. Wind and snow damage in a thinning and fertilization experiment in Picea abies in southern Sweden. Int. J. For. Res. 1996, 69, 25–33. [Google Scholar] [CrossRef]
  100. Fries, A. Damage by pathogens and insects to Scots pine and lodgepole pine 25 years after reciprocal plantings in Canada and Sweden. Scand. J. For. Res. 2017, 32, 459–472. [Google Scholar] [CrossRef] [Green Version]
  101. Taylor, S.W.; Carroll, A.L.; Alfaro, R.I.; Safranyik, L. Forest, Climate, and Mountain Pine Beetle Outbreak Dynamics in Western Canada. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. 2006, pp. 67–94. Available online: https://d1ied5g1xfgpx8.cloudfront.net/pdfs/26040.pdf (accessed on 31 August 2020).
  102. Negrón, J.F.; Huckaby, L. Reconstructing historical outbreaks of mountain pine beetle in lodgepole pine forests in the Colorado Front Range. For. Ecol. Manag. 2020, 473, 118270. [Google Scholar] [CrossRef]
  103. Westwood, A.R.; Conciatori, F.; Tardif, J.C.; Knowles, K. Effects of Armillaria root disease on the growth of Picea mariana trees in the boreal plains of central Canada. For. Ecol. Manag. 2012, 266, 1–10. [Google Scholar] [CrossRef]
  104. Cruickshank, M.G.; Morrison, D.J.; Punja, Z.K. Incidence of Armillaria species in precommercial thinning stumps and spread of Armillaria ostoyae to adjacent Douglas-fir trees. Can. J. For. Res. 1997, 27, 481–490. [Google Scholar] [CrossRef]
  105. Morrison, D.J.; Pellow, K.W.; Nemec, A.F.; Norris, D.J.; Semenoff, P. Effects of selective cutting on the epidemiology of armillaria root disease in the southern interior of British Columbia. Can. J. For. Res. 2001, 31, 59–70. [Google Scholar] [CrossRef]
  106. Filip, G.M.; Goheen, D.J.; Johnson, D.W.; Thompson, J.H. Precommercial thinning in a ponderosa pine stand affected by Armillaria root disease: 20 years of growth and mortality in central Oregon. West. J. Appl. For. 1989, 4, 58–59. [Google Scholar] [CrossRef]
  107. Filip, G.M.; Fitzgerald, S.A.; Ganio, L.M. Precommercial thinning in a ponderosa pine stand affected by Armillaria root disease in central Oregon: 30 years of growth and mortality. West. J. Appl. For. 1999, 14, 144–148. [Google Scholar] [CrossRef] [Green Version]
  108. Filip, G.M.; Ganio, L.M. Early thinning in mixed-species plantations of Douglas-fir, hemlock, and true fir affected by Armillaria root disease in west central Oregon and Washington: 20-year results. West. J. Appl. For. 2004, 19, 25–33. [Google Scholar] [CrossRef] [Green Version]
  109. Wang, L.; Zhang, J.; Drobyshev, I.; Cleary, M.; Rönnberg, J. Incidence and impact of root infection by Heterobasidion spp., and the justification for preventative silvicultural measures on Scots pine trees: A case study in southern Sweden. For. Ecol. Manag. 2014, 315, 153–159. [Google Scholar] [CrossRef]
  110. Blenis, P.V.; Duncan, I. Management implications of western gall rust in pre-commercially thinned lodgepole pine stands. Can. J. For. Res. 1997, 27, 603–608. [Google Scholar] [CrossRef]
  111. Anderson, R.L.; Mistretta, P.A. Management Strategies for Reducing Losses Caused by Fusiform Rust, Annosus Root Rot, and Littleleaf Disease; Agriculture Handbook No. 597; Department of Agriculture: Washington, DC, USA, 1982; 30p. [Google Scholar]
  112. Le Goff, N.; Ottorini, J.M. Leaf development and stem growth of ash (Fraxinus excelsior L.), as affected by tree competitive status. J. Appl. Ecol. 1996, 33, 793–802. [Google Scholar] [CrossRef]
  113. Hood, I.A.; Kimberley, M.O. Impact of armillaria root disease and the effect of thinning in a late-rotation Pinus radiata plantation. For. Pathol. 2009, 39, 415–427. [Google Scholar] [CrossRef]
  114. Oliva, J.; Thor, M.; Stenlid, J. Long-term effects of mechanized stump treatment against Heterobasidion annosum root rot in Picea abies. Can. J. For. Res. 2010, 40, 020–1033. [Google Scholar] [CrossRef]
  115. Mason, R.R. Behavior of Ips populations after summer thinning in a loblolly pine plantation. For. Sci. 1969, 15, 390–398. [Google Scholar]
  116. Hood, S.M.; Baker, S.; Sala, A. Fortifying the forest: Thinning and burning increase resistance to a bark beetle outbreak and promote forest resilience. Ecol. Appl. 2016, 26, 1984–2000. [Google Scholar] [CrossRef]
  117. Scheller, R.M.; Kretchun, A.M.; Loudermilk, E.L.; Hurteau, M.D.; Weisberg, P.J.; Skinner, C. Interactions among fuel management, species composition, bark beetles, and climate change and the potential effects on forests of the Lake Tahoe Basin. Ecosystems 2018, 21, 643–656. [Google Scholar] [CrossRef] [Green Version]
  118. Stadelmann, G.; Bugmann, H.; Meier, F.; Wermelinger, B.; Bigler, C. Effects of salvage logging and sanitation felling on bark beetle (Ips typographus L.) infestations. For. Ecol. Manag. 2013, 305, 273–281. [Google Scholar] [CrossRef]
  119. Negrón, J.F.; Allen, K.K.; Ambourn, A.; Cook, B.; Marchand, K. Large-scale thinnings, ponderosa pine, and mountain pine beetle in the Black Hills, USA. For. Sci. 2017, 63, 529–536. [Google Scholar] [CrossRef] [Green Version]
  120. Steel, Z.L.; Goodwin, M.J.; Meyer, M.D.; Fricker, G.A.; Zald, H.S.J.; Hurteau, M.D.; North, M.P. Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality? Ecosphere 2021, 12, 03344. [Google Scholar] [CrossRef]
  121. Régolini, M.; Castagneyrol, B.; Dulaurent-Mercadal, A.M.; Piou, D.; Samalens, J.C.; Jactel, H. Effect of host tree density and apparency on the probability of attack by the pine processionary moth. For. Ecol. Manag. 2014, 334, 185–192. [Google Scholar] [CrossRef] [Green Version]
  122. Six, D.L.; Biber, E.; Long, E. Management for mountain pine beetle outbreak suppression: Does relevant science support current policy? Forests 2014, 5, 103–133. [Google Scholar] [CrossRef] [Green Version]
  123. Morris, J.E.; Buonanduci, M.S.; Agne, M.C.; Battaglia, M.A.; Harvey, B.J. Does the legacy of historical thinning treatments foster resilience to bark beetle outbreaks in subalpine forests? Ecol. Appl. 2022, 32, e02474. [Google Scholar] [CrossRef]
  124. Nebeker, T.E.; Leininger, T.D.; Meadows, J.S.; Warriner, M.D. Thinning Southern Bottomland Hardwoods Stands: Insect and Disease Considerations. In Ecology and Management of Bottomland Hardwood Systems: The State of Our Understanding; University of Missouri-Columbia: Puxico, MO, USA, 2005; pp. 467–477. [Google Scholar]
  125. Dodds, K.J.; Cooke, R.R.; Hanavan, R.P. The effects of silvicultural treatment on Sirex noctilio attacks and tree health in northeastern United States. Forests 2014, 5, 2810–2824. [Google Scholar] [CrossRef] [Green Version]
  126. Fajvan, M.A.; Gottschalk, K.W. The effects of silvicultural thinning and Lymantria dispar L. defoliation on wood volume growth of Quercus spp. Am. J. Plant Sci. 2012, 3, 276–282. [Google Scholar] [CrossRef] [Green Version]
  127. Dale, V.H.; Joyce, L.A.; McNulty, S.; Neilson, R.P.; Ayres, M.P.; Flannigan, M.D.; Hanson, P.J.; Irland, L.C.; Lugo, A.E.; Peterson, C.J.; et al. Climate change and forest disturbances: Climate change can affect forests by altering the frequency, intensity, duration, and timing of fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, windstorms, ice storms, or landslides. BioScience 2001, 51, 723–734. [Google Scholar] [CrossRef] [Green Version]
  128. McKenney, D.W.; Pedlar, J.H.; Lawrence, K.; Campbell, K.; Hutchinson, M.F. Potential impacts of climate change on the distribution of North American trees. BioScience 2007, 57, 939–948. [Google Scholar] [CrossRef]
  129. Weed, A.S.; Ayres, M.P.; Hicke, J.A. Consequences of climate change for biotic disturbances in North American forests. Ecol. Monogr. 2013, 83, 441–470. [Google Scholar] [CrossRef]
  130. Conly, F.M.; Van der Kamp, G. Monitoring the hydrology of Canadian prairie wetlands to detect the effects of climate change and land use changes. Environ. Monit. Assess. 2001, 67, 195–215. [Google Scholar] [CrossRef]
  131. Vardy, M.; Oppenheimer, M.; Dubash, N.K.; O’Reilly, J.; Jamieson, D. The intergovernmental panel on climate change: Challenges and opportunities. Annu. Rev. Environ. Resour. 2017, 42, 55–75. [Google Scholar] [CrossRef] [Green Version]
  132. Monserud, R.A.; Yang, Y.; Huang, S.; Tchebakova, N. Potential change in lodgepole pine site index and distribution under climatic change in Alberta. Can. J. For. Res. 2008, 38, 343–352. [Google Scholar] [CrossRef]
  133. Liepe, K.J.; Hamann, A.; Smets, P.; Fitzpatrick, C.R.; Aitken, S.N. Adaptation of lodgepole pine and interior spruce to climate: Implications for reforestation in a warming world. Evol. Appl. 2016, 9, 409–419. [Google Scholar] [CrossRef] [Green Version]
  134. Nienstaedt, H.; Zasada, J.C. White Spruce. In Silvics of North America: Volume 1. Conifers; Agriculture Handbook No., 654; Burns, R.M., Honakala, B.H., Eds.; U.S. Department of Agriculture: Washington, DC, USA, 1990. [Google Scholar]
  135. Hogg, E.H.; Michaelian, M.; Hook, T.I.; Undershultz, M.E. Recent climatic drying leads to age-independent growth reductions of white spruce stands in western Canada. Glob. Chang. Biol. 2017, 23, 5297–5308. [Google Scholar] [CrossRef]
  136. Lagergren, F.; Lankreijer, H.; Kučera, J.; Cienciala, E.; Mölder, M.; Lindroth, A. Thinning effects on pine-spruce forest transpiration in central Sweden. For. Ecol. Manag. 2008, 255, 2312–2323. [Google Scholar] [CrossRef]
  137. Gebhardt, T.; Häberle, K.H.; Matyssek, R.; Schulz, C.; Ammer, C. The more, the better. Water relations of Norway spruce stands after progressive thinning. Agric. For. Meteorol. 2014, 197, 235–243. [Google Scholar] [CrossRef]
  138. Calev, A.; Zoref, C.; Tzukerman, M.; Moshe, Y.; Zangy, E.; Osem, Y. High-intensity thinning treatments in mature Pinus halepensis plantations experiencing prolonged drought. Eur. J. For. Res. 2016, 135, 551–563. [Google Scholar] [CrossRef]
  139. Zamora-Pereira, J.C.; Yousefpour, R.; Cailleret, M.; Bugmann, H.; Hanewinkel, M. Magnitude and timing of density reduction are key for the resilience to severe drought in conifer-broadleaf mixed forests in Central Europe. Ann. For. Sci. 2021, 78, 68. [Google Scholar] [CrossRef]
  140. Cabon, A.; Mouillot, F.; Lempereur, M.; Ourcival, J.M.; Simioni, G.; Limousin, J.M. Thinning increases tree growth by delaying drought-induced growth cessation in a Mediterranean evergreen oak coppice. For. Ecol. Manag. 2018, 409, 333–342. [Google Scholar] [CrossRef]
  141. Bello, J.; Vallet, P.; Perot, T.; Balandier, P.; Seigner, V.; Perret, S.; Couteau, C.; Korboulewsky, N. How do mixing tree species and stand density affect seasonal radial growth during drought events? For. Ecol. Manag. 2019, 432, 436–445. [Google Scholar] [CrossRef]
  142. Bottero, A.; Forrester, D.I.; Cailleret, M.; Kohnle, U.; Gessler, A.; Michel, D.; Bose, A.K.; Bauhus, J.; Bugmann, H.; Cuntz, M.; et al. Growth resistance and resilience of mixed silver fir and Norway spruce forests in central Europe: Contrasting responses to mild and severe droughts. Glob. Chang. Biol. 2021, 27, 4403–4419. [Google Scholar] [CrossRef]
  143. Dempster, W.R. Impact of climate on juvenile mortality and Armillaria root disease in lodgepole pine. For. Chron. 2017, 93, 148–160. [Google Scholar] [CrossRef] [Green Version]
  144. Dodson, E.K.; Peterson, D.W.; Harrod, R.J. Understory vegetation response to thinning and burning restoration treatments in dry conifer forests of the eastern Cascades, USA. For. Ecol. Manag. 2008, 255, 3130–3140. [Google Scholar] [CrossRef]
  145. Ares, A.; Berryman, S.D.; Puettmann, K.J. Understory vegetation response to thinning disturbance of varying complexity in coniferous stands. Appl. Veg. Sci. 2009, 12, 472–487. [Google Scholar] [CrossRef]
  146. Moore, M.M.; Jenness, J.S.; Laughlin, D.C.; Strahan, R.T.; Bakker, J.D.; Dowling, H.E.; Springer, J.D. Cover and density of southwestern ponderosa pine understory plants in permanent chart quadrats (2002–2020). Ecology 2022, 103, e3661. [Google Scholar] [CrossRef]
  147. Pollock, M.M.; Beechie, T.J. Does Riparian Forest Restoration Thinning Enhance Biodiversity? The Ecological Importance of Large Wood. J. Am. Water Resour. Assoc. 2014, 50, 543–559. [Google Scholar] [CrossRef]
  148. Busse, M.; Gerrard, R. Thinning and burning effects on long-term litter accumulation and function in young ponderosa pine forests. For. Sci. 2020, 66, 761–769. [Google Scholar] [CrossRef]
  149. Canadian Forest Service (CFS). The State of Canada’s Forests: Annual Report 2022; Canadian Forest Service: Burnaby, BC, Canada, 2022. [Google Scholar]
  150. Wilson, D.S.; Puettmann, K.J. Density management and biodiversity in young Douglas-fir forests: Challenges of managing across scales. For. Ecol. Manag. 2007, 246, 123–134. [Google Scholar] [CrossRef]
  151. Sohn, J.A.; Saha, S.; Bauhus, J. Potential of forest thinning to mitigate drought stress: A meta-analysis. For. Ecol. Manag. 2016, 380, 261–273. [Google Scholar] [CrossRef]
Land 12 01261 g001 550
Figure 1. Demonstration of the trade-off between value and volume using an example of TASS runs for Douglas fir with a harvest age of 60 years (from BC Ministry of Forests 1999). Note the highest volume is produced when thinning to 600 to 800 stems ha−1, yet the highest value (NPV) occurs at a residual density of 200 stems ha−1.
Figure 1. Demonstration of the trade-off between value and volume using an example of TASS runs for Douglas fir with a harvest age of 60 years (from BC Ministry of Forests 1999). Note the highest volume is produced when thinning to 600 to 800 stems ha−1, yet the highest value (NPV) occurs at a residual density of 200 stems ha−1.
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Figure 2. Commercially thinned lodgepole pine stands in northern Alberta in June 2020 (a) un-thinned (~20,000 stems ha−1) and (b) thinned to 2000 stems ha−1.
Figure 2. Commercially thinned lodgepole pine stands in northern Alberta in June 2020 (a) un-thinned (~20,000 stems ha−1) and (b) thinned to 2000 stems ha−1.
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Figure 3. Commercially thinned white spruce stands in northern Alberta in July 2020 (a) unthinned (~10,000 stems ha−1) and (b) thinned to 1500 stems ha−1.
Figure 3. Commercially thinned white spruce stands in northern Alberta in July 2020 (a) unthinned (~10,000 stems ha−1) and (b) thinned to 1500 stems ha−1.
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Table
Table 1. Summary of long-term lodgepole pine stands thinning and fertilization trial trials in Alberta in this review.
Table 1. Summary of long-term lodgepole pine stands thinning and fertilization trial trials in Alberta in this review.
Trial NameAuthor(s)Age At ThinningSpecies *% Basal Area
Removed		MethodData Collection
(Post-Treatment)		Target VariableEffect
K-57Quaite [29],Commercial Lodgepole66% Above ǂ22 years Total Volume growth−
Stewart et al.77 yearspine(heavy)below Merchantable volume−
[30] Mean height−
Top height−
Basal area −
Quadratic mean diameterǂ
Mean DBHǂ
Slenderness (ht/dbh)−
Spacing factor (SF%)ǂ
58 yearsTotal Volume growthǂ
Merchantable volumeǂ
Mean heightǂ
Top heightǂ
Basal areaǂ
Quadratic mean diameter ǂ
Mean DBHǂ
Slenderness (ht/dbh)−
Spacing factor (SF%)ǂ
MacKay Smithers [25]Pre- Lodgepole64–76%Future crop 49 years Total Volume growth−
Stewart et al.commercial pinetwo thinningtree Merchantable volumeǂ
[30]22 years entries at 22(selective) Mean heightǂ
and 37 years Basal area−
Mean DBHǂ
Slenderness (ht/dbh)0
StrachanCrossley [24]Pre- Lodgepole 50% (heavy)Below54 years Total Volume growth−
Stewart et al.commercial pine Merchantable volume−
[30]85 years Mean height−
Basal area ǂ
Mean DBH0
Above Total Volume growth−
(crown) Merchantable volume−
Mean height−
Basal areaǂ
Mean DBH−
Sanitation * Total Volume growth−
thinning Merchantable volume−
Mean height−
Basal area/Mean DBHǂ/−
Swan Lake Pre-Lodgepole>80%Below26 yearsTotal Volume growth−
Stewart et al.commercialpine(heavy) Merchantable volumeǂ
[30]
Bella [31]		9 years Mean heightǂ
Basal area (BA)−
Mean DBHǂ
Slenderness (ht/dbh)0
Gregg Burn Stewart et al.Pre- Lodgepole >50% Below36 years Total Volume growth0
[30]commercial pine(heavy) Merchantable volumeǂ
7 years Mean heightǂ
Basal area0
Mean DBHǂ
Slenderness (ht/dbh)0
Ricinus Stewart et al.Pre- Lodgepole>80%Below39 years Total Volume growth−
Thinning ǂ[30]commercial pine (heavy) Merchantable volume−
fertilization 15 years Mean height0
Basal area0
Mean DBH0
Slenderness (ht/dbh)0
McCardell Stewart et al.CommercialLodgepole >50% Below20 years Total Volume growth−
Creek[30]40 yearspine(heavy) Merchantable volume−
Thinning ǂ Mean height0
fertilization Basal areaǂ
Mean DBH0
TakyiStewart et al.Pre- Lodgepole 75%Below19 years Total Volume growth−
Thinning ǂ[30]commercial pine (heavy) Merchantable volumeǂ
fertilization 24 years Mean heightǂ
Basal areaǂ
Mean DBHǂ
Slenderness (ht/dbh)0
TakyiStewart et al.Pre- Lodgepole75%Future crop Total Volume growth−
Thinning ǂ[30]commercial pine(heavy)tree Merchantable volumeǂ
fertilization (selective) Mean heightǂ
Basal areaǂ
Mean DBHǂ
Slenderness (ht/dbh)0
* Positive and negative effects are indicated by ǂ and −, respectively; 0 indicates no effect; The sanitation cut did not consider final density or spacing; instead, trees with a diameter at breast height of over 7.6 cm that was badly suppressed, diseased, or deformed, such that they would be unmerchantable as poles or pilings at final harvest, were all removed.
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MDPI and ACS Style

Baah-Acheamfour, M.; Schoonmaker, A.; Dewey, M.; Roth, B. Lodgepole Pine and White Spruce Thinning in Alberta―A Review of North American and European Best Practices. Land 2023, 12, 1261. https://doi.org/10.3390/land12061261

AMA Style

Baah-Acheamfour M, Schoonmaker A, Dewey M, Roth B. Lodgepole Pine and White Spruce Thinning in Alberta―A Review of North American and European Best Practices. Land. 2023; 12(6):1261. https://doi.org/10.3390/land12061261

Chicago/Turabian Style

Baah-Acheamfour, Mark, Amanda Schoonmaker, Mark Dewey, and Brian Roth. 2023. "Lodgepole Pine and White Spruce Thinning in Alberta―A Review of North American and European Best Practices" Land 12, no. 6: 1261. https://doi.org/10.3390/land12061261

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