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Article

Freezing-Rain- and Snow-Induced Bending and Recovery of Birch in Young Hemiboreal Stands

by
Jānis Donis
,
Guntars Šņepsts
,
Leonīds Zdors
,
Jānis Vuguls
and
Āris Jansons
*
Latvian State Forest Research Institute ‘Silava’, Rigas 111, LV 2169 Salaspils, Latvia
*
Author to whom correspondence should be addressed.
Forests 2024, 15(2), 275; https://doi.org/10.3390/f15020275
Submission received: 16 November 2023 / Revised: 23 January 2024 / Accepted: 27 January 2024 / Published: 31 January 2024
(This article belongs to the Section Natural Hazards and Risk Management)
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Abstract

:
Damage to birch (Betula spp.) trees after an extensive freezing rain and snowing event was assessed in hemiboreal stands aged from 2 to 30 years. Tree diameter, height, and stem bending were measured, stand age and time of thinning were obtained from inventory data, and the relative distance from a sample plot to the nearest adjacent stand was calculated. Stem bending was remeasured after one and three growing seasons to assess tree recovery potential. Stem damage was found for 31.0% of birch trees, with 93.7% of them bent. The probability of being bent was increased (p < 0.001) for trees of a lower social position within the stand and was higher in stands with larger growing stock (p < 0.05) and relative distance to the adjacent stand (p < 0.001). The probability of damage was significantly (p < 0.05) affected by recent tree removal, with decreasing susceptibility until five to six years after the last thinning. After one growing season, 31.3% of trees had less intense bending, and 8.2% had more intense bending compared with the initial assessment. A tree’s ability to recover stem bending to less than 15° was linked (p < 0.001) to its damage intensity, whereas the magnitude of the recovery was affected by both the damage intensity and the relative diameter (p < 0.001 and p < 0.01, respectively). The results indicate the importance of timely thinning that maintains a stable tree form and is performed considering the spatial planning of heterogeneity in the heights of adjacent stands to avoid damage at a young age.

1. Introduction

Natural disturbances serve important ecological functions by affecting forest structures and the dynamics of forest stands [1]. They also strongly, and almost always negatively, affect economic returns for landowners [2] and carbon sequestration [3]. In European forests, the main focus has been put on studies of wind-, insect-, and fire-caused damage [4,5], whereas the impact of extreme precipitation events is relatively rarely studied. Snow is an important ecological component that protects soil, wildlife, and vascular plants from cold and severe freezing, thus contributing to spring growth and vitality and limiting the spread of diseases and insects [6,7]. The snow cover duration and depth of forests in Northern Europe are expected to decrease due to the changing climate, disrupting light, heat, water, and nutrient availability for trees and leading to changes in soil properties and carbon emissions [6,7]. However, snow can also have a destructive effect on trees when its loading is excessively heavy. During recent years, damage from snow loading has been found in 7% of the forest land in Finland [8], whereas, in European forests, snow has caused damage to about one million cubic meters annually during the period 1950–2000 [4].
The occurrence of damaging snow in Northern Europe is projected to change in the future, but studies have revealed controversial trends. The frequency of damaging snow accumulation across Finland compared to the baseline period 1961–1990 was predicted to decrease by 23% and by 56% for the periods 2021–2050 and 2070–2099 [9]. In the most affected regions, the number of risk days was predicted to decrease from over 30 days to 8 days per year at the end of the century [9]. A more recent study, in contrast, has projected an increase in annual maximum loads of wet and frozen snow in eastern and northern Finland of up to 60% for the period 2070–2099 compared to 1980–2009 under the high-emission RCP8.5 scenario [10]. In addition to the effects related to meteorological factors, damage severity is affected by the response of trees linked to individual tree and stand parameters, the site’s physical conditions, and past silvicultural treatments (e.g., [11,12,13,14]). Thus, even under the present frequency and intensity of snow accumulation, the damage might rise as forests become more vulnerable due to an increase in the total growing stock and age [4,15].
Based on an analysis of snow damage in southern Finland, a snow load of at least 40 mm water equivalent over a five-day period is necessary to break individual large stems [16]. However, the nature of damaging events is complex. If the temperature is near freezing point, snow loading might be accompanied by other precipitation, such as freezing rain or rime and wind. Wet snow and freezing rain may interchange depending on temperature fluctuations, and the effect of these types of precipitation is altered by wind that has complex spatial and temporal dynamics within the forest canopy [17]. The moisture content of snow depends on the temperature, and while dry snow is easy to shed, wet snow is more likely to accumulate on tree crowns [18], whereas freezing rain forms an adherent glaze on branches. An increasing wind speed typically enhances the accumulation of snow until the wind speed exceeds 9 m s−1 when the removal of unfrozen snow starts to dominate [16,18]. In the case of wet, attached snow or glaze, however, a strong wind is likely to facilitate damage. In this field, the effect of a certain factor is problematic to distinguish; therefore, studies typically assess the combined effect of wind, snow, and freezing rain [11,13,14,19,20,21,22,23,24,25,26,27,28].
Birch (Betula spp.) is the dominant broadleaved tree species in hemiboreal Europe, and qualitative birch wood resources are very important for the timber industry, especially the plywood industry [29]. A recent study based on the Norwegian National Forest Inventory has shown that snow is the most frequent damaging agent in birch stands, especially at an intermediate and mature age [30]. Compared to coniferous species, deciduous trees are considered to be less susceptible to snow damage [11,25,27] due to their smaller crown area during winter when snow damage mostly occurs [10]. However, these studies have accounted only for stem- and root failure to resist snow loading. Stem bending is the least studied among the snow damage types, presumably because it does not cause tree death and most frequently occurs in young stands [31]. However, stem bending might be irreversible [32] and cause internal wood defects both from the initial disturbance [33,34] and the formation of reaction wood for trees that continue to grow leaning [35,36]. The status of bent trees is rarely quantified; however, this would provide valuable information to support decision-making for landowners. If a large proportion of trees is permanently bent, starting over might be considered.
Extensive snowing and freezing rain events cannot be prevented; however, stand resistance and resilience can be increased through silvicultural measures that control the stability of trees. Moreover, the shelter effect of neighboring stands may also reduce wind- and snow-induced damage [37]. Acknowledging these factors and implementing them into forest management might mitigate the potential impact of freezing rain and snow loading on trees. This study aimed to (1) characterize the damage caused by freezing rain and snow accumulation in young birch stands, (2) assess the factors affecting the damage severity concerning individual tree and stand parameters, (3) characterize tree status after damage as bending recovery or deterioration after one and three growing seasons, and (4) assess individual tree parameters that affect tree recovery or deterioration.

2. Materials and Methods

Cold, moist winters and warm, wet summers characterize the climate in Latvia, with an average of 686 mm of the mean annual precipitation and a mean annual temperature of +7 °C, according to the Latvia Environmental Geology and Meteorology Centre. According to the climate change scenarios for Latvia (RCP4.5 and RCP8.5), the most significant increase in precipitation (increasing by 24%–37% and 35%–51%) is expected during the winter period, along with a temperature increase by 4 and 8 °C [38]. Extremes of freezing precipitation have also caused severe damage to coniferous hemiboreal forests in the same region a year after the studied event [39,40], corresponding to the overall trend of more frequent winter precipitation extremes in northern Europe [41].
In the last ten-day period of December 2010, the upper layer of the air mass passing through Latvia was warm and resulted in snow along with drizzle, rain, and freezing rain. The amount of precipitation exceeded the 30-year average by 2.7 times, mainly in the form of wet snow, according to the data of the Latvian Environment, Geology, and Meteorology Centre. Ice/snow damage was assessed in young stands in the spring of 2011 in the southeastern part of Latvia, i.e., the region that was most severely affected by extensive snow loading (Figure 1).
We selected 99 stands (a total area of 215 ha) aged from 2 to 29 years, where birch (Betula pendula Roth. and Betula pubescens Ehrh.), according to the latest inventory data, composed at least 30% of the number of trees (if the height of the dominant species < 12 m) or growing stock (if the height of the dominant species ≥ 12 m) (Table 1). Some admixture of common aspen (Populus tremula L.), grey alder (Alnus incana (L.) Moench), and black alder (Alnus glutinosa (L.) Gaertn.) were also present in the studied stands. In each stand, 10, 15, or 20 sample plots were established for a stand size of 0.5–1.0 ha, 1.1–2.0 ha, or 2.1–10 ha, respectively. Plot spatial distribution (coordinates) within the stand was generated using the Repeating Shapes tool (v. 1.5.152) in ArcGIS [42]. The size of a sample plot was 25, 50, or 75 m2 for stands aged <10, 11 to 20, or 21 to 30 years, respectively. In total, 1477 sample plots were established, and 14,617 trees (9525 birch trees) were measured. The total area of the sample plots was 8.1 ha.
The species and tree position (dominant or co-dominant) were recorded, and the diameter at breast height (DBH) was measured for all trees that were at least half of the height of the dominant canopy. Height (H) was measured for four birch trees per sample plot, selecting one tree that was larger than the average, two trees that were near the average size, and one tree that was smaller than the average tree within the stand. If less than 4 birches were in the sample plot, all tree heights were measured. The height of the vertically straight trees was measured using a ruler or Vertex clinometer with an accuracy of 0.1 m. The height (stem length) of the bent trees was calculated assuming that the length of the stem is the length of the arc between the tree top and stem base. The arc length was calculated based on Huygens’ formula, considering (a) the height of the treetop above the ground level at the stem base, (b) the horizontal distance between the stem base and treetop vertical projection, and (c) the height of the stem in the half of the distance of assumption b.
For each sample plot, the distance from the center to the nearest adjacent stand and the height of the dominant canopy of this stand were measured. Primarily, wind speed, wind direction, and air temperature, which are, in turn, influenced by the location, topography, and structure of the nearest stand, impact tree stability and susceptibility to snow and wind damage [18,27]. A ratio between this distance and height was used as a measure of the relative distance from the center of the sample plot to the nearest adjacent stand. The stem slenderness coefficient (the ratio between tree height (m) and DBH (cm) (HD−1)) and relative diameter (the ratio between the DBH of an individual tree and the stand mean DBH) were calculated to characterize the tree social position within the stand. The measured data were used to calculate stand density (number of trees per ha), growing stock, and actual stand composition. The stand composition was expressed as the share of a tree species within the overstory on a scale from 1 to 10 based on (i) the proportion of the number of species if the stand height was <12 m or (ii) growing stock if the stand height was ≥12 m (10 = 90%…100%, 9 = 80%…89%, 8 = 70%…79%, etc.). Stand age and time since thinning (2010 to 2009, 2008 to 2007, 2006 to 2005, 2004 to 2003, and before 2003 or none) were obtained from the stand inventory data. Stand inventory data were also used to obtain site-type groups, according to classification by Bušs [43]: mesic (freely draining) and wet (periodically waterlogged) mineral soil, peat soil (peat layer in ≥30 cm depth), and drained wet mineral and peat soil.
For each birch tree, the stem-damage type—bent, broken, or uprooted—was noted. The measured trees were physically marked so they could be found for a follow-up measurement. The stem bending was measured as a deviation from the vertical axis measured from the tree stem base to the top using a protractor and ruler. Trees that were bent up to 15° were regarded as undamaged. In total, 9526 birch trees were measured.
We monitored the status of bent trees to assess the damage recovery or deterioration after one growing season in autumn 2011 and after three growing seasons in winter 2013/2014. In the first follow-up, stem bending was remeasured for 2009 birch trees in 20 randomly selected stands. In the second follow-up, stem bending was remeasured for 1616 birch trees in 23 stands. For both follow-up measurements, 16 stands with 1068 trees overlapped.
We used a binary logistic generalized linear mixed-effect model (GLMM) to assess the effect of individual tree and stand parameters on damage probability, assuming a tree is damaged if the bending angle exceeds 15 degrees. The stand and sample plot ID were incorporated in the model as nested random factors to consider the design of the sampled stands–accounted sampling plots coming from the same sampled stand.
A tree’s stability is typically affected by its social position within the stand (the effect of topography and the vertical and lateral position of trees in a stand [44]), and this can be characterized by the slenderness ratio and relative diameter that are mutually correlated. The tree social position determines the tree living space and the availability of water, nutrients, and radiation [27,44]. As tree height was measured for a limited number of trees per sample plot, a considerably larger data set could be used to test the effect of the relative diameter (6090 trees) compared to the slenderness ratio (3022 trees), hence affecting the results. Therefore, we first compared the performance of the model (m_Drel), including the tree relative diameter, position within the canopy, stand age, growing stock, number of trees, share of birch within the stand composition, site-type group, relative distance to the nearest adjacent stand, and time since thinning, with the model (m_HD), including all the same factors but substituting the relative diameter with the slenderness coefficient. Both models were applied to the data set of 3022 trees that had the height measurements needed to obtain slenderness ratios and DBH measurements to obtain the relative diameter. According to the Akaike information criterion (AIC), the model that included relative diameter showed a better relative quality for a given set of data, with AIC values of 2821 for m_Drel and 2940 for m_HD. Then, we tested the model m_Drel on the full data set to determine factors that significantly affect the probability of a tree being damaged. We performed a post hoc comparison of the levels of time since thinning using a pairwise Tukey-adjusted comparison of the estimated marginal means from the model.
The effect of the tree social position within the stand and bending angle at the time of the initial measurements on the probability of trees recovering a vertically straight position after one and three growing seasons was assessed using the GLMM. The stand and sample plot ID were incorporated as nested random factors to account for the possible correlation of observations from the same sampling unit. Two models were compared for each remeasurement period containing either the relative diameter or slenderness ratio as a measure of the tree social position. Both models showed a close fit for both remeasurement periods: after one growing season, the AIC values were 235 for the model containing HD and 234 for the model containing Drel; after three growing seasons, the AIC values were 209 for the model containing HD and 210 for the model containing Drel. Considering the tight relation between both parameters and the better fit of the tree relative diameter to determine initial damage than the slenderness ratio, we present results only from the models that contained the relative diameter as a measure of the tree social position.
Then, we used a linear mixed-effect (LME) model containing the relative diameter and the bending angle at the time of the initial measurements to assess the magnitude of changes in the bending angle as the difference between the initial measurement and remeasurements. The stand and sample plot ID were used in the model as nested random factors.
The data analysis was implemented using R 4.0.4 [45] libraries lme4 [46] for the GLMM and LME model, lmerTest [47] for obtaining the p-values of models, and emmeans [48] for post hoc tests. All tests were performed at α = 0.05.

3. Results

3.1. Initial Assessment

The ice/snow-induced damage, i.e., stem deviation larger than 15° from the vertical axis, stem breakage, or uprooting, were observed for 31.0% of the 9525 assessed birch trees. Stem bending was the prevailing damage type, accounting for 93.7% of all damaged birch trees. Among the damaged trees, 5.1% had broken stems, and 1.2% were uprooted.
The probability of a tree being bent was significantly affected by its relative diameter, relative distance from the nearest adjacent stand (both p < 0.001), stand growing stock, and the time since thinning (both p < 0.05). The relative diameter, relative distance from the nearest adjacent stand, and the stand growing stock all had a negative link to damage probability (Figure 2A–C). The timing of thinning showed a curved relationship to the damage probability, first by decreased probability as the time since the tree removal increased, followed by increased probability in stands where thinning was undertaken more than six years ago or not performed at all (Figure 2D).

3.2. Remeasuring after One Growing Season

After one growing season, 60.5% of the remeasured birch trees had the same damage intensity (the difference between measurements was smaller than ±5°), 8.2% had more intense bending, and 31.3% of trees had less intense bending compared with the initial assessment. Among the remeasured trees, 45.6% were initially classified as undamaged, and this figure increased to 57.6% after one growing season.
Whether a tree remains bent or is able to recover a vertically straight position was significantly (p < 0.001) affected by its damage intensity, i.e., the bending angle at the time of the initial measurement (Figure 3A), whereas the relative diameter was non-significant (p > 0.05; Figure 3B).
However, both factors had a significant effect (p < 0.01 for the relative diameter and p < 0.001 for the initial damage intensity) on the magnitude of a tree’s ability to recover. Trees with a lower social position (a lower relative diameter) were able to recover better, i.e., to decrease their bending angle more than relatively larger trees (Figure 4B). Simultaneously, trees that were initially bent more severely (with a higher bending angle) had larger changes after one growing season than the initially slightly bent trees (Figure 4A).

3.3. Remeasuring after Three Growing Seasons

After three growing seasons, 49.0% of the remeasured birch trees had the same damage intensity (the difference between measurements was smaller than ±5°) compared with the initial assessment, 47.7% had reduced the bending angle, and 3.6% had more intense bending. Among the remeasured trees, the proportion of trees that were classified as undamaged increased from 56.7% at the time of initial measurement to 78.0% after three growing seasons.
Consistent with the results obtained after one growing season, the probability of birches recovering a bending angle < 15° after three growing seasons was significantly affected by the initial bending angle (p < 0.001) but not by the tree relative diameter (p > 0.05), whereas the magnitude of changes in the bending intensity was affected by both of these parameters (p < 0.05 for the tree relative diameter and p < 0.001 for the initial bending angle).
The majority of trees had reduced the initial bending angle (Figure 5) to a greater extent than during the first remeasuring: the correlation coefficient between initial bending and remeasurement decreased from rho = 0.82 (p < 0.001) after one growing season to rho = 0.77 (p < 0.001) after three growing seasons. Additionally, the majority of trees (57.0%) that had increased bending between the initial damage assessment and the first remeasurement now had decreased damage intensity (Figure 6). Yet, 22.0% of remeasured trees were still classified as bent, i.e., with a bending angle larger than 15°. The average bending angle at the initial assessment was 12.4 ± 1.0° (±95% confidence interval) for trees that were able to recover and 67.9 ± 2.0° for trees that remained bent. Among the permanently bent trees, 88.2% had initial damage larger than 45°.

4. Discussion

In this study, we assessed snow-induced damage on birch in young hemiboreal stands. We found stem bending to be the prevailing type of damage. The high level of damage we observed corresponds to the previous findings of birch being more prone to bending than other economically important hemiboreal tree species [31]. Periodically, this can cause fatal damage to young stands. For instance, Martiník and Mauer [31] reported an even higher level of snow-induced bending than in our study in birch stands aged from four to 20 years in Central Europe, with 58% to 89% of all birches bent.
The tree social position was the main individual tree parameter that affected the probability of a tree being bent. Trees from a lower social position within a stand were damaged more frequently, as indicated by their smaller relative diameter. A similar pattern was reported from stands damaged by freezing rain [49] and might be explained by secondary damage (the domino effect) from bent neighboring trees [50,51]. Several other studies, however, have indicated less damage from ice accumulation to trees of lower social strata due to a sheltering effect [40,52,53,54,55,56].
We also assume that the effect of the relative diameter reflects the effect of the slenderness ratio, as both of these parameters are typically tightly intercorrelated. Trees of a lower social position are typically characterized by a high slenderness ratio and smaller, asymmetric crowns that facilitate an imbalance in applied loading and increase the lever arm; thus, these trees are more easily bent [25]. Indeed, the tree slenderness ratio had a significant effect on the probability of damage when used in the model instead of the relative diameter (results not shown). Contradicting results are presented by a study that analyzed the bending of birch and found no differences in the slenderness ratio between intact and damaged trees [31]. However, the discrepancy with our results might be related to stand density, as the analyzed stands had an extremely high number of trees (18,400 to 50,600 trees ha−1). In such tightly spaced stands, both damaged and intact trees had high slenderness ratios (mean values of about 1.5 to 1.7 HD−1), thus diminishing the effect of the stable stem taper.
Among the parameters related to the stand developmental stage, damage probability was best explained by the stand growing stock. Smaller trees were able to recover better after bending. Less damage to trees at a younger age is likely due to smaller tree dimensions [57], as smaller tree crowns can accumulate a smaller amount of snow. For this reason, some studies claim that young trees are resilient to snow-induced damage in contrast to the resistance seen in larger trees [12]. For instance, the bending of maple and aspen was almost exclusively found in stems smaller than 18 cm [58]. Higher susceptibility to bending for younger trees as opposed to stem breakage in middle-aged and mature stands is also observed for ice accumulation in coniferous stands [39,50].
Several other studies have found a non-linear link between the proportion of bent trees and the stand developmental stage, with the highest proportion of damaged trees for small trees compared to seedlings and big trees [59]. The same trend is observed for stand age: in a stand younger than six years, trees were least damaged, whereas in stands aged from 7 to 10 years, almost every (91% to 97%) birch was bent, followed by a gradual decrease in the proportion of damaged birches in older stands [31]. A similar trend but biased to a larger stand age was observed in a deciduous forest after freezing rain [51]. Stands aged 14 years old had only a few bent trees along the openings, while adjacent 24- to 28-year-old stands had 36% of damaged stems, with 78% of damaged trees severely bent.
Some studies have considered wind within open stand edges as a factor for storm damage severity [25,27,37,60]. Our results show that the close proximity of a mature stand is linked to a higher proportion of trees bent in a young stand as well. When wind flows from the direction of a lower canopy (young stand) to a higher forest edge (mature stand), wind-facilitated damage is expected to increase closer to the mature stand. The open area of the lower canopy promotes an increase in wind drag [37,61,62] and is followed by a turbulent flow at the edge of the downwind mature stand [61,63]. Instead, a mature upwind stand provides shelter to the downwind young stand [37,64] at a distance of one to two heights of the upwind stand canopy [26,61]. Additionally, wind could also cause shedding snow in a windward direction from a higher to lower canopy, thus increasing loading to young trees [16,18]. As the wind acts differently depending on the spatial arrangement of the adjacent stands, the opposite spatial order might diminish the effect of the wind. Unfortunately, we lack field data on wind direction to confirm a clear link between damage severity and the stand position toward the wind.
More damage near an adjacent mature stand might be the result of competition with the adjacent stand for light availability. Trees exhibit phototropic growth, i.e., lateral crown expansion toward more light exposure, that results in crown asymmetry [65,66] and leaning stems [67] to avoid competition. Such characteristics are more pronounced for early successional species [68] and could be typically observed for a birch at the roadside. In the same way, young trees lean away from adjacent mature stands. Trees with asymmetric crowns are more easily bent due to unbalanced loading, thus possibly explaining why trees closer to a mature stand were more susceptible. However, this could be an additional factor promoting the susceptibility of trees at a close distance to the mature stand and is unlikely to exceed a distance of one to two heights of a mature stand. On the other hand, this helps to explain the increased damage in a sheltered (downwind) young stand, as after an ice storm, trees tend to bend in the direction of enlarged crowns rather than in the direction of the wind [58].
Likewise, stem bending is an unfoundedly overlooked type of snow- and ice-accumulation damage. The recovery of bent trees has received little attention, with studies mostly focusing on the survival of broken trees and their crown-rebuilding capacity [59,69,70]. Our results show a decent recovery of bent trees, with the majority being able to regain a vertically straight stem position; yet, the results after three growing seasons showed about one-fifth of trees being permanently bent.
Damage intensity was the main factor affecting the ability to recover. Trees that did not recover were mainly trees with severe initial bending that, presumably, had broken internal structures after exceeding their elastic limit. Similar results are reported in a study by Rustad et al. [71] that noted understory trees of American beech (Fagus grandifolia Ehrh.) remained bent over the three study years. In extreme cases, severe bending causes tree death. A mortality rate of 7.6% after one growing season was reported for bent trees after a severe ice storm in the southeast United States [56].
The magnitude of changes in the bending angle between initial assessment and repeated measurements, however, was affected by both the initial damage and the tree relative diameter, indicating more flexibility for relatively smaller trees. This is in accordance with an intuitive explanation for tree recovery after ice loading by Greene et al. [72]. They assert that small trees (<1 cm DBH) recover rapidly, whereas larger trees (1–10 cm DBH) might remain permanently bent if the loading is maintained for a prolonged period, but such trees tend to bend back the low-order branches. The increase in living crowns for arched trees hinders unbending, as it applies additional force due to gravity [26]. Additionally, the most dynamic changes within the canopy structure occur within the first two to three years following an ice storm [70]; thus, we assume our results to present the recovery capacity with no considerable changes in tree recovery following the studied period.
Our results show that snow- and ice-accumulation damage might be mitigated by stand management, as the probability of bending decreased during the first years after tree removal in precommercial thinning. This is in accordance with other studies that have shown a link between the increased proportion of damaged trees and stand density [22,31] due to consequent changes in stem taper and crown shape. This is a particularly important issue for birch, as it is mainly established naturally, forming very dense stands.
Studies have shown that management decisions could diminish the negative effect of such disturbances, as individual tree and stand parameters affect the tree response to loading and the probability of stem breakage and uprooting [25,26,27]. Precommercial thinning is an efficient measure to promote growth of the diameter and reduce slenderness for birch [73,74], thus increasing the stability of individual trees. However, our results indicate that this effect diminishes after five to six years or if the reduction of competition is delayed. Previous studies have shown that stem slenderness for densely growing trees increases significantly and, even after competition release, cannot recover to the level of timely thinned trees [73]. Instead, open stands with highly tapering individuals that have lost support from neighboring trees are formed [52]. Trees in such an open stand might suffer more damage, as more snow and ice could accumulate on an individual tree [52], and the wind speed within the canopy increases, presumably facilitating damage.
In this study, only the distance from the nearest adjacent stand from the sampling plot was considered when calculating tree-bent probability. An improvement would be to consider several neighboring stands. However, this would complicate the calculation and data analysis, as the management and stand development of these stands will change over time. Another improvement would be to establish a monitoring system, observing snow loads over a longer period of time, and combine it with continuous precipitation and temperature records, thus reflecting the actual snow accumulation and bending impact, not only the single heavy-snow- or freezing-rain-accumulation episodes.

5. Conclusions

Our results add new knowledge on the importance of individual tree and stand parameters on stem bending. Susceptibility to wind- and snow-induced bending was linked to the tree social position within the stand, the stand developmental stage, the timing of silvicultural intervention, and spatial heterogeneity of adjacent stands’ height. Although the trees possessed a decent recovery rate in terms of regaining vertical posture, further studies on the bending impact on stem and wood quality are needed to justify forthcoming management decisions.

Author Contributions

Conceptualization, J.D. and Ā.J.; methodology, J.D. and J.V.; formal analysis, J.D., G.Š. and J.V.; data curation, L.Z. and G.Š.; writing—original draft preparation, J.D. and J.V.; writing—review and editing, Ā.J. and G.Š.; project administration, J.V.; funding acquisition, J.D. and Ā.J. All authors have read and agreed to the published version of the manuscript.

Funding

Latvian Council of Science project “Reduction of wind storm damage risk in private forest management”, No. lzp-2018/2-0349, expanded and improved in the ERDF project “Tool for assessment of carbon turnover and greenhouse gas fluxes in broadleaved tree stands with consideration of internal stem decay” (No. 1.1.1.1/21/A/063).

Data Availability Statement

Data are available upon request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the measured birch stands in southeastern Latvia.
Figure 1. Location of the measured birch stands in southeastern Latvia.
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Figure 2. Probability of birch being bent in relation to (A) tree relative diameter, (B) stand growing stock, (C) relative distance to the nearest adjacent stand, and (D) time since last thinning. The grey area represents a 95% confidence interval.
Figure 2. Probability of birch being bent in relation to (A) tree relative diameter, (B) stand growing stock, (C) relative distance to the nearest adjacent stand, and (D) time since last thinning. The grey area represents a 95% confidence interval.
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Figure 3. Probability of birch to recover bending angle < 15° after one growing season in relation to (A) initial bending angle and (B) tree relative diameter. The grey area represents a 95% confidence interval.
Figure 3. Probability of birch to recover bending angle < 15° after one growing season in relation to (A) initial bending angle and (B) tree relative diameter. The grey area represents a 95% confidence interval.
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Figure 4. Changes in bending intensity after one growing season in relation to (A) tree relative diameter and (B) initial bending angle. Positive value change indicates more intense arching; negative value change indicates ability to recover vertically straight position. The grey area represents a 95% confidence interval.
Figure 4. Changes in bending intensity after one growing season in relation to (A) tree relative diameter and (B) initial bending angle. Positive value change indicates more intense arching; negative value change indicates ability to recover vertically straight position. The grey area represents a 95% confidence interval.
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Figure 5. Bending angle after (a) one and (b) three growing seasons in relation to the initial bending angle. The bullets under the black line (1:1) indicate trees that have reduced the damage intensity.
Figure 5. Bending angle after (a) one and (b) three growing seasons in relation to the initial bending angle. The bullets under the black line (1:1) indicate trees that have reduced the damage intensity.
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Figure 6. The change between the bending angle between remeasurements after one and three growing seasons in relation to this change between initial assessment and remeasurement after one growing season. The bullets under the black line (1:1) indicate trees that have reduced the damage intensity.
Figure 6. The change between the bending angle between remeasurements after one and three growing seasons in relation to this change between initial assessment and remeasurement after one growing season. The bullets under the black line (1:1) indicate trees that have reduced the damage intensity.
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Table 1. Characteristics of sampled stands. Sampled stands according to the forest type (from dry, moderate nutrient-rich mineral or peat soils to drained soils). Number of measured stands (N), mean (±standard deviation) diameter at breast height (DBH), tree height (H), stem volume (V), and stand age (A) of sampled stands.
Table 1. Characteristics of sampled stands. Sampled stands according to the forest type (from dry, moderate nutrient-rich mineral or peat soils to drained soils). Number of measured stands (N), mean (±standard deviation) diameter at breast height (DBH), tree height (H), stem volume (V), and stand age (A) of sampled stands.
Forest TypeNDBH, cmH, mV, m3 ha−1A, YearsSoil Type
Hylocomiosa210.9 ± 1.012.8 ± 0.590.7 ± 4.424 ± 1dry, moderate nutrient-rich mineral soil
Oxalidosa286.8 ± 3.29.2 ± 4.046.0 ± 41.315 ± 6dry, nutrient-rich mineral soil
Aegopodiosa57.1 ± 1.810.7 ± 2.150.2 ± 24.215 ± 2dry, nutrient-rich mineral soil
Myrtilloso-sphagnosa210.5 ± 2.014.5 ± 2.0167.2 ± 48.321 ± 5periodically wet, moderate nutrient-rich mineral soils
Myrtilloso-polytrichosa87.7 ± 3.310.3 ± 4.262.9 ± 57.115 ± 4periodically wet, nutrient-rich mineral soils
Sphagnosa24.0 ± 0.66.9 ± 0.516.6 ± 12.020 ± 8wet, nutrient-poor peat soils
Caricoso-phragmitosa156.2 ± 4.17.8 ± 4.451.6 ± 52.320 ± 7moderate nutrient-rich, wet peat soils
Dryopterioso-caricosa610.6 ± 2.314.0 ± 2.3137.9 ± 50.623 ± 5nutrient-rich, wet peat soils
Myrtillosa mel.87.2 ± 4.98.3 ± 6.260.6 ± 91.414 ± 8moderate nutrient-rich, drained mineral soils
Mercurialiosa mel.107.9 ± 2.010.1 ± 2.148.2 ± 33.414 ± 3nutrient-rich, drained mineral soils
Myrtillosa turf. mel.88.7 ± 4.010.8 ± 6.071.5 ± 68.916 ± 9moderate nutrient-rich, drained peat soils
Oxalidosa turf. mel.53.9 ± 3.15.7 ± 4.314.6 ± 15.910 ± 4nutrient-rich, drained peat soils
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Donis, J.; Šņepsts, G.; Zdors, L.; Vuguls, J.; Jansons, Ā. Freezing-Rain- and Snow-Induced Bending and Recovery of Birch in Young Hemiboreal Stands. Forests 2024, 15, 275. https://doi.org/10.3390/f15020275

AMA Style

Donis J, Šņepsts G, Zdors L, Vuguls J, Jansons Ā. Freezing-Rain- and Snow-Induced Bending and Recovery of Birch in Young Hemiboreal Stands. Forests. 2024; 15(2):275. https://doi.org/10.3390/f15020275

Chicago/Turabian Style

Donis, Jānis, Guntars Šņepsts, Leonīds Zdors, Jānis Vuguls, and Āris Jansons. 2024. "Freezing-Rain- and Snow-Induced Bending and Recovery of Birch in Young Hemiboreal Stands" Forests 15, no. 2: 275. https://doi.org/10.3390/f15020275

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