Intraspecific Variation in Wood Anatomical, Hydraulic, and Foliar Traits in Ten European Beech Provenances Differing in Growth Yield

Plasticity of wood and leaf traits related to hydraulic efficiency and safety is linked to evaporative demand and not soil moisture in rubber (Hevea brasiliensis)

The predicted increase of drought intensity in South-East Asia has raised concern about the sustainability of rubber (Hevea brasiliensis Müll. Arg.) cultivation. In order to quantify the degree of phenotypic plasticity in this important tree crop species, we analysed a set of wood and leaf traits related to the hydraulic safety and efficiency in PB260 clones from eight small-holder plantations in Jambi province, Indonesia, representing a gradient in local microclimatic and edaphic conditions. Across plots, branch embolism resistance (P50) ranged from -2.14 to -2.58 MPa. The P50 and P88 values declined, and the hydraulic safety margin increased, with an increase in the mean annual vapour pressure deficit (VPD). Among leaf traits, only the changes in specific leaf area were related to the differences in evaporative demand. These variations of hydraulic trait values were not related to soil moisture levels. We did not find a trade-off between hydraulic safety and efficiency, but vessel density (VD) emerged as a major trait associated with both safety and efficiency. The VD, and not vessel diameter, was closely related to P50 and P88 as well as to specific hydraulic conductivity, the lumen-to-sapwood area ratio and the vessel grouping index. In conclusion, our results demonstrate some degree of phenotypic plasticity in wood traits related to hydraulic safety in this tropical tree species, but this is only in response to the local changes in evaporative demand and not soil moisture. Given that VPD may increasingly limit plant growth in a warmer world, our results provide evidence of hydraulic trait changes in response to a rising evaporative demand.

Xylem structure and hydraulic function in roots and stems of chaparral shrub species from high and low elevation in the Sierra Nevada, California

Xylem structure and hydraulics were compared between individuals at lower and upper elevation distribution limits for five chaparral shrub species along a steep transect in the southern Sierra Nevada, California, USA. Higher-elevation plants experienced frequent winter freeze-thaw events and increased precipitation. We hypothesized that environmental differences would lead to xylem trait differences between high and low elevations, but predictions were complicated because both water stress (low elevation) and freeze-thaw events (high elevation) may select for similar traits, such as narrow vessel diameter. We found significant changes in the ratio of stem xylem area to leaf area (Huber value) between elevations, with more xylem area required to support leaves at low elevations. Co-occurring species significantly differed in their xylem traits, suggesting diverse strategies to cope with the highly seasonal environment of this Mediterranean-type climate region. Roots were more hydraulically efficient and more vulnerable to embolism relative to stems, potentially due to roots being buffered from freeze-thaw stress, which allows them to maintain wider diameter vessels. Knowledge of the structure and function of both roots and stems is likely important in understanding whole-plant response to environmental gradients.

Higher local intra- than interspecific variability in water- and carbon-related leaf traits among Neotropical tree species

BACKGROUND AND AIMS

Intraspecific variability in leaf water-related traits remains little explored despite its potential importance in the context of increasing drought frequency and severity. Studies comparing intra- and interspecific variability of leaf traits often rely on inappropriate sampling designs that result in non-robust estimates, mainly owing to an excess of the species/individual ratio in community ecology or, on the contrary, to an excess of the individual/species ratio in population ecology.

METHODS

We carried out virtual testing of three strategies to compare intra- and interspecific trait variability. Guided by the results of our simulations, we carried out field sampling. We measured nine traits related to leaf water and carbon acquisition in 100 individuals from ten Neotropical tree species. We also assessed trait variation among leaves within individuals and among measurements within leaves to control for sources of intraspecific trait variability.

KEY RESULTS

The most robust sampling, based on the same number of species and individuals per species, revealed higher intraspecific variability than previously recognized, higher for carbon-related traits (47-92 and 4-33 % of relative and absolute variation, respectively) than for water-related traits (47-60 and 14-44 % of relative and absolute variation, respectively), which remained non-negligible. Nevertheless, part of the intraspecific trait variability was explained by variation of leaves within individuals (12-100 % of relative variation) or measurement variations within leaf (0-19 % of relative variation) and not only by individual ontogenetic stages and environmental conditions.

CONCLUSIONS

We conclude that robust sampling, based on the same number of species and individuals per species, is needed to explore global or local variation in leaf water- and carbon-related traits within and among tree species, because our study revealed higher intraspecific variation than previously recognized.

Addressing controversies in the xylem embolism resistance-vessel diameter relationship

Although xylem embolism is a key process during drought-induced tree mortality, its relationship to wood anatomy remains debated. While the functional link between bordered pits and embolism resistance is known, there is no direct, mechanistic explanation for the traditional assumption that wider vessels are more vulnerable than narrow ones. We used data from 20 temperate broad-leaved tree species to study the inter- and intraspecific relationship of water potential at 50% loss of conductivity (P₅₀ ) with hydraulically weighted vessel diameter (D_h ) and tested its link to pit membrane thickness (T_PM ) and specific conductivity (K_s ) on species level. Embolism-resistant species had thick pit membranes and narrow vessels. While D_h was weakly associated with T_PM , the P₅₀ -D_h relationship remained highly significant after accounting for T_PM . The interspecific pattern between P₅₀ and D_h was mirrored by a link between P₅₀ and K_s , but there was no evidence for an intraspecific relationship. Our results provide robust evidence for an interspecific P₅₀ -D_h relationship across our species. As a potential cause for the inconsistencies in published P₅₀ -D_h relationships, our analysis suggests differences in the range of trait values covered, and the level of data aggregation (species, tree or sample level) studied.

Traits plasticity of Sodom Apple (Calotropis procera) along the environmental gradient in the semi-arid environment

Biomass and morphological characteristics of plant species provide essential insight into how well a species adapts to its environment. The study aims to evaluate how environmental variables (viz., altitude, slope, aspect degree, and soil properties) influence the morphological traits and biomass variability of Calotropis procera (Aiton) W.T. Aiton in a semi-arid environment. C. procera sample locations were divided into 39 permanent sites (5×5 = 25 m²). Slope, aspect degree, slope aspect, altitude, and soil variables (soil moisture, organic matter, nitrogen (N %), and phosphorus (P) gradients were used to quantify morphological parameters (height, diameters, canopy area, volume, and leave/branch biomass) and aboveground biomass. Environmental variables, i.e., altitude and aspect degree, were the most important factor influencing the biomass variation and affecting soil moisture content; however, they did not directly affect the total biomass of the species. The results also reveal significant plasticity in morphological traits exists concerning elevation and aspect degree at (p< 0.05). Plant volume was a better indicator of species' total biomass revealed from the regression model showing significant at p< 0.05. The study also reveals that soil properties such as soil moisture and Phosphorus have an important role in enhancing the productivity of the studied plant species. The results concluded that plants functional traits and biomass shows significant variation across the altitude and these parameters could be consider in the conservation of this native species.

Leaf trait modification in European beech trees in response to climatic and edaphic drought

Leaf morphological and physiological traits control the carbon and water relations of mature trees and are determinants of drought tolerance, but it is not well understood how they are modified in response to water deficits. We analysed five sun-canopy leaf traits (mean leaf size (LS), specific leaf area (SLA), Huber value (HV), water potential at turgor loss point (Ψ_tlp ) and foliar carbon isotope signature (δ¹³ C)) in European beech (Fagus sylvatica L.) across three precipitation gradients sampled in moist (2010), dry (2019) and very dry (2018) summers, and tested their response to short-term water deficits (climatic water balance (CWB) preceding sample collection) and long-term water availability (mean annual precipitation (MAP), plant-available soil water capacity (AWC) and neighbourhood competition). Across the 34 sites, LS varied seven-fold (3.9-27.0 cm² ), SLA four-fold (77.1-306.9 cm²·g^-1 ) and HV six-fold (1.0-6.65 cm² ·m^-2 ). In the 2018 dataset, LS showed a negative and HV a positive relationship to MAP, which contradicts relations found in multi-species samples. Average Ψ_tlp ranged from -1.90 to -2.62 MPa and decreased across the sites with decreasing CWB in the month prior to measurement, as well as with decreasing MAP and AWC in 2019. Studied leaf traits varied considerably between years, suggesting that mast fruiting and the severe 2018 drought caused the formation of smaller leaves. We conclude that sun-canopy leaf traits of European beech exhibit considerable plasticity in response to climatic and edaphic aridity, and that osmotic adjustment may be an important element in the drought response strategy of this anisohydric tree species.

Interannual adjustments in stomatal and leaf morphological traits of European beech (Fagus sylvatica L.) demonstrate its climate change acclimation potential

The current projections of climate change might exceed the ability of European forest trees to adapt to upcoming environmental conditions. However, stomatal and leaf morphological traits could greatly influence the acclimation potential of forest tree species subjected to global warming, including the single most important forestry species in Europe, European beech. We analysed stomatal (guard cell length, stomatal density and potential conductance index) and leaf (leaf area, leaf dry weight and leaf mass per area) morphological traits of ten provenances from two provenance trials with contrasting climates between 2016 and 2020. The impact of meteorological conditions of the current and preceding year on stomatal and leaf traits was tested by linear and quadratic regressions. Ecodistance was used to capture the impact of adaptation after the transfer of provenances to new environments. Interactions of trial-provenance and trial-year factors were significant for all measured traits. Guard cell length was lowest and stomatal density was highest across beech provenances in the driest year, 2018. Adaptation was also reflected in a significant relationship between aridity ecodistance and measured traits. Moreover, the meteorological conditions of the preceding year affected the interannual variability of stomatal and leaf traits more than the meteorological conditions of the spring of the current year, suggesting the existence of plant stress memory. High intraspecific variability of stomatal and leaf traits controlled by the interaction of adaptation, acclimation and plant memory suggests a high acclimation potential of European beech provenances under future conditions of global climate change.

Recovery after long-term summer drought: Hydraulic measurements reveal legacy effects in trunks of Picea abies but not in Fagus sylvatica

Climate change is expected to increase the frequency and intensity of summer droughts. Sufficient drought resistance, the ability to acclimate to and/or recover after drought, is thus crucial for forest tree species. However, studies on the hydraulics of mature trees during and after drought in natura are scarce. In this study, we analysed trunk water content (electrical resistivity: ER) and further hydraulic (water potential, sap flow density, specific hydraulic conductivity, vulnerability to embolism) as well as wood anatomical traits (tree ring width, conduit diameter, conduit wall reinforcement) of drought-stressed (artificially induced summer drought via throughfall-exclusion) and unstressed Picea abies and Fagus sylvatica trees. In P. abies, ER indicated a strong reduction in trunk water content after 5 years of summer drought, corresponding to significantly lower pre-dawn leaf water potential and xylem sap flow density. Vulnerability to embolism tended to be higher in drought-stressed trees. In F. sylvatica, only small differences between drought-stressed and control trees were observed. Re-watering led to a rapid increase in water potentials and xylem sap flow of both drought-stressed trees, and to increased growth rates in the next growing season. ER analyses revealed lower trunk water content in P. abies trees growing on throughfall-exclusion plots even 1 year after re-watering, indicating a limited capacity to restore internal water reserves. Results demonstrated that P. abies is more susceptible to recurrent summer drought than F. sylvatica, and can exhibit long-lasting and pronounced legacy effects in trunk water reserves.

Branch xylem vascular adjustments in European beech in response to decreasing water availability across a precipitation gradient

Crucial for the climate adaptation of trees is a xylem anatomical structure capable of adjusting to changing water regimes. Although species comparisons across climate zones have demonstrated anatomical change in response to altered water availability and tree height, less is known about the adaptability of tree vascular systems to increasing water deficits at the intraspecific level. Information on the between-population and within-population variability of xylem traits helps assessing a species' ability to cope with climate change. We investigated the variability of wood anatomical and related hydraulic traits in terminal branches of European beech (Fagus sylvatica L.) trees across a precipitation gradient (520-890 mm year-1) and examined the influence of climatic water balance (CWB), soil water capacity (AWC), neighborhood competition (CI), tree height and branch age on these traits. Furthermore, the relationship between xylem anatomical traits and embolism resistance (P50) was tested. Within-population trait variation was larger than between-population variation. Vessel diameter, lumen-to-sapwood area ratio and potential conductivity of terminal branches decreased with decreasing CWB, but these traits were not affected by AWC, whereas vessel density increased with an AWC decrease. In contrast, none of the studied anatomical traits were influenced by variation in tree height (21-34 m) or CI. Branch age was highly variable (2-22 years) despite equal diameter and position in the flow path, suggesting different growth trajectories in the past. Vessel diameter decreased, and vessel density increased, with increasing branch age, reflecting negative annual radial growth trends. Although vessel diameter was not related to P50, vessel grouping index and lumen-to-sapwood area ratio showed a weak, though highly significant, positive relationship to P50. We conclude that the xylem anatomy of terminal tree-top branches in European beech is modified in response to increasing climatic aridity and/or decreasing soil water availability, independent of a tree height effect.

Aridity and cold temperatures drive divergent adjustments of European beech xylem anatomy, hydraulics and leaf physiological traits

Understanding plant trait coordination and variance across climatic gradients is critical for assessing forests' adaptive potential to climate change. We measured 11 hydraulic, anatomical and leaf-level physiological traits in European beech (Fagus sylvatica L.) along a moisture and temperature gradient in the French Alps. We assessed how traits covaried, and how their population-level variances shifted along the gradient. The intrapopulation variances of vessel size and xylem-specific conductivity reduced in colder locations as narrow vessels were observed in response to low temperature. This decreased individual-level water transport capacity compared with the warmer and more xeric sites. Conversely, the maximum stomatal conductance and Huber value variances were constrained in the arid and warm locations, where trees showed restricted gas exchange and higher xylem-specific conductivity. The populations growing under drier and warmer conditions presented wide variance for the xylem anatomical and hydraulic traits. Our results suggest that short-term physiological acclimation to raising aridity and heat in southern beech populations may occur mainly at the leaf level. Furthermore, the wide variance of the xylem anatomical and hydraulic traits at these sites may be advantageous since more heterogeneous hydraulic conductivity could imply populations' greater tree-tree complementarity and resilience against climatic variability. Our study highlights that both intrapopulation trait variance and trait network analysis are key approaches for understanding species adaptation and the acclimation potential to a shifting environment.

Tree height effects on vascular anatomy of upper-canopy twigs across a wide range of tropical rainforest species

Vessel diameter variation along the hydraulic pathway determines how much water can be moved against the force of gravity from roots to leaves. While it is well-documented that tree size scales with vessel diameter variation at the stem base due to the effect of basipetal vessel widening, much less is known whether this likewise applies to terminal sun-exposed twigs. To analyze the effect of tree height on twig xylem anatomy, we compiled data for 279 tropical rainforest tree species belonging to 56 families in the lowlands of Jambi Province, Indonesia. Terminal upper-canopy twigs of fully grown individuals were collected and used for wood anatomical analysis.

We show that hydraulically weighted vessel diameter (Dh) and potential hydraulic conductivity (Kp) of upper canopy twigs increase with tree height across species although the relationship was weak. When averaged across given tree height classes irrespectively of species identity, however, a strong dependency of tree height on Dh and Kp was observed, but not on the lumen-to-sapwood area ratio (Al:Ax) or vessel density (VD).

According to the comparison between actual tree height and the maximum tree height reported for a given species in the stand, we show that the vascular xylem anatomy of their terminal twigs reflects their canopy position and thus ecological niche (understory versus overstory) at maturity. We conclude that the capacity to move large quantities of water during the diurnal peak in evaporative demand is a prerequisite for growing tall in a humid tropical environment.

Influence of Juvenile Growth on Xylem Safety and Efficiency in Three Temperate Tree Species

The evolution of the internal water transport system was a prerequisite for high plant productivity. In times of climate change, understanding the dependency of juvenile growth on xylem hydraulic physiology is therefore of high importance. Here, we explored various wood anatomical, hydraulic, and leaf morphological traits related to hydraulic safety and efficiency in three temperate broadleaved tree species (Acer pseudoplatanus, Betula pendula, and Sorbus aucuparia). We took advantage of a severe natural heat wave that resulted in different climatic growing conditions for even-aged plants from the same seed source growing inside a greenhouse and outside. Inside the greenhouse, the daily maximum vapour pressure deficit was on average 36% higher than outside during the growing seasons. Because of the higher atmospheric moisture stress, the biomass production differed up to 5.6-fold between both groups. Except for one species, a high productivity was associated with a high hydraulic efficiency caused by large xylem vessels and a large, supported leaf area. Although no safety-efficiency trade-off was observed, productivity was significantly related to P50 in two of the tree species but without revealing any clear pattern. A considerable plasticity in given traits was observed between both groups, with safety-related traits being more static while efficiency-related traits revealed a higher intra-specific plasticity. This was associated with other wood anatomical and leaf morphological adjustments. We confirm that a high hydraulic efficiency seems to be a prerequisite for a high biomass production, while our controversial results on the growth–xylem safety relationship confirm that safety-efficiency traits are decoupled and that their relationship with juvenile growth and water regime is species-specific.

Lack of hydraulic recovery as a cause of post-drought foliage reduction and canopy decline in European beech

European beech (Fagus sylvatica) was among the most affected tree species during the severe 2018 European drought. It not only suffered from instant physiological stress but also showed severe symptoms of defoliation and canopy decline in the following year. To explore the underlying mechanisms, we used the Swiss-Canopy-Crane II site and studied in branches of healthy and symptomatic trees the repair of hydraulic function and concentration of carbohydrates during the 2018 drought and in 2019. We found loss of hydraulic conductance in 2018, which did not recover in 2019 in trees that developed defoliation symptoms in the year after drought. Reduced branch foliation in symptomatic trees was associated with a gradual decline in wood starch concentration throughout summer 2019. Visualization of water transport in healthy and symptomatic branches in the year after the drought confirmed the close relationship between xylem functionality and supported branch leaf area. Our findings showed that embolized xylem does not regain function in the season following a drought and that sustained branch hydraulic dysfunction is counterbalanced by the reduction in supported leaf area. It suggests acclimation of leaf development after drought to mitigate disturbances in canopy hydraulic function.

Effects of Provenance, Growing Site, and Growth on Quercus robur Wood Anatomy and Density in a 12-Year-Old Provenance Trial

Vessels are responsible for an efficient and safe water transport in angiosperm xylem. Whereas large vessels efficiently conduct the bulk of water, small vessels might be important under drought stress or after winter when large vessels are embolized. Wood anatomy can adjust to the environment by plastic adaptation, but is also modified by genetic selection, which can be driven by climate or other factors. To distinguish between plastic and genetic components on wood anatomy, we used a Quercus robur trial where trees from ten Central European provenances were planted in three locations in Austria along a rainfall gradient. Because wood anatomy also adjusts to tree size and in ring-porous species, the vessel size depends on the amount of latewood and thereby ring width, we included tree size and ring width in the analysis. We found that the trees' provenance had a significant effect on average vessel area (VA), theoretical specific hydraulic conductivity (Ks), and the vessel fraction (VF), but correlations with annual rainfall of provenances were at best weak. The trial site had a strong effect on growth (ring width, RW), which increased from the driest to the wettest site and wood density (WD), which increased from wet to dry sites. Significant site x provenance interactions were seen only for WD. Surprisingly, the drier site had higher VA, higher VF, and higher Ks. This, however, is mainly a result of greater RW and thus a greater proportion of latewood in the wetter forest. The average size of vessels > 70 μm diameter increased with rainfall. We argue that Ks, which is measured per cross-sectional area, is not an ideal parameter to compare the capacity of ring-porous trees to supply leaves with water. Small vessels (<70 μm) on average contributed only 1.4% to Ks, and we found no evidence that their number or size was adaptive to aridity. RW and tree size had strong effect on all vessel parameters, likely via the greater proportion of latewood in wide rings. This should be accounted for when searching for wood anatomical adaptations to the environment.

Relationships Between Wood-Anatomical Features and Resistance Drilling Density in Norway Spruce and European Beech

Environmental conditions affect tree-ring width (TRW), wood structure, and, consequently, wood density, which is one of the main wood quality indicators. Although studies on inter- and intra-annual variability in tree-ring features or density exist, studies demonstrating a clear link between wood structure on a cellular level and its effect on wood density on a macroscopic level are rare. Norway spruce with its simple coniferous structure and European beech, a diffuse-porous angiosperm species were selected to analyze these relationships. Increment cores were collected from both species at four sites in Slovenia. In total, 24 European beech and 17 Norway spruce trees were sampled. In addition, resistance drilling measurements were performed just a few centimeters above the increment core sampling. TRW and quantitative wood anatomy measurements were performed on the collected cores. Resistance drilling density values, tree-ring (TRW, earlywood width-EWW, transition-TWW, and latewood width-LWW) and wood-anatomical features (vessel/tracheid area and diameter, cell density, relative conductive area, and cell wall thickness) were then averaged for the first 7 cm of measurements. We observed significant relationships between tree-ring and wood-anatomical features in both spruce and beech. In spruce, the highest correlation values were found between TRW and LWW. In beech, the highest correlations were observed between TRW and cell density. There were no significant relationships between wood-anatomical features and resistance drilling density in beech. However, in spruce, a significant negative correlation was found between resistance drilling density and tangential tracheid diameter, and a positive correlation between resistance drilling density and both TWW + LWW and LWW. Our findings suggest that resistance drilling measurements can be used to evaluate differences in density within and between species, but they should be improved in resolution to be able to detect changes in wood anatomy.

Adaptive plasticity in plant traits increases time to hydraulic failure under drought in a foundation tree

The viability of forest trees, in response to climate change-associated drought, will depend on their capacity to survive through genetic adaptation and phenotypic plasticity in drought tolerance traits. Genotypes with enhanced plasticity for drought tolerance (adaptive plasticity) will have a greater ability to persist and delay the onset of hydraulic failure. By examining populations from different climate-origins grown under contrasting soil water availability, we tested for genotype (G), environment (E) and genotype-by-environment (G × E) effects on traits that determine the time it takes for saplings to desiccate from stomatal closure to 88% loss of stem hydraulic conductance (time to hydraulic failure, THF). Specifically, we hypothesized that: (i) THF is dependent on a G × E interaction, with longer THF for warm, dry climate populations in response to chronic water deficit treatment compared with cool, wet populations, and (ii) hydraulic and allometric traits explain the observed patterns in THF. Corymbia calophylla saplings from two populations originating from contrasting climates (warm-dry or cool-wet) were grown under well-watered and chronic soil water deficit treatments in large containers. Hydraulic and allometric traits were measured and then saplings were dried-down to critical levels of drought stress to estimate THF. Significant plasticity was detected in the warm-dry population in response to water-deficit, with enhanced drought tolerance compared with the cool-wet population. Projected leaf area and total plant water storage showed treatment variation, and minimum conductance showed significant population differences driving longer THF in trees from warm-dry origins grown in water-limited conditions. Our findings contribute information on intraspecific variation in key drought traits, including hydraulic and allometric determinants of THF. It highlights the need to quantify adaptive capacity in populations of forest trees in climate change-type drought to improve predictions of forest die-back.

Soil water availability and branch age explain variability in xylem safety of European beech in Central Europe

Xylem embolism resistance has been identified as a key trait with a causal relation to drought-induced tree mortality, but not much is known about its intra-specific trait variability (ITV) in dependence on environmental variation. We measured xylem safety and efficiency in 300 European beech (Fagus sylvatica L.) trees across 30 sites in Central Europe, covering a precipitation reduction from 886 to 522 mm year⁻¹. A broad range of variables that might affect embolism resistance in mature trees, including climatic and soil water availability, competition, and branch age, were examined. The average P₅₀ value varied by up to 1 MPa between sites. Neither climatic aridity nor structural variables had a significant influence on P₅₀. However, P₅₀ was less negative for trees with a higher soil water storage capacity, and positively related to branch age, while specific conductivity (K_s) was not significantly associated with either of these variables. The greatest part of the ITV for xylem safety and efficiency was attributed to random variability within populations. We conclude that the influence of site water availability on P₅₀ and K_s is low in European beech, and that the high degree of within-population variability for P₅₀, partly due to variation in branch age, hampers the identification of a clear environmental signal.

Hydraulic Function Analysis of Conifer Xylem Based on a Model Incorporating Tracheids, Bordered Pits, and Cross-Field Pits

Wood has a highly complex and anisotropic structure. Its xylem characteristics are key in determining the hydraulic properties of plants to transport water efficiently and safely, as well as the permeability in the process of wood impregnation modification. Previous studies on the relationship between the xylem structure and hydraulic conductivity of conifer have mainly focused on tracheids and bordered pits, with only a few focusing on the conduction model of cross-field pits which connect tracheids and rays. This study takes the xylem structure of conifer as an example, drawing an analogy between water flow under tension and electric current, and extends the model to the tissue scale, including cross-field pits by establishing isometric scaling. The structure parameters were collected by scanning electron microscopy and transmission electron microscopy. The improved model can quantify the important hydraulic functional characteristics of xylem only by measuring the more easily obtained tracheid section size. Then, this model was applied to quantify the relationship between the xylem anatomical structure and hydraulic properties in the pine (Pinus sylvestris L. var. mongholica Litv.) and the spruce (Picea koraiensis Nakai), and also to evaluate the effects of the number and size of cross-field pits on xylem conduction. The results showed that the growth ring conduction value of the pine was more than twice that of the spruce for the two tree species with similar growth widths in this study. The tracheid wall resistance of the pine reflected the result of the interaction of the size and number of cross-field pits, in comparison, the wall resistance of the spruce was more sensitive to the number of cross-field pits. Finally, the calculation output of the new model was cross-validated with the literature, which verified the accuracy and effectiveness of the model. This study provides an effective and complete solution for xylem conductivity measurement and the study of wood ecophysiological diversity and processing.

Hydraulic variability of three temperate broadleaf tree species along a water availability gradient in central Europe

Plant hydraulic traits are key for understanding and predicting tree drought responses. Information about the degree of the traits' intra-specific variability may guide the selection of drought-resistant genotypes and is crucial for trait-based modelling approaches. For the three temperate minor broadleaf tree species Acer platanoides, Carpinus betulus and Tilia cordata, we measured xylem embolism resistance (P₅₀ ), leaf turgor loss point (P_TLP ), specific hydraulic conductivity (K_S ), Huber values (HVs), and hydraulic safety margins in adult trees across a precipitation gradient. We further quantified trait variability on different organizational levels (inter-specific to within-canopy variation), and analysed its relationship to climatic and soil water availability. Although we observed a certain intra-specific trait variability (ITV) in safety-related traits (P₅₀ , P_TLP ) with higher within-tree and between-tree than between populations variability, the magnitude was small compared to inter-specific differences, which explained 78.4% and 58.3% of the variance in P₅₀ and P_TLP , respectively. In contrast, efficiency-related traits (K_S , HV) showed a high ITV both within populations and within the crowns of single trees. Surprisingly, the observed ITV of all traits was neither driven by climatic nor soil water availability. In conclusion, the high degree of conservatism in safety-related traits highlights their potential for trait-based modelling approaches.

Limited plasticity in embolism resistance in response to light in leaves and stems in species with considerable vulnerability segmentation

Xylem resistance to embolism is a key metric determining plant survival during drought. Yet, we have a limited understanding of the degree of plasticity in vulnerability to embolism. Here, we tested whether light availability influences embolism resistance in leaves and stems. The optical vulnerability method was used to assess stem and leaf resistance to embolism in Phellodendron amurense and Ilex verticillata acclimated to sun and shade microenvironments within the same canopy. In both species, we found considerable segmentation in xylem resistance to embolism between leaves and stems, but only minor acclimation in response to light availability. With the addition of a third species, Betula pubescens, which shows no vulnerability segmentation, we sought to investigate xylem anatomical traits that might correlate with strong vulnerability segmentation. We found a correlation between the area fraction of vessels in the xylem and embolism resistance across species and tissue types. Our results suggest that minimal acclimation of embolism resistance occurs in response to light environment in the same individual and that the degree of vulnerability segmentation between leaves and stems might be determined by the vessel lumen fraction of the xylem.

Different Wood Anatomical and Growth Responses in European Beech (Fagus sylvatica L.) at Three Forest Sites in Slovenia

European beech (Fagus sylvatica L.) adapts to local growing conditions to enhance its performance. In response to variations in climatic conditions, beech trees adjust leaf phenology, cambial phenology, and wood formation patterns, which result in different tree-ring widths (TRWs) and wood anatomy. Chronologies of tree ring width and vessel features [i.e., mean vessel area (MVA), vessel density (VD), and relative conductive area (RCTA)] were produced for the 1960-2016 period for three sites that differ in climatic regimes and spring leaf phenology (two early- and one late-flushing populations). These data were used to investigate long-term relationships between climatic conditions and anatomical features of four quarters of tree-rings at annual and intra-annual scales. In addition, we investigated how TRW and vessel features adjust in response to extreme weather events (i.e., summer drought). We found significant differences in TRW, VD, and RCTA among the selected sites. Precipitation and maximum temperature before and during the growing season were the most important climatic factors affecting TRW and vessel characteristics. We confirmed differences in climate-growth relationships between the selected sites, late flushing beech population at Idrija showing the least pronounced response to climate. MVA was the only vessel trait that showed no relationship with TRW or other vessel features. The relationship between MVA and climatic factors evaluated at intra-annual scale indicated that vessel area in the first quarter of tree-ring were mainly influenced by climatic conditions in the previous growing season, while vessel area in the second to fourth quarters of tree ring width was mainly influenced by maximum temperature and precipitation in the current growing season. When comparing wet and dry years, beech from all sites showed a similar response, with reduced TRW and changes in intra-annual variation in vessel area. Our findings suggest that changes in temperature and precipitation regimes as predicted by most climate change scenarios will affect tree-ring increments and wood structure in beech, yet the response between sites or populations may differ.

Drought elicits contrasting responses on the autumn dynamics of wood formation in late successional deciduous tree species

Research on wood phenology has mainly focused on reactivation of the cambium in spring. In this study we investigated if summer drought advances cessation of wood formation and if it has any influence on wood structure in late successional forest trees of the temperate zone. The end of xylogenesis was monitored between August and November in stands of European beech and pedunculate oak in Belgium for two consecutive years, 2017 and 2018, with the latter year having experienced an exceptional summer drought. Wood formation in oak was affected by the drought, with oak trees ceasing cambial activity and wood maturation about 3 weeks earlier in 2018 compared with 2017. Beech ceased wood formation before oak, but its wood phenology did not differ between years. Furthermore, between the 2 years, no significant difference was found in ring width, percentage of mature fibers in the late season, vessel size and density. In 2018, beech did show thinner fiber walls, whereas oak showed thicker walls. In this paper, we showed that summer drought can have an important impact on late season wood phenology xylem development. This will help to better understand forest ecosystems and improve forest models.

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

Understanding how tree species will respond to a future climate requires reliable and quantitative estimates of intra-specific variation under current climate conditions. We studied three 10-year-old common garden experiments established across a rainfall and drought gradient planted with nearly 10,000 pedunculate oak (Quercus robur L.) trees from ten provenances with known family structure. We aimed at disentangling adaptive and plastic responses for growth (height and diameter at breast height) as well as for leaf and wood functional traits related to adaptation to dry environments. We used restricted maximum likelihood approaches to assess additive genetic variation expressed as narrow-sense heritability (h2), quantitative trait differentiation among provenances (QST), and genotype-by-environment interactions (GxE). We found strong and significant patterns of local adaptation in growth in all three common gardens, suggesting that transfer of seed material should not exceed a climatic distance of approximately 1°C under current climatic conditions, while transfer along precipitation gradients seems to be less stringent. Moreover, heritability reached 0.64 for tree height and 0.67 for dbh at the dry margin of the testing spectrum, suggesting significant additive genetic variation of potential use for future selection and tree breeding. GxE interactions in growth were significant and explained less phenotypic variation than origin of seed source (4% versus 10%). Functional trait variation among provenances was partly related to drought regimes at provenances origins but had moderate explanatory power for growth. We conclude that directional selection, either naturally or through breeding, is the most likely and feasible outcome for pedunculate oak to adapt to warmer and drier climate conditions in the future.

Intraspecific Variations in Functional and Molecular Traits of Near-Endemic Onopordum alexandrinum Boiss. in Natural and Anthropogenic Habitats along the Western Mediterranean Coast of Egypt: Implications for Conservation

Onopordum alexandrinum is a near-endemic to Egypt and neighboring countries. Its habitats are designated as priority sites for conservation in the south and east Mediterranean regions. We hypothesize that variation in morphological, reproductive, and molecular traits could provide a survival strategy that allows the species to occupy landscapes including anthropogenic habitats (barley and abandoned fields) and natural habitats (sand dunes and desert plateau) with different soil resources along the western Mediterranean coast of Egypt. The results indicated that plant functional traits associated with high rates of resource acquisition and growth (e.g., high values of vegetative height and specific leaf area, and low values of leaf dry matter content) occurred in populations located in abandoned fields with high soil resources. The genetic diversity analyses indicated similarity in genetic diversity of the present populations of O. alexandrinum in barley and abandoned fields with those of sand dunes. However, the genetic structures of these populations were different from those of natural desert plateau, which suggests reduced rates of gene flow. In this framework, it is essential to monitor and reduce the anthropogenic activity which will not only support the conservation of genetic diversity within populations but will also help ensure the resilience of O. alexandrinum in the face of environmental and climatic changes.

Microgeographic adaptation and the effect of pollen flow on the adaptive potential of a temperate tree species

In species with long-distance dispersal capacities and inhabiting a large ecological niche, local selection and gene flow are expected to be major evolutionary forces affecting the genetic adaptation of natural populations. Yet, in species such as trees, evidence of microgeographic adaptation and the quantitative assessment of the impact of gene flow on adaptive genetic variation are still limited. Here, we used extensive genetic and phenotypic data from European beech seedlings collected along an elevation gradient, and grown in a common garden, to study the signature of selection on the divergence of eleven potentially adaptive traits, and to assess the role of gene flow in resupplying adaptive genetic variation. We found a significant signal of adaptive differentiation among plots separated by < 1 km, with selection acting on growth and phenological traits. Consistent with theoretical expectations, our results suggest that pollen dispersal contributes to increase genetic diversity for these locally differentiated traits. Our results thus highlight that local selection is an important evolutionary force in natural tree populations and suggest that management interventions to facilitate movement of gametes along short ecological gradients would boost genetic diversity of individual tree populations, and enhance their adaptive potential to rapidly changing environments.

Wood Architecture and Composition Are Deeply Remodeled in Frost Sensitive Eucalyptus Overexpressing CBF/DREB1 Transcription Factors

Eucalypts are the most planted trees worldwide, but most of them are frost sensitive. Overexpressing transcription factors for CRT-repeat binding factors (CBFs) in transgenic Eucalyptus confer cold resistance both in leaves and stems. While wood plays crucial roles in trees and is affected by environmental cues, its potential role in adaptation to cold stress has been neglected. Here, we addressed this question by investigating the changes occurring in wood in response to the overexpression of two CBFs, taking advantage of available transgenic Eucalyptus lines. We performed histological, biochemical, and transcriptomic analyses on xylem samples. CBF ectopic expression led to a reduction of both primary and secondary growth, and triggered changes in xylem architecture with smaller and more frequent vessels and fibers exhibiting reduced lumens. In addition, lignin content and syringyl/guaiacyl (S/G) ratio increased. Consistently, many genes of the phenylpropanoid and lignin branch pathway were upregulated. Most of the features of xylem remodeling induced by CBF overexpression are reminiscent of those observed after long exposure of Eucalyptus trees to chilling temperatures. Altogether, these results suggest that CBF plays a central role in the cross-talk between response to cold and wood formation and that the remodeling of wood is part of the adaptive strategies to face cold stress.

Phenotypic plasticity and genetic adaptation of functional traits influences intra-specific variation in hydraulic efficiency and safety

Understanding which hydraulic traits are under genetic control and/or are phenotypically plastic is essential in understanding how tree species will respond to rapid shifts in climate. We quantified hydraulic traits in Eucalyptus obliqua L'Her. across a precipitation gradient in the field to describe (i) trait variation in relation to long-term climate and (ii) the short-term (seasonal) ability of traits to adjust (i.e., phenotypic plasticity). Seedlings from each field population were raised under controlled conditions to assess (iii) which traits are under strong genetic control. In the field, drier populations had smaller leaves with anatomically thicker xylem vessel walls, a lower leaf hydraulic vulnerability and a lower water potential at turgor loss point, which likely confers higher hydraulic safety. Traits such as the water potential at turgor loss point and ratio of sapwood to leaf area (Huber value) showed significant adjustment from wet to dry conditions in the field, indicating phenotypic plasticity and importantly, the ability to increase hydraulic safety in the short term. In the nursery, seedlings from drier populations had smaller leaves and a lower leaf hydraulic vulnerability, suggesting that key traits associated with hydraulic safety are under strong genetic control. Overall, our study suggests a strong genetic control over traits associated with hydraulic safety, which may compromise the survival of wet-origin populations in drier future climates. However, phenotypic plasticity in physiological and morphological traits may confer sufficient hydraulic safety to facilitate genetic adaptation.

Plant functional traits differ in adaptability and are predicted to be differentially affected by climate change

Climate change is testing the resilience of forests worldwide pushing physiological tolerance to climatic extremes. Plant functional traits have been shown to be adapted to climate and have evolved patterns of trait correlations (similar patterns of distribution) and coordinations (mechanistic trade-off). We predicted that traits would differentiate between populations associated with climatic gradients, suggestive of adaptive variation, and correlated traits would adapt to future climate scenarios in similar ways.We measured genetically determined trait variation and described patterns of correlation for seven traits: photochemical reflectance index (PRI), normalized difference vegetation index (NDVI), leaf size (LS), specific leaf area (SLA), δ13C (integrated water-use efficiency, WUE), nitrogen concentration (NCONC), and wood density (WD). All measures were conducted in an experimental plantation on 960 trees sourced from 12 populations of a key forest canopy species in southwestern Australia.Significant differences were found between populations for all traits. Narrow-sense heritability was significant for five traits (0.15-0.21), indicating that natural selection can drive differentiation; however, SLA (0.08) and PRI (0.11) were not significantly heritable. Generalized additive models predicted trait values across the landscape for current and future climatic conditions (>90% variance). The percent change differed markedly among traits between current and future predictions (differing as little as 1.5% (δ13C) or as much as 30% (PRI)). Some trait correlations were predicted to break down in the future (SLA:NCONC, δ13C:PRI, and NCONC:WD).Synthesis: Our results suggest that traits have contrasting genotypic patterns and will be subjected to different climate selection pressures, which may lower the working optimum for functional traits. Further, traits are independently associated with different climate factors, indicating that some trait correlations may be disrupted in the future. Genetic constraints and trait correlations may limit the ability for functional traits to adapt to climate change.

Towards a New Generation of Trait-Flexible Vegetation Models

Plant trait variability, emerging from eco-evolutionary dynamics that range from alleles to macroecological scales, is one of the most elusive, but possibly most consequential, aspects of biodiversity. Plasticity, epigenetics, and genetic diversity are major determinants of how plants will respond to climate change, yet these processes are rarely represented in current vegetation models. Here, we provide an overview of the challenges associated with understanding the causes and consequences of plant trait variability, and review current developments to include plasticity and evolutionary mechanisms in vegetation models. We also present a roadmap of research priorities to develop a next generation of vegetation models with flexible traits. Including trait variability in vegetation models is necessary to better represent biosphere responses to global change.

The nature and way of root adaptation of juvenile woody plants Sorbus and Pyrus to drought

The functional root traits of Pyrus pyraster (L.) Burgsd. and Sorbus domestica L. during early growth stages were evaluated. The aim of the study was to identify the functional traits of root systems that determine the adaptability of these woody species to drought conditions. The experiment was carried out under the controlled environment of a growth chamber. The root systems were analyzed using WinRhizo software. Several functional root traits were identified, including specific root length, root surface area, root length, root volume, root-to-shoot mass ratio (R:S), fine root (ϕ ˂ 2 mm) volume, coarse root (ϕ > 2 mm) volume, and fine-to-coarse root volume ratio (F/C). In drought, P. pyraster maintained the absorptive root surface unchanged, when increased the volume of the fine root fraction. The different strategy of adaptation to drought has been confirmed for S. domestica, which accumulated more dry mass in the root system in comparison to aboveground organs (significant increase of R:S ratio). The functional root traits analyzed here were species-dependent. The key functional traits that indicate the responses of studied tree taxa to drought conditions include root thickening, F/C, and R:S. Increased values of these parameters indicate the investment of the plant towards root extension. A higher proportion of fine roots increases the absorbing surface of the root system, thereby promoting water uptake from the soil.

Soil moisture regime and palm height influence embolism resistance in oil palm

With the prospect of climate change and more frequent El Niño-related dry spells, the drought tolerance of oil palm (Elaeis guineensis Jacq.), one of the most important tropical crop species, is of major concern. We studied the influence of soil water availability and palm height on the plasticity of xylem anatomy of oil palm fronds and their embolism resistance at well-drained and seasonally flooded riparian sites in lowland Sumatra, Indonesia. We found overall mean P12 and P50 values, i.e., the xylem pressures at 12% or 50% loss of hydraulic conductance, of -1.05 and - 1.86 MPa, respectively, indicating a rather vulnerable frond xylem of oil palm. This matches diurnal courses of stomatal conductance, which in combination with the observed low xylem safety evidence a sensitive water loss regulation. While the xylem anatomical traits vessel diameter (Dh), vessel density and potential hydraulic conductivity (Kp) were not different between the sites, palms in the moister riparian plots had on average by 0.4 MPa higher P50 values than plants in the well-drained plots. This could largely be attributed to differences in palm height between systems. As a consequence, palms of equal height had 1.3 MPa less negative P50 values in the moister riparian plots than in the well-drained plots. While palm height was positively related to P50, Dh and Kp decreased with height. The high plasticity in embolism resistance may be an element of the drought response strategy of oil palm, which, as a monocot, has a relatively deterministic hydraulic architecture. We conclude that oil palm fronds develop a vulnerable water transport system, which may expose the palms to increasing drought stress in a warmer and drier climate. However, the risk of hydraulic failure may be reduced by considerable plasticity in the hydraulic system and the environmental control of embolism resistance, and a presumably large stem capacitance.

Adjustments and coordination of hydraulic, leaf and stem traits along a water availability gradient

Trait variability in space and time allows plants to adjust to changing environmental conditions. However, we know little about how this variability is distributed and coordinated at different organizational levels. For six dominant tree species in northeastern Spain (three Fagaceae and three Pinaceae) we quantified the inter- and intraspecific variability of a set of traits along a water availability gradient. We measured leaf mass per area (LMA), leaf nitrogen (N) concentration, carbon isotope composition in leaves (δ¹³ C), stem wood density, the Huber value (Hv, the ratio of cross-sectional sapwood area to leaf area), sapwood-specific and leaf-specific stem hydraulic conductivity, vulnerability to xylem embolism (P₅₀ ) and the turgor loss point (P_tlp ). Differences between families explained the largest amount of variability for most traits, although intraspecific variability was also relevant. Species occupying wetter sites showed higher N, P₅₀ and P_tlp , and lower LMA, δ¹³ C and Hv. However, when trait relationships with water availability were assessed within species they held only for Hv and P_tlp . Overall, our results indicate that intraspecific adjustments along the water availability gradient relied primarily on changes in resource allocation between sapwood and leaf area and in leaf water relations.

Plant Functional Traits Are the Mediators in Regulating Effects of Abiotic Site Conditions on Aboveground Carbon Stock-Evidence From a 30 ha Tropical Forest Plot

Understanding the relative contribution of abiotic and biotic factors to the formation of ecosystem functioning across scales is vital to evaluate ecosystem services. Here, we elucidate the effects of abiotic site conditions (i.e., soil and topographic properties) and plant functional traits on variations of stand aboveground carbon (AGC) stock in an old-growth tropical montane rain forest. The response-effect framework in functional ecology is adopted in examining how plant functional traits respond to environmental changes and affect ecosystem functioning. We measured specific leaf area and wood density of 270 woody plant species and estimated stand AGC stocks in a 30-ha forest plot. The relationships among environmental factors (ENVIRONMENT), community-weighted means of functional traits (TRAITS) and stand AGC stocks across nested spatial scales were disentangled by structural equation modeling. The results showed that the stands composed of 'acquisitive' species (high specific leaf area and low wood density) had low AGC, whereas stands composed of 'conservative' species (low specific leaf area and high wood density) had high AGC. TRAITS responded to ENVIRONMENT and affected AGC directly. ENVIRONMENT had an indirect effect on AGC through its direct effect on TRAITS. TRAITS were more important than ENVIRONMENT in driving variations of AGC. The effects of TRAITS on AGC increased, while the effects of ENVIRONMENT on AGC decreased with the increase of spatial scales in the tropical montane rain forest. Our study suggests that plant functional traits are the mediators in regulating effects of abiotic site conditions on ecosystem functions.

Dependence of functional traits related to growth rates and their CO2 response on multiple habitat climate factors across Arabidopsis thaliana populations

The values of many plant traits are often different even within a species as a result of local adaptation. Here, we studied how multiple climate variables influence trait values in Arabidopsis thaliana grown under common conditions. We examined 9 climate variables and 29 traits related to vegetative growth rate in 44 global A. thaliana accessions grown at ambient or elevated CO₂ concentration ([CO₂]) and applied a multiple regression analysis. We found that genetic variations in the traits related to growth rates were associated with various climate variables. At ambient [CO₂], plant size was positively correlated with precipitation in the original habitat. This may be a result of larger biomass investment in roots at the initial stage in plants adapting to a lower precipitation. Stomatal conductance and photosynthetic nitrogen use efficiency were negatively correlated with vapor pressure deficit, probably as a result of the trade-off between photosynthetic water- and nitrogen-use efficiency. These results suggest that precipitation and air humidity influence belowground and aboveground traits, respectively. Elevated [CO₂] altered climate dependences in some of the studied traits. The CO₂ response of relative growth rate was negatively correlated with altitude, indicating that plants inhabiting a higher altitude have less plasticity to changing [CO₂]. These results are useful not only for understanding evolutionary process but also to predict the plant species that are favored under future global change.

Assessing inter- and intraspecific variability of xylem vulnerability to embolism in oaks

The genus Quercus comprises important species in forestry not only for their productive value but also for their ability to withstand drought. Hence an evaluation of inter- and intraspecific variation in drought tolerance is important for selecting the best adapted species and provenances for future afforestation. The presence of long vessels makes it difficult to assess xylem vulnerability to embolism in oak. Thanks to the development of an in situ flow centrifuge equipped with a large rotor, we quantified (i) the between species variability of embolism resistance in four native and two exotic species of oaks in Europe and (ii) the within species variability in Quercus petraea. Embolism resistance varied significantly among species, with the pressure inducing 50% loss of hydraulic conductivity (P50 ) ranging between - 7.0 and -4.2 MPa. Species native to the Mediterranean region were more resistant than pan-European species. In contrast, intraspecific variability in embolism resistance in Q. petraea was low within provenances and null among provenances. A positive correlation between P50 and vessel diameter among the six oak species indicates that the more embolism resistant species had narrower xylem vessels. However, this tradeoff between hydraulic efficiency and safety was not observed between Q. petraea provenances.

Do atmospheric CO2 concentration increase, climate and forest management affect iWUE of common beech? Evidences from carbon isotope analyses in tree rings

Beech is one of the most important forest tree species in Europe, hence possible adverse factors affecting its physiology and productivity can have strong ecological and economic impacts. In this context, four beech forests along a latitudinal gradient from southern Apennines to middle European lowlands were selected for chronological determinations of carbon isotope composition (δ13C) in tree-ring cellulose. The main objectives of this study were to assess (i) the effect of climate on the carbon signature of tree-ring cellulose (δ13C); (ii) the physiological response to recent CO2 concentration increment and to climatic variation; and (iii) the relationship between intrinsic water-use efficiency (iWUE, here the average long-term ratio of net photosynthesis to stomatal conductance) and growth of trees in different sites since 1950. Our results demonstrated that site climatic conditions peculiarly affect δ13C. In northern sites, a climatic control of summer precipitation and temperature on stomatal conductance was demonstrated by their opposite correlations with δ13C, negative and positive, respectively. Furthermore, an 'earliness effect' was suggested by a significant relationship between spring temperature and δ13C in the coldest sites and by a positive one between winter temperature and δ13C in the warmest ones. In all the study sites, during the maturity phase, a positive correlation between the increment of CO2 and iWUE was observed, due to an active response of trees to CO2 increment. This increment of CO2 was the main driver of the long term increasing trend of iWUE, resulting by an active response of trees to CO2 fertilization. Moreover, precipitation mostly influences positively and negatively the inter-annual variations of iWUE of the southernmost and northernmost sites, respectively. Overall, we observed a mean increment of 40% of iWUE. Moreover, the sensitivity of iWUE to the increase of CO2 was different between the northernmost and southernmost sites. Increasing iWUE was correlated to growth in the two sites during the release phase and we hypothesize a positive effect of silvicultural treatments.

Phenotypic trait variation measured on European genetic trials of Fagus sylvatica L

We present BeechCOSTe52; a database of European beech (Fagus sylvatica) phenotypic measurements for several traits related to fitness measured in genetic trials planted across Europe. The dataset was compiled and harmonized during the COST-Action E52 (2006–2010), and subsequently cross-validated to ensure consistency of measurement data among trials and provenances. Phenotypic traits (height, diameter at breast height, basal diameter, mortality, phenology of spring bud burst and autumn–leaf discoloration) were recorded in 38 trial sites where 217 provenances covering the entire distribution of European beech were established in two consecutive series (1993/95 and 1996/98). The recorded data refer to 862,095 measurements of the same trees aged from 2 to 15 years old over multiple years. This dataset captures the considerable genetic and phenotypic intra-specific variation present in European beech and should be of interest to researchers from several disciplines including quantitative genetics, ecology, biogeography, macroecology, adaptive management of forests and bioeconomy.

Quantifying in situ phenotypic variability in the hydraulic properties of four tree species across their distribution range in Europe

Many studies have reported that hydraulic properties vary considerably between tree species, but little is known about their intraspecific variation and, therefore, their capacity to adapt to a warmer and drier climate. Here, we quantify phenotypic divergence and clinal variation for embolism resistance, hydraulic conductivity and branch growth, in four tree species, two angiosperms (Betula pendula, Populus tremula) and two conifers (Picea abies, Pinus sylvestris), across their latitudinal distribution in Europe. Growth and hydraulic efficiency varied widely within species and between populations. The variability of embolism resistance was in general weaker than that of growth and hydraulic efficiency, and very low for all species but Populus tremula. In addition, no and weak support for a safety vs. efficiency trade-off was observed for the angiosperm and conifer species, respectively. The limited variability of embolism resistance observed here for all species except Populus tremula, suggests that forest populations will unlikely be able to adapt hydraulically to drier conditions through the evolution of embolism resistance.

Intraspecific variation in embolism resistance and stem anatomy across four sunflower (Helianthus annuus L.) accessions

Drought-induced xylem embolism is a key process closely related to plant mortality during extreme drought events. However, this process has been poorly investigated in crop species to date, despite the observed decline of crop productivity under extreme drought conditions. Interspecific variation in hydraulic traits has frequently been reported, but less is known about intraspecific variation in crops. We assessed the intraspecific variability of embolism resistance in four sunflower (Helianthus annuus L.) accessions grown in well-watered conditions. Vulnerability to embolism was determined by the in situ flow-centrifuge method (cavitron), and possible trade-offs between xylem safety, xylem efficiency and growth were assessed. The relationship between stem anatomy and hydraulic traits was also investigated. Mean P₅₀ was -3 MPa, but significant variation was observed between accessions, with values ranging between -2.67 and -3.22 MPa. Embolism resistance was negatively related to growth and positively related to xylem-specific hydraulic conductivity. There is, therefore, a trade-off between hydraulic safety and growth but not between hydraulic safety and efficiency. Finally, we found that a few anatomical traits, such as vessel density and the area of the vessel lumen relative to that of the secondary xylem, were related to embolism resistance, whereas stem tissue lignification was not. Further investigations are now required to investigate the link between the observed variability of embolism resistance and yield, to facilitate the identification of breeding strategies to improve yields in an increasingly arid world.

Variation in xylem vulnerability to embolism in European beech from geographically marginal populations

Climate change is expected to increase the frequency and intensity of droughts and heatwaves in Europe, leading to effects on forest growth and major forest dieback events due to hydraulic failure caused by xylem embolism. Inter-specific variability in embolism resistance has been studied in detail, but little is known about intra-specific variability, particularly in marginal populations. We evaluated 15 European beech populations, mostly from geographically marginal sites of the species distribution range, focusing particularly on populations from the dry southern margin. We found small, but significant differences in resistance to embolism between populations, with xylem pressures causing 50% loss of hydraulic conductivity ranging from -2.84 to -3.55 MPa. Significant phenotypic clines of increasing embolism resistance with increasing temperature and aridity were observed: the southernmost beech populations growing in a warmer drier climate and with lower habitat suitability have higher resistance to embolism than those from Northern Europe growing more favourable conditions. Previous studies have shown that there is little or no difference in embolism resistance between core populations, but our findings show that marginal populations have developed ways of protecting their xylem based on either evolution or plasticity.

Acclimation of branch and leaf hydraulics in adult Fagus sylvatica and Picea abies in a forest through-fall exclusion experiment

Decreasing water availability due to climate change poses the question of whether and to what extent tree species are able to hydraulically acclimate and how hydraulic traits of stems and leaves are coordinated under drought. In a through-fall exclusion experiment, hydraulic acclimation was analyzed in a mixed forest stand of Fagus sylvatica L. and Picea abies (L.) Karst. In drought-stressed (TE, through-fall exclusion over 2 years) and control (CO) trees, hydraulic vulnerability was studied in branches as well as in leaves (F. sylvatica) and end-twigs (P. abies, entirely formed during the drought period) sampled at the same height in sun-exposed portions of the tree crown. In addition, relevant xylem anatomical traits and leaf pressure-volume relations were analyzed. The TE trees reached pre-dawn water potentials down to -1.6 MPa. In both species, water potentials at 50% loss of xylem hydraulic conductivity were ~0.4 MPa more negative in TE than in CO branches. Foliage hydraulic vulnerability (expressed as water potential at 50% loss of leaf/end-twig hydraulic conductance) and water potential at turgor loss point were also, respectively, 0.4 and 0.5 MPa lower in TE trees. Minor differences were observed in conduit mean hydraulic diameter and cell wall reinforcement. Our findings indicate significant and fast hydraulic acclimation under relatively mild drought in both tree species. Acclimation was well coordinated between branches and foliage, which might be essential for survival and productivity of mature trees under future drought periods.

Aridity drove the evolution of extreme embolism resistance and the radiation of conifer genus Callitris

Xylem vulnerability to embolism is emerging as a major factor in drought-induced tree mortality events across the globe. However, we lack understanding of how and to what extent climate has shaped vascular properties or functions. We investigated the evolution of xylem hydraulic function and diversification patterns in Australia's most successful gymnosperm clade, Callitris, the world's most drought-resistant conifers. For all 23 species in this group, we measured embolism resistance (P₅₀ ), xylem specific hydraulic conductivity (K_s ), wood density, and tracheary element size from natural populations. We investigated whether hydraulic traits variation linked with climate and the diversification of this clade using a time-calibrated phylogeny. Embolism resistance varied widely across the Callitris clade (P₅₀ : -3.8 to -18.8 MPa), and was significantly related to water scarcity, as was tracheid diameter. We found no evidence of a safety-efficiency tradeoff; K_s and wood density were not related to rainfall. Callitris diversification coincides with the onset of aridity in Australia since the early Oligocene. Our results highlight the evolutionary lability of xylem traits with climate, and the leading role of aridity in the diversification of conifers. The uncoupling of safety from other xylem functions allowed Callitris to evolve extreme embolism resistance and diversify into xeric environments.