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Article

Impact of Environmental Conditions on Wood Anatomical Traits of Green Alder (Alnus alnobetula) at the Alpine Treeline

by
Andreas Gruber
,
Gerhard Wieser
,
Marion Fink
and
Walter Oberhuber
*
Department of Botany, Leopold-Franzens-University of Innsbruck, Sternwartestrasse 15, A-6020 Innsbruck, Austria
*
Author to whom correspondence should be addressed.
Forests 2024, 15(1), 24; https://doi.org/10.3390/f15010024
Submission received: 7 November 2023 / Revised: 14 December 2023 / Accepted: 15 December 2023 / Published: 21 December 2023
(This article belongs to the Special Issue Tree Growth in Relation to Climate Change)
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Abstract

:
Due to land use change, green alder (Alnus alnobetula), formerly restricted to moist slopes, is now expanding to drier sun-exposed sites at the alpine treeline. The highly productive shrub is forming closed thickets, establishing nitrogen-saturated species poor shrublands. To evaluate wood anatomical adaptations to changing environmental conditions, we analyzed vessel characteristics (mean vessel area, MVA; vessel density, VD; and theoretic conductive area, TCA) and axial parenchyma abundance, as well as their distribution in the annual ring at a moist north-facing and a dry south-facing site at the alpine treeline on Mt. Patscherkofel (Central European Alps, Austria). Results revealed that lower soil water availability and enhanced evaporative demand did not affect MVA while VD and TCA were significantly reduced at the dry south-facing site. This suggests that in green alder, vessel size is a static trait whereas vessel number responds plastic. Limited water availability also triggered a significant increase in axial parenchyma, confirming the important role of xylem parenchyma in water relations. Harsh environmental conditions at the distributional limit of green alder may have affected xylogenesis, leading to a near semi-ring-porous distribution of vessels and an accumulation of parenchyma in the late growing season. We conclude that in a warmer and drier climate, growth limitation and physiological stress may set limits to the distribution of Alnus alnobetula at drought-prone sites in the alpine treeline ecotone.

1. Introduction

Mountain ecosystems are particularly vulnerable and are expected to undergo considerable transformations due to global change [1,2,3]. However, within the European Alps, land abandonment and decreasing grazing pressure may have a greater impact on the treeline ecotone in the coming decades than rising temperatures [4,5]. There are indications that shrubs may benefit more from land use changes than trees and take over abandoned pastures [6,7,8]. During the last decades, in particular, green alder (Alnus alnobetula (Ehrh.) K. Koch = Alnus viridis (Chaix) DC.) has spread rapidly across the Alps [7,9,10,11]. A. alnobetula is a cold-resistant and moderately shade-tolerant species of the subalpine and subarctic zones of the northern hemispheres, growing in mountains, tundra, and river valleys [12]. Due to clonal growth, high seed production, and symbiosis with N2-fixing actinobacteria and ectomycorrhizal fungi, A. alnobetula is spreading rapidly after land abandonment. As green alder forms closed thickets, with canopy heights up to 4 m, it drives former N-poor grassland into nitrogen-saturated-species-poor shrubland and suppresses tree establishment within the treeline ecotone [7].
Although the occurrence of green alder was restricted to north-facing slopes and avalanche gullies, with high water availability [10,13,14], this species is now expanding into moderately steep well-drained subalpine grasslands, as well as sun-exposed sites with shallow soils and impaired water availability [9,15]. Green alder is known to be an anisohydric species, keeping its stomata open even under high vapor pressure deficits [16,17,18], a strategy that can be risky under drought conditions [19]. Nevertheless, the former restriction of A. alnobetula to moist habitats may be due to former land use patterns, while A. alnobetula is able to adapt to drier condition [15].
A. alnobetula is a diffuse-porous species, keeping vessel diameters constant throughout the annual ring [20]. However, drought-induced changes in vessel development in diffuse-porous species have been reported before [21,22,23]. Size, number, and distribution of vessels are closely linked to tree hydraulic conductivity and drought resistance [24,25], but still little is known about intraspecific adaptations in hydraulic properties. The intraspecific studies available to date have shown the effects of water availability on conduit traits [26,27] where decreasing water availability resulted in narrower conduits and higher conduit density in different species [28,29,30,31]. However, the adaptation of wood anatomical traits might vary between species and there may be different adaptation strategies for ring-porous and diffuse-porous angiosperms [32,33].
Furthermore, there is increasing awareness that aside from conductive cells, parenchyma tissues in the secondary xylem also play a critical role in the water relations of woody plants [34,35,36,37]. It is well known that parenchyma in the secondary xylem stores carbohydrates, which can subsequently be used for growth, establishment of freeze tolerance, and protection against or recovery after infestations [38,39,40,41]. Increasing drought stress in the course of global warming emphasizes the role of xylem parenchyma in maintenance of the water transport and embolism repair [33,42,43], as well as water storage and circulation between xylem and phloem [34,36,40,44,45]. Axial parenchyma is reported to be more plastic than ray parenchyma [39,40,46] and its fractions vary with environmental conditions at the intra- and interspecific level [47,48].
Analysis of functional adaptations in the wood anatomy of A. alnobetula provides insights into the ecological limits of this pioneer species. We, therefore, evaluated vessel and axial parenchyma properties and distribution in A. alnobetula at a south-facing windward and a north-facing leeward site at the alpine treeline, differing in soil water availability and evaporative demand. We tested the hypotheses that reduced soil water availability and enhanced evaporative demand on the south-facing site (i) reduces vessel diameter, (ii) increases vessel density, (iii) changes vessel distribution within the annual ring leading to a semi-ring-porous arrangement of vessels, and (iv) causes an increase in axial parenchyma cells.

2. Materials and Methods

2.1. Study Area and Sample Plots

The study was performed within the treeline ecotone of Mt. Patscherkofel in the Central European Alps, Tyrol, Austria (47°12′ N, 11°27′ E), where A. alnobetula stands primarily spread out in avalanche gullies on leeward north-facing slopes between 1950 and 2150 m asl. Even so, during the last decades, A. alnobetula stands spread out to windward sites on south- to southeast-facing slopes [9]. Within the study area, the bedrock is dominated by gneisses and schists [49], and the soils are classified as haplic podzols [50,51].
The mean annual temperature at the meteorological station on top of Mt. Patscherkofel was 0.8 ± 0.7 °C during the period from 1991 to 2020, with February being the coldest month (−6.6 °C) and July the warmest (8.9 °C). Despite the high altitude, air temperature can frequently reach maxima around 20 °C during summer. Mean annual precipitation was 889 ± 128 mm with precipitation maxima occurring in summer (long-term mean: 371 ± 74 mm, June to August) and winters being the driest season (132 ± 60 mm, December to February). The study area is also characterized by the frequent occurrence of strong southerly Föhn-type winds [52], which strongly influences snow depth and snow distribution—[53] and hence, the duration period of the permanent snow cover. At south-facing slopes, snow depth is generally < than 1 m and only small patches of snow are present till early April; while on leeward north-facing slopes, a closed snow cover with snow depths up to 3 m frequently persists from the end of October till the end of May.
Within the study area, we selected two study plots at the upper edge of the treeline ecotone (Figure 1, Table 1): a north-exposed leeward (hereafter N-site) and a south-exposed windward site (hereafter S-site). Although the two plots were only 150 m apart in linear distance, they differed considerably with respect to slope exposure and soil depth (Table 1), soil water availability, and evaporative demand (Table 2, Figure 2). The canopy height of A. alnobetula was higher on the N-site, although shrubs were older on the S-site (Table 1).

2.2. Environmental Data

Environmental conditions were recorded at both study sites from June through September 2022. At each study plot, air temperature and relative air humidity (CS215 Temperature and Relative Humidity Sensor) and solar radiation (SP1110 Pyranometer Sensor) (all sensors, Campbell Scientific, Shepshed, UK) were monitored 2 m above ground, while soil temperature (T 107 temperature probe, Campbell Scientific) and moisture in 10 cm soil depth were monitored using four ThetaProbes ML2 (Delta-T Devices Ltd., Burwell, UK). Precipitation (ARG100 Rain Gauge) was recorded at the S-site. All the environmental data were recorded with a Campbell CR1000 data logger (Campbell Scientific, Shepshed, UK) programmed to record 30-minute averages of measurements taken every minute.
Daily mean air temperature averaged throughout the growing season did not differ significantly between the two study plots (Figure 2, Table 2). During the growing season, daily mean solar radiation and soil temperature in 10 cm soil depth, respectively, were significantly higher on the windward S-site than on the leeward N-site (Table 2). Conversely, soil water content in 10 cm soil depth was significantly lower on the S-site than on the N-site (Table 2), indicating that drier conditions prevail on the S-site compared with the N-site. This assumption is supported by significantly lower annual radial increments and a lower canopy height at the S-site compared with the N-site (Table 1).

2.3. Sample Collection, Preparation, and Wood Anatomical Analysis

Stem discs were sampled from 3 different stocks on the N-site (mean diameter of stem discs: 2.13 ± 0.21 cm) and 4 stocks and the S-site (mean diameter of stem discs: 2.08 ± 0.17 cm). Discs were taken 150 cm from the shoot tip as there is evidence that xylem anatomy in trees and shrubs is changing from tip to base [54,55,56,57], with the narrowest conduits found at the tip of the shoots. Stem discs were air-dried, and subsequently, rectangular pieces were cut from the discs to prepare the probes for microtome sectioning. After the samples were soaked in glycerin for one day to soften the wood, microsections of 10 μm thickness were produced using a sledge microtome (WSL-core microtome, WSL, Birmensdorf, Switzerland). The microsections were stained with safranin and astrablue to differentiate between lignified (red) and unlignified (blue) cells. The stained cross sections were observed under the microscope (Olympus Typ BX50, Olympus Corporation, Tokyo, Japan), and images were taken at 10× magnification with an HD-microscope camera (ProgRes GryphaxR, Jenoptik, Jena, Germany).
Wood anatomic variables were evaluated from the microscopic images using the open-source image analysis program Fiji (ImageJ2). Vessel number, vessel area, and diameter were evaluated for 10 annual rings (2011–2020). In total wood anatomy of 70 annual rings from 7 samples was determined. In total, 5677 and 4402 vessels were measured at the N- and the S-site, respectively. From the collected data, mean vessel area (MVA), vessel density (VD, number of vessels mm−2), and theoretic conductive area (TCA, i.e., percentage of vessel area to total area) were calculated for each sample. Kernel density estimations were used to compare vessel area distributions between sites. All individual vessel measurements were included in this analysis. To compare vessel area distributions related to wood surface area, all measured data were assigned to size classes and the number of vessels per size classes and xylem area was calculated.
To quantify phenological differences between vessels formed in different phases during the growing season, we determined the position within the annual ring for each vessel. Using this positioning data, we analyzed differences in the number and sizes of vessels in the first and second 50% of each annual ring (i.e., early and late growing season). MVA, VD, and TCA were calculated to gain insight into the plasticity of vessel formation throughout the growing period. In addition, vessel area distribution was analyzed for the first and the second 50% of the growth rings. Because some of the inner growth rings were curved, which made reliable positioning difficult, only the outermost 6-year rings (2015–2020) were used for the processing of the position data.
In a second series of measurements, we evaluated the number, area, and position of the diffuse parenchyma within the annual ring. Using these data, we calculated the percentage of the total area occupied by the axial parenchyma (PA). We also used the positioning data to calculate the percentage of PA for the first and the second 50% of the annual ring to get insight into the timing of the formation of diffuse parenchyma in the course of the growing period.

2.4. Data Analysis

Shapiro–Wilk tests and Q-Q plots were used to check for normal distribution of data. Most anatomical and climate datasets were not normally distributed, so we used nonparametric statistical tests for analyses. Because temperature data were consistently normally distributed, differences in air and soil temperature at the N-site and S-site were tested for significance using paired t-tests. For analyzing differences in MVA, VD, and TCA at the two sites, Mann–Whitney U tests were applied. For comparison of vessel and parenchyma properties in the first and second half of the annual rings and for analysis of soil moisture and solar radiation data, we used paired samples Wilcoxon tests. All statistical analyses were performed using SPSS Statistics, Version 26 (IBM, New York, NY, USA).

3. Results

3.1. Wood Anatomy

On an entire growing season basis, MVA was not significantly different between the N-site and S-site (Figure 3, Table 3). VD (p = 0.037) and TCA (p = 0.017), by contrast, were significantly higher on the N-site than on the S-site (Figure 3).
Vessel area distribution calculated by Kernel density estimation (Figure 4) showed similar patterns at both sites. Vessels around 250 µm2 had the highest frequency at both measuring sites, then tailing out at with increasing vessel area (Figure 4). Nevertheless, wood samples at the S-site showed a higher proportion of smaller vessels between 200 µm2 and 800 µm2 and vessels between 1200 and 2000 µm2 when compared with the N-site. (Figure 3). Numbers of vessels per surface area were higher at the N-site in size classes up to 1200 µm2. Values for vessels with areas greater than 1200 µm2 were quite equal at both sites. Vessels up to 2500 µm2 were frequently found in samples from both sampling sites.
MVA, VD, and TCA were significantly higher in the first 50% of the annual ring compared with the second half at both sites (p-values, Figure 5). In the first half of the annual rings, the N-site and the S-site showed no significant differences in MVA while VD and TCA were significantly higher in N-site (p ≤ 0.006). The same accounted for the second half of the annual ring (p ≤ 0.002). Kendal density curves and the calculated numbers of vessels per area for the first and second half of the annual ring (Figure 6) provided a more detailed insight into the differences in vessel production throughout the growing season. In the first half of the annual ring, mean Kendal density curves were bimodal in shape at both sampling sites. There were years where the bimodality of distribution was quite pronounced, even in the second half of the annual ring, while it was weak or missing in other annual rings. The first half of the annual ring showed a higher density of midsize and large vessels. Vessels in the second half of the annual ring showed higher density in small vessels up to a size of 500 and 750 µm2 at the N-site and S-site, respectively. The number of vessels per size class and area was higher in the first 50% of the annual ring throughout all size classes, with the greatest differences found for vessel areas between 200 and 1000 µm2. At the S-site, the numbers of vessels per size class were overall significantly smaller compared with the N-site.

3.2. Percentage of Axial Parenchyma

The percentage of axial parenchyma in the total annual ring was significantly higher (p < 0.001) at the S-site (16.5 ± 5.2%) when compared with the N-site (10.1 ± 2.0%) (Figure 7). At both sites, the percentage of axial parenchyma was significantly higher in the second half of the annual ring, (p < 0.001 for N- and S-site), and the highest percentages of axial parenchyma (19.6 ± 5.7%) were found in the second half of the annual ring at the S-site. Furthermore, for both sections of the annual ring, the percentage of axial parenchyma was significantly higher at the S-site (p < 0.001 and p = 0.002 for the first and second half of the annual ring) compared with the N-site.

4. Discussion

Analysis of site-specific functional adaptations of A. alnobetula enables us to determine the effects of global warming on the limits of the expansion of this pioneer species. Intraspecific studies provide insights into the impact of environmental factors on species-specific anatomical traits, disentangling genotypic and environmental effects contributing to xylem development is a difficult task. There is evidence that provenances can have more impact on xylem anatomy than environmental conditions [58,59,60]. Therefore, anatomical variations in plants growing at faraway sites or along wide-ranging environmental gradients may result from genetic variations. However, in the presented study, the examined plants were growing in close proximity, and A. alnobetula has expanded from the north-facing to the south-facing site only during the last decades [9], ruling out differences in provenience. Even so, the two sites differed significantly with respect to environmental conditions. The S-site experienced significantly higher solar radiation and was more exposed to the prevailing southerly winds while soil water availability was significantly lower than at the N-site (Figure 2, Table 2; c.f. [53]). Moreover, in anisohydric A. alnobetula, stomata remain open even under high evaporative demand [16,17,18], which, in combination with frequently occurring strong winds reducing boundary layer resistance (e.g., [61,62]), may lead to drought stress during periods of reduced soil water availability at the S-site. This is confirmed by a lower canopy height (Table 1), a significantly lower radial growth (Table 2; [53]), and premature leaf wilting, occurring at the S-site in mid-August after a dry period in summer 2022. At the N-site, by contrast, leaf wilting did not occur until early October.
Xylem cell division and differentiation are driven by environmental and internal factors [63]. As a key factor, temperature controls cambial cell division and plays a major role in cell wall lignification [64,65]. Conversely, as a turgor-driven procees cell enlargement is mainly controlled by cell water status [66,67] and sugar availability acting as an osmotic agent [68]. Cell enlargement and cambial cell division are seriously hampered under drought, often leading to a decline in radial growth [69] and vessel diameter [70,71,72]. Air temperature did not significantly differ between the two sampling sites (Table 2), and due to a high surface roughness, A. alnobetula experienced strong aerodynamic coupling to the free atmosphere [16]. On the other hand, significantly higher solar radiation at the S-site, especially during cloudless periods, enhanced solar heating of the ground. Nevertheless, as samples were taken in the upper branch section, temperature effects are less likely to explain observed differences in anatomical traits between the N- and the S-site.
Fast-growing species like Populus sp. show high phenotypic plasticity and are used as model species to examine environmental impacts on xylogenesis [73]. In poplar, drought often initiates a reduction in the vessel area [23,29,74,75]. In our study, however, MVA and HD did not differ significantly between the S- and N-site. However, tracheid enlargement might be based on turgor control in gymnosperms [66], while in more complex angiosperms, cell differentiation is probably actively regulated by endogen mechanisms rather than being a result of passive processes [76]. Anatomical traits are often under strong genetic control [58,73] suggesting species-specific adaptations to environmental impacts. In beech, for example, like in our study, no change in MVA under drought was detected [22,77,78]. Despite significant differences in environmental conditions, plant height, and annual ring width, Kernel density estimations of vessel areas (Figure 4a) showed no differences between the two sampling sites, confirming our assumption that vessel size is a static trait in green alder. Nevertheless, the control of cell formation and the patterns of cell distribution and size are poorly understood and the role of auxin and other morphogens are still under debate [24,79,80].
The number of vessels per size class and wood surface area (Figure 4b) showed a lower absolute number of small and midsize vessels (up to 1200 µm2) at the dry S-site, which is in accordance with the significantly lower VD and TCA at the S-site. On the contrary, an increase in VD and TCA under drought conditions has been reported in several studies [21,58,77]. However, Arnic et al. [21] and Giagli et al. [22] pointed out that drier summer conditions resulted in narrower annual rings and, consequently, higher VD and TCA. Environmental and provenance effects on anatomical traits often result from stem height and ring width [58]. In beech, VD was positively related to the above-ground biomass and increased with tree height [77]. Plant height and crown size might often explain anatomical properties like VD and TCA better than climatic conditions [81], and leaf area has been found to be correlated with stem hydraulic conductivity [74]. In our study, the green alder stand at the N-site reached greater height, had a denser crown and, thus, higher leaf area compared with the S-site. Considering this, higher stem hydraulic conductivity was needed at the N-site to adjust the balance between the water supply and transpiring surface, which explains higher VD and TCA.
A. alnobetula is classified as a diffuse-porous species, with constant vessel diameters throughout the annual ring [20]. Nevertheless, anatomical parameters and density curves in our study estimate a near semi-ring-porous vessel distribution at both sampling sites. Environmentally induced changes in vessel distribution in diffuse-porous species have been reported—e.g., a near semi-ring-porous distribution of vessels in dry years and continuous decrease in vessel size under dry conditions have been reported for diffuse-porous beech [21,22,23]. As in diffuse-porous species, wide and narrow vessels can be formed during the entire growing period, short-term adaptations to changing environmental conditions are possible in these species [82,83]. The bimodal shape of density curves over several years might be another reference for frequent semi-ring-porous vessel distribution at our treeline site [84]. Schreiber et al. [29] linked the bimodal distribution of vessel diameters in trembling aspen, which was found at a boreal site, to rough environmental conditions and a short growing period. Therefore, the harsh conditions at the altitudinal limit of A. alnobetula and not water deficiency might be the reason for hampered vessel formation in the second half of the growing season.
In addition to vascular tissue, axial parenchyma seems to play a key role in hydraulic optimization [33,44]. Xylem parenchyma is known to modulate xylem flow and hydraulic resistance through osmotic exudation into the xylem [34,39] and is involved in in embolism repair [34,85]. Moreover, water stored in xylem parenchyma cells [86,87] buffers a decline in water potential to sustain water transport under drought stress [34,88]. Aritsara et al. [44] found that species with more axial parenchyma were close to their hydraulic limits and [38] hypothesized that axial parenchyma fractions potentially keep vessels hydrated during drought periods. We, therefore, assume that the significantly higher amount of axial parenchyma at the S-site might be an adaptation to drought conditions.
However, there are still gaps in knowledge, especially when it comes to diffuse axial parenchyma in temperate species as most of the available studies concentrate on paratracheal parenchyma and species from subtropical and tropical regions (e.g., [38,44,89,90,91]). An accumulation of diffuse apotracheal parenchyma in latewood has been described for several conifer species [92] and European beech and pedunculate oak [20,93]. Nevertheless, in our study, the accumulation of diffuse parenchyma is very pronounced and not restricted to latewood. Therefore, we suggest that the higher fraction of parenchyma cells in the second half of the growing season, when vessel production is declining, indicates an enhanced investment in carbohydrate storage triggered by harsh environmental conditions at the upper distributional limit of the species.

5. Conclusions

In contrast to our first and second hypotheses, lower soil water availability and enhanced evaporative demand did not affect the vessel diameter. However, vessel density and the theoretic conductive area were reduced at the drier south-facing site. This is confirmed by the higher number of vessels per size class at equal distribution, suggesting that vessel size is a static trait in green alder. Vessel numbers, by contrast, demonstrate plastic response. In accordance with our third hypothesis, limited water availability triggered a significant increase in axial parenchyma. This confirms the important role of xylem parenchyma when it comes to drought stress. Harsh environmental conditions at the distributional limit of green alder have affected xylogenesis, leading to a semi-ring-porous distribution of vessels and accumulation of parenchyma in the late growing season. We conclude from the present study that reduced annual increment, early leaf fall, and an accumulation of parenchyma indicate growth limitation and physiological stress, setting limits to the distribution of A. alnobetula in the treeline ecotone in a future drier environment.

Author Contributions

W.O., G.W. and A.G. conceived the study. W.O. coordinated the overall project. A.G., W.O. and M.F. collected, prepared, and analyzed the data. W.O., G.W., A.G. and M.F. interpreted and discussed the data. A.G. wrote the manuscript and G.W. and W.O. provided editorial advice. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Austrian Science Fund (FWF), P34706-B. For the purpose of open access, the author has applied a CC BY public copyright license to any author accepted manuscript version arising from this submission.

Data Availability Statement

The wood anatomical data presented in this study are openly available on Zenodo at: 10.5281/zenodo.10405899.

Acknowledgments

We thank Bernhard Nairz and Michaela Schweinschwaller for their help with anatomical measurements.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Verrall, B.; Pickering, C.M. Alpine vegetation in the context of climate change: A global review of past research and future directions. Sci. Total Environ. 2020, 748, 141344. [Google Scholar] [CrossRef] [PubMed]
  2. Gottfried, M.; Pauli, H.; Futschik, A.; Akhalkatsi, M.; Barančok, P.; Benito Alonso, J.L.; Coldea, G.; Dick, J.; Erschbamer, B.; Fernández Calzado, M.R.; et al. Continent-wide response of mountain vegetation to climate change. Nat. Clim. Chang. 2012, 2, 111–115. [Google Scholar] [CrossRef]
  3. Grabherr, G.; Gottfried, M.; Pauli, H. Climate change impacts in alpine environments. Geogr. Compass 2010, 4, 1133–1153. [Google Scholar] [CrossRef]
  4. Gruber, A.; Oberhuber, W.; Wieser, G. Treeline-Quo Vadis? An Ecophysiological Approach. Forests 2022, 13, 857. [Google Scholar] [CrossRef]
  5. Gehring-Fasel, J.; Gusian, A.; Zimmermann, N.E. Tree line shifts in the Swiss Alps: Climate change or land abandonment. J. Veg. Sci. 2007, 18, 571–582. [Google Scholar] [CrossRef]
  6. Francon, L.; Roussel, E.; Lopez-Saez, J.; Saulnier, M.; Stoffel, M.; Corona, C. Alpine shrubs have benefited more than trees from 20th century warming at a treeline ecotone site in the French Pyrenees. Agric. For. Meteorol. 2023, 329, 109284. [Google Scholar] [CrossRef]
  7. Bühlmann, T.; Körner, C.; Hiltbrunner, E. Shrub expansion of Alnus viridis drives former montane grassland into nitrogen saturation. Ecosystems 2016, 19, 968–985. [Google Scholar] [CrossRef]
  8. Dullinger, S.; Dirnböck, T.; Grabherr, G. Patterns of shrub invasion into high mountain grasslands of the Northern Calcareous Alps, Austria. Arct. Antarct. Alp. Res. 2003, 35, 434–441. [Google Scholar] [CrossRef]
  9. Oberhuber, W.; Wieser, G.; Bernich, F.; Gruber, A. Radial stem growth of the clonal shrub Alnus alnobetula at treeline is constrained by summer temperature and winter desiccation and differs in carbon allocation strategy compared to co-occurring Pinus cembra. Forests 2022, 13, 440. [Google Scholar] [CrossRef]
  10. Bühlmann, T.; Hiltbrunner, E.; Körner, C. Alnus viridis expansion contributes to excess reactive nitrogen release, reduces biodiversity and constrains forest succession in the Alps. Alp. Bot. 2014, 124, 187–191. [Google Scholar] [CrossRef]
  11. Anthelme, F.; Villaret, J.C.; Brun, J.J. Shrub encroachment in the Alps gives rise to the convergence of sub-alpine communities on a regional scale. J. Veg. Sci. 2007, 18, 355–362. [Google Scholar] [CrossRef]
  12. Hantemirova, E.; Marchuk, E. Phylogeography and genetic structure of a subarctic-alpine shrub species, Alnus alnobetula (Ehrh.) K. Koch s. l., inferred from chloroplast DNA markers. Tree Genet. Genomes 2021, 17, 18. [Google Scholar] [CrossRef]
  13. Richard, L. Ecologie de l’Aulne Vert (Alnus viridis Chaix): Facteurs climatiques et édaphiques. Doc. Carte. Veg. Alpes. 1968, 6, 107–158. [Google Scholar]
  14. Schröter, C. Das Pflanzenleben der Alpen: Eine Schilderung der Hochgebirgsflora; Albert Raustein: Zürich, Switzerland, 1908. [Google Scholar]
  15. Caviezel, C.; Hunziker, M.; Kuhn, N.J. Green alder encroachment in the European Alps: The need for analyzing the spread of a native-invasive species across spatial data. Catena 2017, 159, 149–158. [Google Scholar] [CrossRef]
  16. Van den Bergh, T.; Körner, C.; Hiltbrunner, E. Alnus shrub expansion increases evapotranspiration in the Swiss Alps. Reg. Environ. Chang. 2018, 18, 1375–1385. [Google Scholar] [CrossRef]
  17. McDowell, N.; Pockman, W.T.; Allen, C.D.; Breshears, D.D.; Cobb, N.; Kolb, T.; Yepez, E.A. Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought? New Phytolog. 2008, 178, 719–739. [Google Scholar] [CrossRef]
  18. Körner, C.; Jussel, U.; Schiffer, K. Transpiration, diffusionswiderstand und wasserpotential in verschiedenen schichten eines grünerlenbestandes. In Ökologische Analysen von Almflächen im Gasteiner Tal. Veröffentlichungen des Österreichischen MaB-Hochgebirgsprogramms Hohe Tauern, Band 2; Cernusca, A., Ed.; Universitätsverlag Wagner: Innsbruck, Austria, 1978; pp. 81–98. [Google Scholar]
  19. Sade, N.; Gebremedhin, A.; Moshelion, M. Risk-taking plants: Anisohydric behavior as a stress-resistance trait. Plant Signal. Behav. 2012, 7, 767–770. [Google Scholar] [CrossRef]
  20. Schweingruber, F.H.; Börner, A.; Schulze, E.D. Atlas of Stem Anatomy in Herbs, Shrubs and Trees; Springer Science & Business Media: Berlin/Heidelberg, Germany, 2011; Volume 1. [Google Scholar]
  21. Arnič, D.; Gričar, J.; Jevšenak, J.; Božič, G.; von Arx, G.; Prislan, P. Different wood anatomical and growth responses in European beech (Fagus sylvatica L.) at three forest sites in Slovenia. Front. Plant Sci. 2021, 12, 669229. [Google Scholar] [CrossRef]
  22. Giagli, K.; Gričar, J.; Vavrčík, H.; Menšík, L.; Gryc, V. The effects of drought on wood formation in Fagus sylvatica during two contrasting years. IAWA J. 2016, 37, 332–348. [Google Scholar] [CrossRef]
  23. Schume, H.; Grabner, M.; Eckmüllner, O. The influence of an altered groundwater regime on vessel properties of hybrid poplar. Trees 2004, 18, 184–194. [Google Scholar] [CrossRef]
  24. Hacke, U.G.; Lachenbruch, B.; Pittermann, J.; Mayr, S.; Domec, J.-C.; Schulte, P.J. The Hydraulic Architecture of Conifers BT—Functional and Ecological Xylem Anatomy; Springer International Publishing: Cham, Switzerland, 2015. [Google Scholar]
  25. Tyree, M.T.; Zimmermann, M.H. Xylem Structure and the Ascent of Sap; Springer Series in Wood Science; Springer: Berlin/Heidelberg, Germany, 2013. [Google Scholar]
  26. Chambi-Legoas, R.; Tomazello-Filho, M.; Laclau, J.P.; Chaix, G. Potassium fertilization enhances xylem plasticity and growth recovery of Eucalyptus grandis trees in response to drastic changes in water availability. For. Ecol. Manag. 2023, 528, 120656. [Google Scholar] [CrossRef]
  27. Chenlemuge, T.; Schuldt, B.; Dulamsuren, C.; Hertel, D.; Leuschner, C.; Hauck, M. Stem increment and hydraulic architecture of a boreal conifer (Larix sibirica) under contrasting macroclimates. Trees 2015, 29, 623–636. [Google Scholar] [CrossRef]
  28. Schuldt, B.; Knutzen, F.; Delzon, S.; Jansen, S.; Müller-Haubold, H.; Burlett, R.; Leuschner, C. How adaptable is the hydraulic system of European beech in the face of climate change-related precipitation reduction? New Phytol. 2016, 210, 443–458. [Google Scholar] [CrossRef] [PubMed]
  29. Schreiber, S.G.; Hacke, U.G.; Hamann, A. Variation of xylem vessel diameters across a climate gradient: Insight from a reciprocal transplant experiment with a widespread boreal tree. Funct. Ecol. 2015, 29, 1392–1401. [Google Scholar] [CrossRef]
  30. Montwé, D.; Spiecker, H.; Hamann, A. An experimentally controlled extreme drought in a Norway spruce forest reveals fast hydraulic response and subsequent recovery of growth rates. Trees 2014, 28, 891–900. [Google Scholar] [CrossRef]
  31. Sterck, F.J.; Zweifel, R.; Sass-Klaassen, U.; Chowdhury, Q. Persisting soil drought reduces leaf specific conductivity in Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens). Tree Physiol. 2008, 28, 529–536. [Google Scholar] [CrossRef]
  32. Bader, M.K.F.; Scherrer, D.; Zweifel, R.; Körner, C. Less pronounced drought responses in ring-porous than in diffuse-porous temperate tree species. Agric. For. Meteorol. 2022, 327, 109184. [Google Scholar] [CrossRef]
  33. Kiorapostolou, N.; Da Sois, L.; Petruzzellis, F.; Savi, T.; Trifilò, P.; Nardini, A.; Petit, G. Vulnerability to xylem embolism correlates to wood parenchyma fraction in angiosperms but not in gymnosperms. Tree Physiol. 2019, 39, 1675–1684. [Google Scholar] [CrossRef]
  34. Slupianek, A.; Dolzblasz, A.; Sokołowska, K. Xylem parenchyma—Role and relevance in wood functioning in trees. Plants 2021, 10, 1247. [Google Scholar] [CrossRef]
  35. Hearn, D.J.; Poulsen, S.; Spicer, R. The evolution of growth forms with expanded root and shoot parenchymatous storage is correlated across the eudicots. Int. J. Plant Sci. 2013, 174, 1049–1061. [Google Scholar] [CrossRef]
  36. Sevanto, S.; Hölttä, T.; Holbrook, N.M. Effects of the hydraulic coupling between xylem and phloem on diurnal phloem diameter variation. Plant Cell Environ. 2011, 34, 690–703. [Google Scholar] [CrossRef] [PubMed]
  37. Höltta, T.; Vesala, T.; Sevanto, S.; Peramaki, M.; Nikinmaa, E. Modeling xylem and phloem water flows in trees according to cohesion theory and Munch hypothesis. Trees 2006, 20, 67–78. [Google Scholar] [CrossRef]
  38. Morris, H.; Gillingham, M.A.; Plavcová, L.; Gleason, S.M.; Olson, M.E.; Coomes, D.A.; Fichtler, E.; Klepsch, M.M.; Martínez-Cabrera, H.I.; McGlinn, D.J.; et al. Vessel diameter is related to amount and spatial arrangement of axial parenchyma in woody angiosperms. Plant Cell Environ. 2018, 41, 245–260. [Google Scholar] [CrossRef] [PubMed]
  39. Morris, H.; Plavcová, L.; Cvecko, P.; Fichtler, E.; Gillingham, M.A.; Martínez-Cabrera, H.I.; Jansen, S. A global analysis of parenchyma tissue fractions in secondary xylem of seed plants. New Phytol. 2016, 209, 1553–1565. [Google Scholar] [CrossRef] [PubMed]
  40. Spicer, R. Symplasmic networks in secondary vascular tissues: Parenchyma distribution and activity supporting long-distance transport. J. Exp. Bot. 2014, 65, 1829–1848. [Google Scholar] [CrossRef] [PubMed]
  41. Höll, W. Distribution, fluctuation and metabolism of food reserves in the wood of trees. In Cell and Molecular Biology of Wood Formation; Savidge, R., Barnett, J., Napier, R., Eds.; BIOS Scientific Publishers: Oxford, UK, 2000; pp. 347–362. [Google Scholar]
  42. Secchi, F.; Zwieniecki, M.A. Sensing embolism in xylem vessels: The role of sucrose as a trigger for refilling. Plant Cell Environ. 2011, 34, 514–524. [Google Scholar] [CrossRef] [PubMed]
  43. Salleo, S.; Trifilò, P.; Esposito, S.; Nardini, A.; Gullo, M.A.L. Starch-to-sugar conversion in wood parenchyma of field-growing Laurus nobilis plants: A component of the signal pathway for embolism repair? Funct. Plant Biol. 2009, 36, 815–825. [Google Scholar] [CrossRef]
  44. Aritsara, A.N.A.; Razakandraibe, V.M.; Ramananantoandro, T.; Gleason, S.M.; Cao, K.F. Increasing axial parenchyma fraction in the Malagasy Magnoliids facilitated the co-optimisation of hydraulic efficiency and safety. New Phytol. 2021, 229, 1467–1480. [Google Scholar] [CrossRef]
  45. Mencuccini, M.; Hölttä, T.; Sevanto, S.; Nikinmaa, E. Concurrent measurements of change in the bark and xylem diameters of trees reveal a phloem-generated turgor signal. New Phytol. 2013, 198, 1143–1154. [Google Scholar] [CrossRef]
  46. Pratt, R.B.; Jacobsen, A.L. Conflicting demands on angiosperm xylem: Tradeoffs among storage, transport and biomechanics. Plant Cell Environ. 2017, 40, 897–913. [Google Scholar] [CrossRef]
  47. Alves, E.S.; Angyalossy-Alfonso, V. Ecological trends in the wood anatomy of some Brazilian species. 2. Axial parenchyma, rays and fibres. IAWA J. 2002, 23, 391–418. [Google Scholar] [CrossRef]
  48. Wheeler, E.A.; Baas, P. A survey of the fossil record for dicotyledonous wood and its significance for evolutionary and ecological wood anatomy. IAWA J. 1991, 12, 275–318. [Google Scholar] [CrossRef]
  49. Tollmann, A. Geologie von Österreich; Band 1. Die Zentralalpen; Deuticke: Wien, Austria, 1977. [Google Scholar]
  50. FAO. World Reference Base for Soil Resources; FAO: Rome, Italy, 1998. [Google Scholar]
  51. Neuwinger, I. Böden der subalpinen und alpinen Stufe in den Tiroler Alpen. Mitteilungen Ostalpin Dinarischen Ges. Veg. 1970, 11, 135–150. [Google Scholar]
  52. Richner, H.; Hächler, P. Understanding and forecasting Alpine foehn. In Mountain Weather Research and Forecasting: Recent Progress and Current Challenges; Chow, F.K., De Wekker, S.F.J., Snyder, B.J., Eds.; Springer: Berlin/Heidelberg, Germany, 2013; pp. 219–260. [Google Scholar]
  53. Oberhuber, W.; Dobler, A.-L.; Heinzle, T.; Scandurra, F.; Gruber, A.; Wieser, G. Climate overrides the influence of microsite conditons on radial growth of the tall multi-stemmed shrub Alnus alnobetula at treeline. Plants 2023, 12, 1708. [Google Scholar] [CrossRef] [PubMed]
  54. Olson, E.M.; Anfodillo, T.; Gleason, M.S.; McCulloh, K.A. Tip-to-base xylem conduit widening as an adaptation: Causes, consequences, and empirical priorities. New Phytol. 2020, 229, 1877–1893. [Google Scholar] [CrossRef] [PubMed]
  55. McCulloh, K.A.; Sperry, J.S. Patterns in hydraulic architecture and their implications for transport efficiency. Tree Physiol. 2005, 25, 257–267. [Google Scholar] [CrossRef] [PubMed]
  56. Anfodillo, T.; Carraro, V.; Carrer, M.; Fior, C.; Rossi, S. Convergent tapering of xylem conduits in different woody species. New Phytol. 2006, 169, 279–290. [Google Scholar] [CrossRef] [PubMed]
  57. Meinzer, F.C.; Clearwater, M.J.; Goldstein, G. Water transport in trees: Current perspectives, new insights and some controversies. Environ. Exp. Bot. 2001, 45, 239–262. [Google Scholar] [CrossRef]
  58. Hietz, P.; Rungwattana, K.; Scheffknecht, S.; George, J.P. Effects of Provenance, Growing Site, and Growth on Quercus robur Wood Anatomy and Density in a 12-Year-Old Provenance Trial. Front. Plant Sci. 2022, 13, 795941. [Google Scholar] [CrossRef]
  59. George, J.P.; Theroux-Rancourt, G.; Rungwattana, K.; Scheffknecht, S.; Momirovic, N.; Neuhauser, L.; Hietz, P. Assessing adaptive and plastic responses in growth and functional traits in a 10-year-old common garden experiment with pedunculate oak (Quercus robur L.) suggests that directional selection can drive climatic adaptation. Evol. Appl. 2020, 13, 2422–2438. [Google Scholar] [CrossRef]
  60. Bayramzadeh, V.; Funada, R.; Kubo, T. Relationships between vessel element anatomy and physiological as well as morphological traits of leaves in Fagus crenata seedlings originating from different provenances. Trees 2008, 22, 217–224. [Google Scholar] [CrossRef]
  61. Schymanski, S.J.; Or, D. Wind increases leaf water use efficiency. Plant Cell Environ. 2016, 39, 1448–1459. [Google Scholar] [CrossRef] [PubMed]
  62. Körner, C. Alpine Plant Life: Functional Plant Ecology of High Mountain Ecosystems; Springer: New York, NY, USA, 2003. [Google Scholar]
  63. Rathgeber, C.B.K.; Perez-de-Lis, G.; Fernandez-de-Una, L.; Fonti, P.; Rossi, S.; Treydte, K.; Ponton, S. Anatomical, developmental and physiological bases of tree-ring formation in relation to environmental factors. In Stable Isotopes in Tree Rings: Inferring Physiological, Climatic and Environmental Responses; Siegwolf, R.T.W., Brooks, J.R., Roden, J., Saurer, M., Eds.; Springer International: Cham, Switzerland, 2022; pp. 61–99. [Google Scholar]
  64. Chaffey, N.J. (Ed.) Wood Formation in Trees—Cell and Molecular Biology Techniques; Taylor and Francis: London, UK; New York, NY, USA, 2002; p. 364. [Google Scholar]
  65. Donaldson, L.A. Lignification and lignin topochemistry—An ultrastructural view. Phytochemistry 2001, 57, 859–873. [Google Scholar] [CrossRef] [PubMed]
  66. Cabon, A.; Fernandez-de-Una, L.; Gea-Izquierdo, G.; Meinzer, F.C.; Woodruff, D.R.; Martınez-Vilalta, J.; de Caceres, M. Water potential control of turgor driven tracheid enlargement in Scots pine at its xeric distribution edge. New Phytol. 2020, 225, 209–221. [Google Scholar] [CrossRef] [PubMed]
  67. Fromm, J. Xylem development in trees: From cambial divisions to mature wood cells. In Cellular Aspects of Wood Formation; Springer: Berlin/Heidelberg, Germany, 2013; pp. 3–39. [Google Scholar]
  68. Koch, K. Sucrose metabolism: Regulatory mechanisms and pivotal roles in sugar sensing and plant development. Curr. Opin. Plant Biol. 2004, 7, 235–246. [Google Scholar] [CrossRef] [PubMed]
  69. Gruber, A.; Strobl, S.; Veit, B.; Oberhuber, W. Impact of drought on the temporal dynamics of wood formation in Pinus sylvestris. Tree Physiol. 2010, 30, 490–501. [Google Scholar] [CrossRef] [PubMed]
  70. Balducci, L.; Cuny, H.E.; Rathgeber, C.B.K.; Deslauriers, A.; Giovannelli, A.; Rossi, S. Compensatory mechanisms mitigate the effect of warming and drought on wood formation. Plant Cell Environ. 2016, 39, 1338–1352. [Google Scholar] [CrossRef] [PubMed]
  71. Rossi, S.; Simard, S.; Rathgeber, C.B.K.; Deslauriers, A.; De Zan, C. Effects of a 20-day-long dry period on cambial and apical meristem growth in Abies balsamea seedlings. Trees 2009, 23, 85–93. [Google Scholar] [CrossRef]
  72. Nonami, H.; Boyer, J.S. Turgor and growth at low water potentials. Plant Physiol. 2008, 89, 798–804. [Google Scholar] [CrossRef]
  73. Hacke, U.G.; Spicer, R.; Schreiber, S.G.; Plavcová, L. An ecophysiological and developmental perspective on variation in vessel diameter. Plant Cell Environ. 2017, 40, 831–845. [Google Scholar] [CrossRef]
  74. Plavcova, L.; Hacke, U.G. Phenotypic and developmental plasticity of xylem in hybrid poplar saplings subjected to experimental drought, nitrogen fertilization, and shading. J. Exp. Bot. 2012, 63, 6481–6491. [Google Scholar] [CrossRef] [PubMed]
  75. Awad, H.; Barigah, T.; Badel, E.; Cochard, H.; Herbette, S. Poplar vulnerability to xylem cavitation acclimates to drier soil conditions. Physiol. Plant. 2010, 139, 280–288. [Google Scholar] [CrossRef] [PubMed]
  76. Noyer, E.; Stojanović, M.; Horáček, P.; Pérez-de-Lis, G. Toward a better understanding of angiosperm xylogenesis: A new method for a cellular approach. New Phytol. 2023, 239, 792–805. [Google Scholar] [CrossRef] [PubMed]
  77. Hajek, P.; Kurjak, D.; von Wühlisch, G.; Delzon, S.; Schuldt, B. Intraspecific variation in wood anatomical, hydraulic, and foliar traits in ten European beech provenances differing in growth yield. Front. Plant Sci. 2016, 7, 791. [Google Scholar] [CrossRef] [PubMed]
  78. Oladi, R.; Bräuning, A.; Pourtahmasi, K. “Plastic” and “static” behavior of vessel-anatomical features in Oriental beech (Fagus orientalis Lipsky) in view of xylem hydraulic conductivity. Trees 2014, 28, 493–502. [Google Scholar] [CrossRef]
  79. Buttò, V.; Rozenberg, P.; Deslauriers, A.; Rossi, S.; Morin, H. Environmental and developmental factors driving xylem anatomy and micro-density in black spruce. New Phytol. 2021, 230, 957–971. [Google Scholar] [CrossRef] [PubMed]
  80. Hartmann, F.; Rathgeber, C.; Fournier, M.; Moulia, B. Modelling wood formation and structure: Power and limits of a morphogenetic gradient in controlling xylem cell proliferation and growth. Ann. For. Sci. 2017, 74, 1–15. [Google Scholar] [CrossRef]
  81. Rosell, J.A.; Olson, M.E.; Anfodillo, T. Scaling of xylem vessel diameter with plant size: Causes, predictions, and outstanding questions. Curr. For. Rep. 2017, 3, 46–59. [Google Scholar] [CrossRef]
  82. Schweingruber, F.H. Wood Structure and Environment; Springer: Berlin/Heidelberg, Germany, 2007. [Google Scholar]
  83. Bosshard, H. Holzkunde. Band 1: Mikroskopie und Makroskopie des Holzes; Birkhauser: Basel, Switzerland, 1982. [Google Scholar]
  84. Carlquist, S. Comparative Wood Anatomy; Springer: Berlin/Heidelberg, Germany, 2001. [Google Scholar]
  85. Schenk, H.J.; Jansen, S.; Hölttä, T. Positive pressure in xylem and its role in hydraulic function. New Phytol. 2021, 230, 27–45. [Google Scholar] [CrossRef]
  86. Jupa, R.; Plavcová, L.; Gloser, V.; Jansen, S. Linking xylem water storage with anatomical parameters in five temperate tree species. Tree Physiol. 2016, 36, 756–769. [Google Scholar] [CrossRef]
  87. Holbrook, N.M. Stem water storage. In Plant Stems: Physiology and Functional Morphology; Academic Press: San Diego, CA, USA, 1995; pp. 151–174. [Google Scholar]
  88. Meinzer, F.C.; Johnson, D.M.; Lachenbruch, B.; McCulloh, K.A.; Woodruff, D.R. Xylem hydraulic safety margins in woody plants: Coordination of stomatal control of xylem tension with hydraulic capacitance. Funct. Ecol. 2009, 23, 922–930. [Google Scholar] [CrossRef]
  89. Kawai, K.; Minagi, K.; Nakamura, T.; Saiki, S.T.; Yazaki, K.; Ishida, A. Parenchyma underlies the interspecific variation of xylem hydraulics and carbon storage across 15 woody species on a subtropical island in Japan. Tree Physiol. 2022, 42, 337–350. [Google Scholar] [CrossRef] [PubMed]
  90. Janssen, T.A.; Hölttä, T.; Fleischer, K.; Naudts, K.; Dolman, H. Wood allocation trade-offs between fiber wall, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees. Plant Cell Environ. 2019, 43, 965–980. [Google Scholar] [CrossRef] [PubMed]
  91. Zheng, J.; Martínez-Cabrera, H.I. Wood anatomical correlates with theoretical conductivity and wood density across China: Evolutionary evidence of the functional differentiation of axial and radial parenchyma. Ann. Bot. 2013, 112, 927–935. [Google Scholar] [CrossRef]
  92. Carlquist, S. Living cells in wood. 1. Absence, scarcity and histology of axial parenchyma as keys to function. Bot. J. Linn. Soc. 2015, 177, 291–321. [Google Scholar] [CrossRef]
  93. Gasson, P. Some implications of anatomical variations in the wood of pedunculate oak (Quercus robur L.), including comparisons with common beech (Fagus sylvatica L.). IAWA J. 1987, 8, 149–166. [Google Scholar] [CrossRef]
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Figure 1. Geographical location of the N-site and S-site in the Central Austrian Alps. Source: tirisMaps Land Tirol, (https://maps.tirol.gv.at/synserver?user=guest&project=tmap_master, accessed on 6 November 2023).
Figure 1. Geographical location of the N-site and S-site in the Central Austrian Alps. Source: tirisMaps Land Tirol, (https://maps.tirol.gv.at/synserver?user=guest&project=tmap_master, accessed on 6 November 2023).
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Figure 2. (a) Seasonal variation of soil moisture (solid lines) and precipitation (bars). (b) Air and soil temperature (solid and dotted lines, respectively) and solar radiation (solid lines) at both study plots. Black and red lines indicate the N- and S-site, respectively.
Figure 2. (a) Seasonal variation of soil moisture (solid lines) and precipitation (bars). (b) Air and soil temperature (solid and dotted lines, respectively) and solar radiation (solid lines) at both study plots. Black and red lines indicate the N- and S-site, respectively.
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Figure 3. Boxplots of (a) mean vessel area (MVA), (b) vessel density (VD), and (c) theoretic conductive area (TCA). N-site: dark blue; S-site: light blue. Different letters indicate p < 0.05 among sites. p-values are shown in the upper right corner.
Figure 3. Boxplots of (a) mean vessel area (MVA), (b) vessel density (VD), and (c) theoretic conductive area (TCA). N-site: dark blue; S-site: light blue. Different letters indicate p < 0.05 among sites. p-values are shown in the upper right corner.
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Figure 4. (a) Kernel Gauss distribution of vessel area for the N-site (black line) and S-site (red line). (b) Counts of vessels of different size classes per cross-section area. N-site: black line, S-site: red line; grey and thin red lines represent single measuring years (2011–2020) at the N-site and S-site, respectively.
Figure 4. (a) Kernel Gauss distribution of vessel area for the N-site (black line) and S-site (red line). (b) Counts of vessels of different size classes per cross-section area. N-site: black line, S-site: red line; grey and thin red lines represent single measuring years (2011–2020) at the N-site and S-site, respectively.
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Figure 5. Boxplots of (a) mean vessel area (MVA), (b) vessel density (VD), and (c) theoretic conductive area (TCA), in the first and second half of the annual ring. Left panels: N-site in dark blue; right panels: S-site in light blue. Different letters indicate p < 0.05 between the first and second 50% of the annual ring. p-values are shown in the upper right corner.
Figure 5. Boxplots of (a) mean vessel area (MVA), (b) vessel density (VD), and (c) theoretic conductive area (TCA), in the first and second half of the annual ring. Left panels: N-site in dark blue; right panels: S-site in light blue. Different letters indicate p < 0.05 between the first and second 50% of the annual ring. p-values are shown in the upper right corner.
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Figure 6. Kernel Gauss distribution of vessel area at the (a) N-site and (b) S-site and counts of vessels of different size classes per cross-section area at the (c) N-site and (d) S-site. Black lines and dark red lines represent values for the first half of the annual ring; grey lines and orange lines represent values for the second half of the annual ring. In (c,d), thin lines represent single measuring years (2015–2020).
Figure 6. Kernel Gauss distribution of vessel area at the (a) N-site and (b) S-site and counts of vessels of different size classes per cross-section area at the (c) N-site and (d) S-site. Black lines and dark red lines represent values for the first half of the annual ring; grey lines and orange lines represent values for the second half of the annual ring. In (c,d), thin lines represent single measuring years (2015–2020).
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Figure 7. (a) Boxplots of the percentage of axial parenchyma (PA) at the N-site (orange) and the S-site (light orange). (b) Boxplots of the percentage of axial parenchyma at the N-site in the first and the second half of the annual ring. (c) Boxplots of the percentage of axial parenchyma at the S-site in the first and the second half of the annual ring. Different letters indicate p < 0.05 among groups. p-values are shown in the upper right corner.
Figure 7. (a) Boxplots of the percentage of axial parenchyma (PA) at the N-site (orange) and the S-site (light orange). (b) Boxplots of the percentage of axial parenchyma at the N-site in the first and the second half of the annual ring. (c) Boxplots of the percentage of axial parenchyma at the S-site in the first and the second half of the annual ring. Different letters indicate p < 0.05 among groups. p-values are shown in the upper right corner.
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Table 1. Site description (N-site = north-facing leeward site; S-site = south-exposed windward site) and characteristics of the selected A. alnobetula stands. Mean values ± standard deviation (SD) are shown.
Table 1. Site description (N-site = north-facing leeward site; S-site = south-exposed windward site) and characteristics of the selected A. alnobetula stands. Mean values ± standard deviation (SD) are shown.
Elevation
(m asl)		AspectSlope (°)Soil Depth (cm)
Mean ± SD		Canopy Height (m)
Mean ± SD		Stand Age (yrs)
Mean ± SD
N-site2150N3512 ± 3270 ± 8015 ± 7
S-site2140SE307 ± 3140 ± 4020 ± 8
Table 2. Climatic parameters (mean values ± standard deviation for July and August) obtained during the growing season 2022 and mean ring width on the S- and N-site. Statistical significance (p-values) of the differences between the study plots are indicated.
Table 2. Climatic parameters (mean values ± standard deviation for July and August) obtained during the growing season 2022 and mean ring width on the S- and N-site. Statistical significance (p-values) of the differences between the study plots are indicated.
Tair (°C) 1Solar Radiation (Wm−2)Tsoil (°C) 2 Soil Moisture (% vol.)Ring Width (µm)
S-site10.8 ± 4.0229.5 ± 82.710.3 ± 1.020.1 ± 2.7378 ± 171
N-site10.4 ± 3.6180.7 ± 67.19.8 ± 1.123.0 ± 3.2714 ± 228
p0.421<0.0010.021<0.001<0.001
1 Tair = air temperature, 2 Tsoil = soil temperature.
Table 3. Mean vessel area (MVA), vessel density (VD), theoretical conductive area (TCA), and percentage of axial parenchyma (PA) at the S- and N-site for the total annual ring (ARtot) and the first (ARfirst) and the second half (ARsecond) of the annual ring. Mean values ± standard deviations are shown.
Table 3. Mean vessel area (MVA), vessel density (VD), theoretical conductive area (TCA), and percentage of axial parenchyma (PA) at the S- and N-site for the total annual ring (ARtot) and the first (ARfirst) and the second half (ARsecond) of the annual ring. Mean values ± standard deviations are shown.
MVA (µm2)VD (no./mm−2)TCA (%)PA (%)
N-site (ARtot)659.5 ± 177.4135.6 ± 39.59.3 ± 2.410.1 ± 2.0
S-site (ARtot)708.4 ± 223.5110.9 ± 32.97.7 ± 2.916.5 ± 5.2
N-site (ARfirst)741.6 ± 187.5166.1 ± 65.611.5 ± 3.14.0 ± 2.1
N-site (ARsecond)658,8 ± 153.7133.3 ± 55.48.3 ± 2.314.9 ± 3.5
S-site (ARfirst)823.6 ± 241.8109.7 ± 37.88.8 ± 3.49.5 ± 4.9
S-site (ARsecond)701.1 ± 161.984.9 ± 34.06.5 ± 4.319.6 ± 5.7
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Gruber, A.; Wieser, G.; Fink, M.; Oberhuber, W. Impact of Environmental Conditions on Wood Anatomical Traits of Green Alder (Alnus alnobetula) at the Alpine Treeline. Forests 2024, 15, 24. https://doi.org/10.3390/f15010024

AMA Style

Gruber A, Wieser G, Fink M, Oberhuber W. Impact of Environmental Conditions on Wood Anatomical Traits of Green Alder (Alnus alnobetula) at the Alpine Treeline. Forests. 2024; 15(1):24. https://doi.org/10.3390/f15010024

Chicago/Turabian Style

Gruber, Andreas, Gerhard Wieser, Marion Fink, and Walter Oberhuber. 2024. "Impact of Environmental Conditions on Wood Anatomical Traits of Green Alder (Alnus alnobetula) at the Alpine Treeline" Forests 15, no. 1: 24. https://doi.org/10.3390/f15010024

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