Tree hydraulic traits are coordinated and strongly linked to climate-of-origin across a rainfall gradient

Climate of origin shapes variations in wood anatomical properties of 17 Picea species

BACKGROUND

Variations in hydraulic conductivity may arise from species-specific differences in the anatomical structure and function of the xylem, reflecting a spectrum of plant strategies along a slow-fast resource economy continuum. Spruce (Picea spp.), a widely distributed and highly adaptable tree species, is crucial in preventing soil erosion and enabling climate regulation. However, a comprehensive understanding of the variability in anatomical traits of stems and their underlying drivers in the Picea genus is currently lacking especially in a common garden.

RESULTS

We assessed 19 stem economic properties and hydraulic characteristics of 17 Picea species grown in a common garden in Tianshui, Gansu Province, China. Significant interspecific differences in growth and anatomical characteristics were observed among the species. Specifically, xylem hydraulic conductivity (Ks) and hydraulic diameter exhibited a significant negative correlation with the thickness to span ratio (TSR), cell wall ratio, and tracheid density and a significant positive correlation with fiber length, and size of the radial tracheid. PCA revealed that the first two axes accounted for 64.40% of the variance, with PC1 reflecting the trade-off between hydraulic efficiency and mechanical support and PC2 representing the trade-off between high embolism resistance and strong pit flexibility. Regression analysis and structural equation modelling further confirmed that tracheid size positively influenced Ks, whereas the traits DWT, D_r, and TSR have influenced Ks indirectly. All traits failed to show significant phylogenetic associations. Pearson's correlation analysis demonstrated strong correlations between most traits and longitude, with the notable influence of the mean temperature during the driest quarter, annual precipitation, precipitation during the wettest quarter, and aridity index.

CONCLUSIONS

Our results showed that xylem anatomical traits demonstrated considerable variability across phylogenies, consistent with the pattern of parallel sympatric radiation evolution and global diversity in spruce. By integrating the anatomical structure of the stem xylem as well as environmental factors of origin and evolutionary relationships, our findings provide novel insights into the ecological adaptations of the Picea genus.

Coordinated hydraulic traits influence the two phases of time to hydraulic failure in five temperate tree species differing in stomatal stringency

Worldwide, forests are increasingly exposed to extreme droughts causing tree mortality. Because of the complex nature of the mechanisms involved, various traits have been linked to tree drought responses with contrasting results. This may be due to species-specific strategies in regulating water potential, a process that unfolds in two distinct phases: a first phase until stomatal closure, and a second phase until reaching lethal xylem hydraulic thresholds. We conducted dry-down experiments with five broadleaved temperate tree species differing in their degree of isohydry to estimate the time to stomatal closure (tsc) and subsequent time to critical hydraulic failure (tcrit). We measured various traits linked to tree drought responses, such as the water potentials at turgor loss point (Ptlp), stomatal closure (Pgs90), and 12%, 50% and 88% loss of xylem hydraulic conductance (P12, P50, P88), hydraulic capacitance (C), minimum leaf conductance (gmin), hydroscape area (HSA) and hydraulic safety margins (HSM). We found that Pgs90 followed previously recorded patterns of isohydry and was associated with HSA. Species ranked from more to less isohydric in the sequence Acer pseudoplatanus < Betula pendula < Tilia cordata < Sorbus aucuparia < Fagus sylvatica. Their degree of isohydry was associated with leaf safety (Ptlp and gmin), drought avoidance (C) and tsc, but decoupled from xylem safety (HSM and P88) and tcrit. Regardless of their stomatal stringency, species with wider HSM and lower P88 reached critical hydraulic failure later. We conclude that the duration of the first phase is determined by stomatal regulation, while the duration of the second phase is associated with xylem safety. Isohydry is thus linked to water use rather than to drought survival strategies, confirming the proposed use of HSA as a complement to HSM for describing plant drought responses before and after stomatal closure.

Plasticity and implications of water-use traits in contrasting tropical tree species under climate change

Plants face a trade-off between hydraulic safety and growth, leading to a range of water-use strategies in different species. However, little is known about such strategies in tropical trees and whether different water-use traits can acclimate to warming. We studied five water-use traits in 20 tropical tree species grown at three different altitudes in Rwanda (RwandaTREE): stomatal conductance (gs), leaf minimum conductance (gmin), plant hydraulic conductance (Kplant), leaf osmotic potential (ψo) and net defoliation during drought. We also explored the links between these traits and growth and mortality data. Late successional (LS) species had low Kplant, gs and gmin and, thus, low water loss, while low ψo helped improve leaf water status during drought. Early successional (ES) species, on the contrary, used more water during both moist and dry conditions and exhibited pronounced drought defoliation. The ES strategy was associated with lower mortality and more pronounced growth enhancement at the warmer sites compared to LS species. While Kplant and gmin showed downward acclimation in warmer climates, ψo did not acclimate and gs measured at prevailing temperature did not change. Due to distinctly different water use strategies between successional groups, ES species may be better equipped for a warmer climate as long as defoliation can bridge drought periods.

Herb hydraulics: Variation and correlation for traits governing drought tolerance and efficiency of water transport

Hydraulic traits dictate plant response to drought, thus enabling better understanding of community dynamics under global climate change. Despite being intensively documented in woody species, herbaceous species (graminoids and forbs) are largely understudied, hence the distribution and correlation of hydraulic traits in herbaceous species remains unclear. Here, we collected key hydraulic traits for 436 herbaceous species from published literature, including leaf hydraulic conductivity (Kleaf), water potential inducing 50 % loss of hydraulic conductivity (P50), stomatal closure (Pclose) and turgor loss (Ptlp). Trait variation of herbs was analyzed and contrasted with angiosperm woody species within the existing global hydraulic traits database, as well as between different growth forms within herbs. Furthermore, hydraulic traits coordination was also assessed for herbaceous species. We found that herbs showed overall more negative Pclose but less negative Ptlp compared with angiosperm woody species, while P50 did not differ between functional types, regardless of the organ (leaf and stem). In addition, correlations were found between Kleaf and P50 of leaf (P50leaf), as well as between Pclose, P50leaf and Kleaf. Within herbs, graminoids generally exhibited more negative P50 and Ptlp, but lower Kleaf, relative to forbs. Within herbs, no clear pattern regarding hydraulic traits-climate relationship was found. Our analysis provided insights into herb hydraulic, and highlighted the knowledge gaps need to be filled regarding the response of herbs to drought.

Petiole XLA (xylem to leaf area ratio) integrates hydraulic safety and efficiency across a diverse group of eucalypt leaves

A theoretical trade-off between the efficiency and safety of water transport systems in plants is used to explain diverse ecological patterns, from tree size to community structure. Despite its pervasive influence, this theory has marginal empirical support. This may be partially due to obfuscation of associations by wide phylogenetic sampling or non-standard sampling between studies. To address this, we examine the coordination of structural and anatomical traits linked to hydraulic safety and efficiency in the leaves of an ecologically diverse group of eucalypts. We introduce a new trait for characterising leaf water transport function measured as the cross-sectional XA at the petiole divided by the downstream leaf area (XLApetiole ). Variation in XLApetiole revealed support for a safety-efficiency trade-off in eucalypt leaves. XLApetiole was negatively correlated with theoretical petiole xylem conductivity (Ks_petiole ) and strongly negatively correlated with leaf cavitation vulnerability (Ψ50leaf ). Species with lower Ψ50leaf exhibited petiole xylem with narrower vessels and greater fibre wall area fractions. Our findings highlight XLApetiole as a novel integrative trait that provides insights into the evolution of leaf form and function in eucalypts and holds promise for wider use among diverse species.

Intraspecific variations of leaf hydraulic, economic, and anatomical traits in Cinnamomum camphora along an urban-rural gradient

Urbanization brings numerous benefits to residents, but it also introduces complex, variable, and heterogeneous habitat conditions to urban plants, resulting in an arid and hot urban environment that decreases tree growth and the ecological service capacity of trees. In this study, we evaluated leaf hydraulic, economic, and anatomical traits and their covariations of Cinnamomum camphora along an urban-rural gradient in Hefei, Eastern China. We found that Cinnamomum camphora in urban adopted a conservative hydraulic strategy with low leaf turgor loss point (Tlp), leaf hydraulic conductance (Kleaf), and leaf water potential resulting in 50 % loss of hydraulic conductance (P50), as well as a quick investment-return economic strategy with low unit leaf dry matter content (LMA) and high leaf nitrogen content (Leaf N). P50, Kleaf and LMA were significantly positively correlated with the urban-rural gradient (PC1urban-rural gradient), while Leaf N exhibited a negative correlation with it. The results showed a trade-off between intraspecific safety and efficiency in leaf hydraulic traits along the urban-rural gradient and an intraspecific coordinated variation in leaf hydraulic and economic traits. In addition, based on the analysis of a trait coordination network, it was revealed that leaf mesophyll and stomata were key structures for trait adjustment and coordination. Furthermore, our findings offer a significant theoretical underpinning for the effective management of landscape trees and the strategic planning of urban tree species.

Leaf relative water content at 50% stomatal conductance measured by noninvasive NMR is linked to climate of origin in nine species of eucalypt

Stomata are the gatekeepers of plant water use and must quickly respond to changes in plant water status to ensure plant survival under fluctuating environmental conditions. The mechanism for their closure is highly sensitive to disturbances in leaf water status, which makes isolating their response to declining water content difficult to characterise and to compare responses among species. Using a small-scale non-destructive nuclear magnetic resonance spectrometer as a leaf water content sensor, we measure the stomatal response to rapid induction of water deficit in the leaves of nine species of eucalypt from contrasting climates. We found a strong linear correlation between relative water content at 50% stomatal conductance (RWCgs50 ) and mean annual temperature at the climate of origin of each species. We also show evidence for stomata to maintain control over water loss well below turgor loss point in species adapted to warmer climates and secondary increases in stomatal conductance despite declining water content. We propose that RWCgs50 is a promising trait to guide future investigations comparing stomatal responses to water deficit. It may provide a useful phenotyping trait to delineate tolerance and adaption to hot temperatures and high leaf-to-air vapour pressure deficits.

Drought survival and recovery in grasses: Stress intensity and plant-plant interactions impact plant dehydration tolerance

Plant dehydration tolerance confers drought survival in grasses, but the mortality thresholds according to soil water content (SWC), vapour pressure deficit (VPD) and plant-plant interactions are little explored. We compared the dehydration dynamics of leaf meristems, which are the key surviving organs, plant mortality, and recovery of Mediterranean and temperate populations of two perennial grass species, Dactylis glomerata and Festuca arundinacea, grown in monocultures and mixtures under a low-VPD (1.5 kPa) versus a high-VPD drought (2.2 kPa). The lethal drought index (LD₅₀ ), that is, SWC associated with 50% plant mortality, ranged from 2.87% (ψs = -1.68 MPa) to 2.19% (ψs = -4.47 MPa) and reached the lowest values under the low-VPD drought. Populations of D. glomerata were more dehydration-tolerant (lower LD₅₀ ), survived and recovered better than F. arundinacea populations. Plant-plant interactions modified dehydration tolerance and improved post-drought recovery in mixtures compared with monocultures. Water content as low as 20.7%-36.1% in leaf meristems allowed 50% of plants to survive. We conclude that meristem dehydration causes plant mortality and that drought acclimation can increase dehydration tolerance. Genetic diversity, acclimation and plant-plant interactions are essential sources of dehydration tolerance variability to consider when predicting drought-induced mortality.

Aridity-dependent sequence of water potentials for stomatal closure and hydraulic dysfunctions in woody plants

The sequence of physiological events during drought strongly impacts plants' overall performance. Here, we synthesized the global data of stomatal and hydraulic traits in leaves and stems of 202 woody species to evaluate variations in the water potentials for key physiological events and their sequence along the climatic gradient. We found that the seasonal minimum water potential, turgor loss point, stomatal closure point, and leaf and stem xylem vulnerability to embolism were intercorrelated and decreased with aridity, indicating that water stress drives trait co-selection. In xeric regions, the seasonal minimum water potential occurred at lower water potential than turgor loss point, and the subsequent stomatal closure delayed embolism formation. In mesic regions, however, the seasonal minimum water potential did not pose a threat to the physiological functions, and stomatal closure occurred even at slightly more negative water potential than embolism. Our study demonstrates that the sequence of water potentials for physiological dysfunctions of woody plants varies with aridity, that is, xeric species adopt a more conservative sequence to prevent severe tissue damage through tighter stomatal regulation (isohydric strategy) and higher embolism resistance, while mesic species adopt a riskier sequence via looser stomatal regulation (anisohydric strategy) to maximize carbon uptake at the cost of hydraulic safety. Integrating both aridity-dependent sequence of water potentials for physiological dysfunctions and gap between these key traits into the hydraulic framework of process-based vegetation models would improve the prediction of woody plants' responses to drought under global climate change.

From comfort zone to mortality: Sequence of physiological stress thresholds in Robinia pseudoacacia seedlings during progressive drought

INTRODUCTION

Parameterizing the process of trees from the comfort zone to mortality during progressive drought is important for, but is not well represented in, vegetation models, given the lack of appropriate indices to gauge the response of trees to droughts. The objective of this study was to determine reliable and readily available tree drought stressindices and the thresholds at which droughts activate important physiological responses.

METHODS

We analyzed the changes in the transpiration (T), stomatal conductance, xylem conductance, and leaf health status due to a decrease in soil water availability (SWA), predawn xylem water potential (ψ_pd), and midday xylem water potential (ψ_md) in Robinia pseudoacacia seedlings during progressive drought.

RESULTS

The results showed that ψ_md was a better indicator of drought stress than SWA and ψ_pd, because ψ_md was more closely related to the physiological response (defoliation and xylem embolization) during severe drought and could be measured more conveniently. We derived the following five stress levels from the observed responses to decreasing ψ_md: comfort zone (ψ_md > -0.9 MPa), wherein transpiration and stomatal conductance are not limited by SWA; moderate drought stress (-0.9 to -1.75 MPa), wherein transpiration and stomatal conductance are limited by drought; high drought stress (-1.75 to -2.59 MPa), wherein transpiration decreases significantly (T< 10%) and stomata closes completely; severe drought stress (-2.59 to -4.02 MPa), wherein transpiration ceases (T< 0.1%) and leaf shedding orwilting is > 50%; and extreme drought stress (< -4.02 MPa), leading to tree mortality due to xylem hydraulic failure.

DISCUSSION

To our knowledge, our scheme is the first to outline the quantitative thresholds for the downregulation of physiological processes in R. pseudoacacia during drought, therefore, can be used to synthesize valuable information for process-based vegetation models.

Stem hydraulic conductivity and embolism resistance of Quercus species are associated with their climatic niche

The hydraulic traits of a plant species may reflect its climate adaptations. Southwest China is considered as a biodiversity hotpot of the genus Quercus (oak). However, the hydraulic adaptations of Asian oaks to their climate niches remain unclear. Ten common garden-grown oak species with distinct natural distributions in eastern Asia were used to determine their stem xylem embolism resistance (water potential at 50% loss of hydraulic conductivity, P50), stem hydraulic efficiency (vessel anatomy and sapwood specific hydraulic conductivity (Ks)) and leaf anatomical traits. We also compiled four key functional traits: wood density, hydraulic-weighted vessel diameter, Ks and P50 data for 31 oak species from previous literature. We analyzed the relationship between hydraulic traits and climatic factors over the native ranges of 41 oak species. Our results revealed that the 10 Asian oak species, which are mainly distributed in humid subtropical habitats, possessed a stem xylem with low embolism resistance and moderate hydraulic efficiency. The deciduous and evergreen species of the 10 Asian oaks differed in the stem and leaf traits related to hydraulic efficiency. Ks differed significantly between the two phenological groups (deciduous and evergreens) in the 41-oak dataset. No significant difference in P50 between the two groups was found for the 10 Asian oaks or the 41-oak dataset. The oak species that can distribute in arid habitats possessed a stem xylem with high embolism resistance. Ks negatively related to the humidity of the native range of the 10 Asian oaks, but showed no trend when assessing the entire global oak dataset. Our study suggests that stem hydraulic conductivity and embolism resistance in Quercus species are shaped by their climate niche. Our findings assist predictions of oak drought resistance with future climate changes for oak forest management.

Local diversity of drought resistance and resilience in Populus tomentosa correlates with the variation of DNA methylation

Little information is known about DNA methylation variation in shaping environment-specific drought resistance and resilience for tree adaptation. In this study, we leveraged RNA sequencing and whole-genome bisulfite sequencing data to dissect the distinction of epigenetic regulation under drought stress and rewater condition of Populus tomentosa accessions from three geographical regions. We demonstrated low resistance and high resilience for accessions from South. Non-CG methylation levels in promoter regions of Southern accessions were lower than accessions from higher latitudes and negatively regulated gene expression. CHH context methylation was more sensitive to drought stress, and the geographical-specific differentially methylated regions were scarcely changed by environmental fluctuation. We identified 60 conserved hub genes within the co-expression networks that correlate with photosynthetic and stomatal morphological traits. Epigenome-wide association studies and genome-wide association studies of these 60 hub genes revealed the interdependency between genetic and epigenetic variation in GATA9 and LECRK-VIII.2, which was associated with stomatal morphology and chlorophyll content. The natural epigenetic variation in GATA9 was also faithfully transmitted to progenies in two family-based F1 populations. This study indicates a functional relationship of DNA methylation diversity with drought resistance and resilience which offers new insights into plants' local adaptation to a stressful environment.

At least it is a dry cold: the global distribution of freeze-thaw and drought stress and the traits that may impart poly-tolerance in conifers

Conifers inhabit some of the most challenging landscapes where multiple abiotic stressors (e.g., aridity, freezing temperatures) often co-occur. Physiological tolerance to multiple stressors ('poly-tolerance') is thought to be rare because exposure to one stress generally limits responses to another through functional trade-offs. However, the capacity to exhibit poly-tolerance may be greater when combined abiotic stressors have similar physiological impacts, such as the disruption of hydraulic function imposed by drought or freezing. Here, we reviewed empirical data in light of theoretical expectations for conifer adaptations to drought and freeze-thaw cycles with particular attention to hydraulic traits of the stem and leaf. Additionally, we examined the commonality and spatial distribution of poly-stress along indices of these combined stressors. We found that locations with the highest values of our poly-stress index (PSi) are characterized by moderate drought and moderate freeze-thaw, and most of the global conifer distribution occupies areas of moderate poly-stress. Among traits examined, we found diverse responses to the stressors. Turgor loss point did not correlate with freeze-thaw or drought stress individually, but did with the PSi, albeit inverse to what was hypothesized. Leaf mass per area was more strongly linked with drought stress than the poly-stress and not at all with freeze-thaw stress. In stems, the water potential causing 50% loss of hydraulic conductivity became more negative with increasing drought stress and poly-stress but did not correlate with freeze-thaw stress. For these traits, we identified a striking lack of coverage for substantial portions of species ranges, particularly at the upper boundaries of their respective PSis, demonstrating a critical gap in our understanding of trait prevalence and plasticity along these stress gradients. Future research should investigate traits that confer tolerance to both freeze-thaw and drought stress in a wide range of species across broad geographic scales.

Contributions of phenotypic integration, plasticity and genetic adaptation to adaptive capacity relating to drought in Banksia marginata (Proteaceae)

The frequency and intensity of drought events are predicted to increase because of climate change, threatening biodiversity and terrestrial ecosystems in many parts of the world. Drought has already led to declines in functionally important tree species, which are documented in dieback events, shifts in species distributions, local extinctions, and compromised ecosystem function. Understanding whether tree species possess the capacity to adapt to future drought conditions is a major conservation challenge. In this study, we assess the capacity of a functionally important plant species from south-eastern Australia (Banksia marginata, Proteaceae) to adapt to water-limited environments. A water-manipulated common garden experiment was used to test for phenotypic plasticity and genetic adaptation in seedlings sourced from seven provenances of contrasting climate-origins (wet and dry). We found evidence of local adaptation relating to plant growth investment strategies with populations from drier climate-origins showing greater growth in well-watered conditions. The results also revealed that environment drives variation in physiological (stomatal conductance, predawn and midday water potential) and structural traits (wood density, leaf dry matter content). Finally, these results indicate that traits are coordinated to optimize conservation of water under water-limited conditions and that trait coordination (phenotypic integration) does not constrain phenotypic plasticity. Overall, this study provides evidence for adaptive capacity relating to drought conditions in B. marginata, and a basis for predicting the response to climate change in this functionally important plant species.

Evidence for phylogenetic signal and correlated evolution in plant-water relation traits

Evolutionary relationships are likely to play a significant role in shaping plant physiological and structural traits observed in contemporary taxa. We review research on phylogenetic signal and correlated evolution in plant-water relation traits, which play important roles in allowing plants to acquire, use, and conserve water. We found more evidence for a phylogenetic signal in structural traits (e.g. stomatal length and stomatal density) than in physiological traits (e.g. stomatal conductance and water potential at turgor loss). Although water potential at turgor loss is the most-studied plant-water relation trait in an evolutionary context, it is the only trait consistently found to not have a phylogenetic signal. Correlated evolution was common among traits related to water movement efficiency and hydraulic safety in both leaves and stems. We conclude that evidence for phylogenetic signal varies depending on: the methodology used for its determination, that is, model-based approaches to determine phylogenetic signal such as Blomberg's K or Pagel's λ vs statistical approaches such as ANOVAs with taxonomic classification as a factor; on the number of taxa studied (size of the phylogeny); and the setting in which plants grow (field vs common garden). More explicitly and consistently considering the role of evolutionary relationships in shaping plant ecophysiology could improve our understanding of how traits compare among species, how traits are coordinated with one another, and how traits vary with the environment.

Leaf habit affects the distribution of drought sensitivity but not water transport efficiency in the tropics

Considering the global intensification of aridity in tropical biomes due to climate change, we need to understand what shapes the distribution of drought sensitivity in tropical plants. We conducted a pantropical data synthesis representing 1117 species to test whether xylem-specific hydraulic conductivity (K_S ), water potential at leaf turgor loss (Ψ_TLP ) and water potential at 50% loss of K_S (Ψ_P50 ) varied along climate gradients. The Ψ_TLP and Ψ_P50 increased with climatic moisture only for evergreen species, but K_S did not. Species with high Ψ_TLP and Ψ_P50 values were associated with both dry and wet environments. However, drought-deciduous species showed high Ψ_TLP and Ψ_P50 values regardless of water availability, whereas evergreen species only in wet environments. All three traits showed a weak phylogenetic signal and a short half-life. These results suggest strong environmental controls on trait variance, which in turn is modulated by leaf habit along climatic moisture gradients in the tropics.

Conduit position and connectivity affect the likelihood of xylem embolism during natural drought in evergreen woodland species

BACKGROUND AND AIMS

Hydraulic failure is considered a main cause of drought-induced forest mortality. Yet, we have a limited understanding of how the varying intensities and long time scales of natural droughts induce and propagate embolism within the xylem.

METHODS

X-ray computed tomography (microCT) images were obtained from different aged branch xylem to study the number, size and spatial distribution of in situ embolized conduits among three dominant tree species growing in a woodland community.

KEY RESULTS

Among the three studied tree species, those with a higher xylem vulnerability to embolism (higher water potential at 50 % loss of hydraulic conductance; P50) were more embolized than species with lower P50. Within individual stems, the probability of embolism was independent of conduit diameter but associated with conduit position. Rather than the occurrence of random or radial embolism, we observed circumferential clustering of high and low embolism density, suggesting that embolism spreads preferentially among conduits of the same age. Older xylem also appeared more likely to accumulate embolisms than young xylem, but there was no pattern suggesting that branch tips were more vulnerable to cavitation than basal regions.

CONCLUSIONS

The spatial analysis of embolism occurrence in field-grown trees suggests that embolism under natural drought probably propagates by air spreading from embolized into neighbouring conduits in a circumferential pattern. This pattern offers the possibility to understand the temporal aspects of embolism occurrence by examining stem cross-sections.

Exploring leaf hydraulic traits to predict drought tolerance of Eucalyptus clones

Ongoing changes in climate, and the consequent mortality of natural and cultivated forests across the globe, highlight the urgent need to understand the plant traits associated with greater tolerance to drought. Here, we aimed at assessing key foliar traits, with a focus on the hydraulic component, that could confer a differential ability to tolerate drought in three commercial hybrids of the most important Eucalyptus species utilized in tropical silviculture: E. urophyla, E. grandis and E. camaldulensis. All genotypes exhibited similar water potential when the 90% stomatal closure (Ψgs90) occurs with Ψgs90 always preceding the start of embolism events. The drought-tolerant hybrid showed a higher leaf resistance to embolism, but the leaf hydraulic efficiency was similar among all genotypes. Other traits presented by the drought-tolerant hybrid were a higher cell wall reinforcement, lower value of osmotic potential at full turgor and greater bulk modulus of elasticity. We also identified that the leaf capacitance after the turgor loss, the ratio between cell wall thickness (t) and lumen breadth (b) ratio (t/b)3, and the minimal conductance might be good proxies for screening drought-tolerant Eucalyptus genotypes. Our findings suggest that xylem resistance to embolism can be an important component of drought tolerance in Eucalyptus in addition to other traits aimed at delaying the development of high tensions in the xylem. Highlight Drought tolerance in tropical Eucalyptus hybrids encompasses a high leaf resistance to embolism and a suite of traits aimed at delaying the development of high tensions in the xylem.

Examining physiological, water relations, and hydraulic vulnerability traits to determine anisohydric and isohydric behavior in almond (Prunus dulcis) cultivars: Implications for selecting agronomic cultivars under changing climate

The search for drought tolerant species or cultivars is important to address water scarcity caused by climate change in Mediterranean regions. The anisohydric-isohydric behavior concept has been widely used to describe stomatal regulation during drought, simply in terms of variation of minimal water potential (Ψ_min) in relation to pre-dawn water potential (Ψ_pd). However, its simplicity has sometimes failed to deliver consistent results in describing a complex behavior that results from the coordination of several plant functional traits. While Prunus dulcis (almond) is known as a drought tolerant species, little information is available regarding consistent metrics to discriminate among cultivars or the mechanisms underlying drought tolerance in almond. Here we show a sequence of plant stomatal, hydraulic, and wilting responses to drought in almonds, and the main differences between anisohydric and isohydric cultivars. In a pot desiccation experiment we observed that stomatal closure in P. dulcis is not driven by loss in turgor or onset of xylem cavitation, but instead, occurs early in response to decreasing Ψ_min that could be related to the protection of the integrity of the hydraulic system, independently of cultivar. Also, we report that anisohydric cultivars of P. dulcis are characterized by maximum stomatal conductance, lower water potentials for stomatal closure and turgor loss, and lower vulnerability to xylem cavitation, which are traits that correlated with metrics to discriminate anisohydric and isohydric behavior. Our results demonstrate that P. dulcis presents a strategy to avoid cavitation by closing stomata during the early stages of drought. Future research should also focus on below-ground hydraulic traits, which could trigger stomatal closure in almond.

Limited hydraulic recovery in seedlings of six tree species with contrasting leaf habits in subtropical China

Subtropical tree species may experience severe drought stress due to variable rainfall under future climates. However, the capacity to restore hydraulic function post-drought might differ among co-occurring species with contrasting leaf habits (e.g., evergreen and deciduous) and have implications for future forest composition. Moreover, the links between hydraulic recovery and physiological and morphological traits related to water-carbon availability are still not well understood. Here, potted seedlings of six tree species (four evergreen and two deciduous) were grown outdoors under a rainout shelter. They grew under favorable water conditions until they were experimentally subjected to a soil water deficit leading to losses of ca. 50% of hydraulic conductivity, and then soils were re-watered to field capacity. Traits related to carbon and water relations were measured. There were differences in drought responses and recovery between species, but not as a function of evergreen or deciduous groups. Sapindus mukorossi exhibited the most rapid drought response, which was associated with a suite of physiological and morphological traits (larger plant size, the lowest hydraulic capacitance (C _branch), higher minimum conductance (g _min) and lower HV (Huber value)). Upon re-watering, xylem water potential exhibited fast recovery in 1-3 days among species, while photosynthesis at saturating light (A _sat) and stomatal conductance (g _s) recovery lagged behind water potential recovery depending on species, with g _s recovery being more delayed than A _sat in most species. Furthermore, none of the six species exhibited significant hydraulic recovery during the 7 days re-watering period, indicating that xylem refilling was apparently limited; in addition, NSC availability had a minimal role in facilitating hydraulic recovery during this short-term period. Collectively, if water supply is limited by insignificant hydraulic recovery post-drought, the observed carbon assimilation recovery of seedlings may not be sustained over the longer term, potentially altering seedling regeneration and shifting forest species composition in subtropical China under climate change.

Structural and functional leaf diversity lead to variability in photosynthetic capacity across a range of Juglans regia genotypes

Similar to other cropping systems, few walnut cultivars are used as scion in commercial production. Germplasm collections can be used to diversify cultivar options and hold potential for improving crop productivity, disease resistance and stress tolerance. In this study, we explored the anatomical and biochemical bases of photosynthetic capacity and response to water stress in 11 Juglans regia accessions in the U.S. department of agriculture, agricultural research service (USDA-ARS) National Clonal Germplasm. Net assimilation rate (A_n ) differed significantly among accessions and was greater in lower latitudes coincident with higher stomatal and mesophyll conductances, leaf thickness, mesophyll porosity, gas-phase diffusion, leaf nitrogen and lower leaf mass and stomatal density. High CO₂ -saturated assimilation rates led to increases in A_n under diffusional and biochemical limitations. Greater A_n was found in lower-latitude accessions native to climates with more frost-free days, greater precipitation seasonality and lower temperature seasonality. As expected, water stress consistently impaired photosynthesis with the highest % reductions in lower-latitude accessions (A3, A5 and A9), which had the highest A_n under well-watered conditions. However, A_n for A3 and A5 remained among the highest under dehydration. J. regia accessions, which have leaf structural traits and biochemistry that enhance photosynthesis, could be used as commercial scions or breeding parents to enhance productivity.

Towards species-level forecasts of drought-induced tree mortality risk

Predicting species-level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections. We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South-Eastern Australia (mean annual precipitation range: 344-1424 mm yr^-1 ). We conducted three experiments: applying CABLE to the 2017-2019 drought; a 20% drier drought; and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO₂ ). The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within-species' ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO₂ in ameliorating drought stress was consistent across species. Our results represent an important advance in our capacity to forecast the resilience of individual tree species, providing an evidence base for decision-making around the resilience of restoration plantings or net-zero emission strategies.

Lack of phenotypic plasticity in leaf hydraulics for 10 woody species common to urban forests of North China

The survival and performance of urban forests are increasingly challenged by urban drought, consequently compromising the sustainability and functionality of urban vegetation. Plant-water relations largely determine species drought tolerance, yet little is known about the hydraulics of urban forest species. Here, we report the leaf hydraulic and carbon traits that govern plant growth and drought resistance, including vulnerability to embolism, hydraulic conductivity and leaf gas exchange characteristics, as well as morphological traits that are potentially linked with these physiological attributes, with the aim of guiding species selection and management in urban forests. Plant materials were collected from mature shrubs and trees on our university campus in Beijing, representing 10 woody species common to urban forests in north China. We found that the leaf embolism resistance, represented by the water potential inducing 50% loss of hydraulic conductivity (P50), as well as the hydraulic safety margin (HSM) defined by P50 and the water potential threshold at the inception of embolism (P12), varied remarkably across species, but was unrelated to growth form. Likewise, stem and leaf-specific hydraulic conductivity (Kstem and kl) was also highly species-specific. Leaf P50 was positively correlated with hydraulic conductivity. However, neither P50 nor hydraulic conductivity was correlated with leaf gas exchange traits, including maximum photosynthetic rate (Amax) and stomatal conductance (gs). Plant morphological and physiological traits were not related, except for specific leaf area, which showed a negative relationship with HSM. Traits influencing plant-water transport were primarily correlated with the mean annual precipitation of species climatic niche. Overall, current common woody species in urban forest environments differed widely in their drought resistance and did not have the capacity to modify these characteristics in response to a changing climate. Species morphology provides limited information regarding physiological drought resistance. Thus, screening urban forest species based on plant physiology is essential to sustain the ecological services of urban forests.

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.

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.

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models.

Canopy damage during a natural drought depends on species identity, physiology and stand composition

Vulnerability to xylem cavitation is a strong predictor of drought-induced damage in forest communities. However, biotic features of the community itself can influence water availability at the individual tree-level, thereby modifying patterns of drought damage. Using an experimental forest in Tasmania, Australia, we determined the vulnerability to cavitation (leaf P₅₀ ) of four tree species and assessed the drought-induced canopy damage of 2944 6-yr-old trees after an extreme natural drought episode. We examined how individual damage was related to their size and the density and species identity of neighbouring trees. The two co-occurring dominant tree species, Eucalyptus delegatensis and Eucalyptus regnans, were the most vulnerable to drought-induced xylem cavitation and both species suffered significantly greater damage than neighbouring, subdominant species Pomaderris apetala and Acacia dealbata. While the two eucalypts had similar leaf P₅₀ values, E. delegatensis suffered significantly greater damage, which was strongly related to the density of neighbouring P. apetala. Damage in E. regnans was less impacted by neighbouring plants and smaller trees of both eucalypts sustained significantly more damage than larger trees. Our findings demonstrate that natural drought damage is influenced by individual plant physiology as well as the composition, physiology and density of the surrounding stand.

The ecology and physiology of fern gametophytes: A methodological synthesis

All green plants alternate between the gametophyte and sporophyte life stages, but only seed-free vascular plants (ferns and lycophytes) have independent, free-living gametophytes. Fern and lycophyte gametophytes are significantly reduced in size and morphological complexity relative to their sporophytic counterparts and have often been overlooked in ecological and physiological studies. Understanding the ecological and physiological factors that directly impact this life stage is of critical importance because the ultimate existence of a sporophyte is dependent upon successful fertilization in the gametophyte generation. Furthermore, previous research has shown that the dual nature of the life cycle and the high dispersibility of spores can result in different geographic patterns between gametophytes and their respective sporophytes. This variation in distribution patterns likely exacerbates the separation of selective pressures acting on gametophyte and sporophyte generations, and can uniquely impact a species' ecology and physiology. Here, we provide a review of historical and contemporary methodologies used to examine ecological and physiological aspects of fern gametophytes, as well as those that allow for comparisons between the two generations. We conclude by suggesting methodological approaches to answer currently outstanding questions. We hope that the information covered herein will serve as a guide to current researchers and stimulate future discoveries in fern gametophyte ecology and physiology.

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.

Tree-to-Shrub Shift Benefits the Survival of Quercus mongolica Fisch. ex Ledeb. at the Xeric Timberline

Woody species are either trees or shrubs, with the exception of approximately 9.2% “trub” species exhibiting both tree and shrub growth forms. Little is known thus far about the ecological importance of plant growth-form plasticity under a drying climate. Quercus mongolica Fisch. ex Ledeb., a dominant tree species of temperate forests in East Asia, typically shows tree-to-shrub growth-form shifts at the xeric timberline and is suitable to test whether growth-form plasticity can physiologically benefit plant drought acclimation. We quantified the tree architecture, drought stress, physiological drought tolerance, and nonstructural carbohydrates of Q. mongolica trees and shrubs on the opposite slope of the same mountain at the xeric timberline in July 2018. Compared with Q. mongolica trees on shady slopes, the shrubs on sunny slopes had less available water to use and were more severely threatened by drought. Moreover, the shrubs had stronger tolerance to drought but still tended to have less nonstructural carbohydrate accumulation. The tree-to-shrub shift effectively benefits the survival of Q. mongolica under a dry climate and may strongly contribute to forest dynamics and even fire regimes under climate drying, especially for sensitive ecosystems such as the xeric timberline, which will be vulnerable under future climate change scenarios.

Limited plasticity of anatomical and hydraulic traits in aspen trees under elevated CO2 and seasonal drought

The timing of abiotic stress elicitors on wood formation largely affects xylem traits that determine xylem efficiency and vulnerability. Nonetheless, seasonal variability of elevated CO2 (eCO2) effects on tree functioning under drought remains largely unknown. To address this knowledge gap, 1-year-old aspen (Populus tremula L.) trees were grown under ambient (±445 ppm) and elevated (±700 ppm) CO2 and exposed to an early (spring/summer 2019) or late (summer/autumn 2018) season drought event. Stomatal conductance and stem shrinkage were monitored in vivo as xylem water potential decreased. Additional trees were harvested for characterization of wood anatomical traits and to determine vulnerability and desorption curves via bench dehydration. The abundance of narrow vessels decreased under eCO2 only during the early season. At this time, xylem vulnerability to embolism formation and hydraulic capacitance during severe drought increased under eCO2. Contrastingly, stomatal closure was delayed during the late season, while hydraulic vulnerability and capacitance remained unaffected under eCO2. Independently of the CO2 treatment, elastic, and inelastic water pools depleted simultaneously after 50% of complete stomatal closure. Our results suggest that the effect of eCO2 on drought physiology and wood traits are small and variable during the growing season and question a sequential capacitive water release from elastic and inelastic pools as drought proceeds.

Drought stress recovery of hydraulic and photochemical processes in Neotropical tree saplings

Climate models predict an increase in the severity and the frequency of droughts. Tropical forests are among the ecosystems that could be highly impacted by these droughts. Here, we explore how hydraulic and photochemical processes respond to drought stress and re-watering. We conducted a pot experiment on saplings of five tree species. Before the onset of drought, we measured a set of hydraulic traits, including minimum leaf conductance, leaf embolism resistance and turgor loss point. During drought stress, we monitored traits linked to leaf hydraulic functioning (leaf water potential (ψmd) and stomatal conductance (gs)) and traits linked to leaf photochemical functioning (maximum quantum yield of photosystem II (Fv/Fm) and maximum electron transport rate (ETRmax)) at different wilting stages. After re-watering, the same traits were measured after 3, 7 and 14 days. Hydraulic trait values decreased faster than photochemical trait values. After re-watering, the values of the four traits recovered at different rates. Fv/Fm recovered very fast close to their initial values only 3 days after re-watering. This was followed by ETRmax, Ψmd and gs. Finally, we show that species with large stomatal and leaf safety margin and low πtlp are not strongly impacted by drought, whereas they have a low recovery on photochemical efficiency. These results demonstrate that πtlp, stomatal and leaf safety margin are a good indicators of plant responses to drought stress and also to recovery for photochemical efficiency.

Effects of Water Availability on the Relationships Between Hydraulic and Economic Traits in the Quercus wutaishanica Forests

Water availability is a key environmental factor affecting plant species distribution, and the relationships between hydraulic and economic traits are important for understanding the species' distribution patterns. However, in the same community type but within different soil water availabilities, the relationships in congeneric species remain ambiguous. In northwest China, Quercus wutaishanica forests in the Qinling Mountains (QM, humid region) and Loess Plateau (LP, drought region) have different species composition owing to contrasting soil water availability, but with common species occurring in two regions. We analyzed eight hydraulic traits [stomatal density (SD), vein density (VD), wood specific gravity (WSG_branch), lower leaf area: sapwood area (Al: As), stomatal length (SL), turgor loss point (Ψ_Tlp), maximum vessel diameter (Vd_max) and height (Height)] and five economic traits [leaf dry matter content (LDMC), leaf tissue density (TD), leaf dry mass per area (LMA), Leaf thickness (LT) and maximum net photosynthetic rate (P_max)] of congeneric species (including common species and endemic species) in Q. wutaishanica forests of QM and LP. We explored whether the congeneric species have different economic and hydraulic traits across regions. And whether the relationship between hydraulic and economic traits was determined by soil water availability, and whether it was related to species distribution and congeneric endemic species composition of the same community. We found that LP species tended to have higher SD, VD, WSG_branch, Al: As, SL, Ψ_Tlp and Vd_max than QM species. There was a significant trade-off between hydraulic efficiency and safety across congeneric species. Also, the relationships between hydraulic and economic traits were closer in LP than in QM. These results suggested that relationships between hydraulic and economic traits, hydraulic efficiency and safety played the role in constraining species distribution across regions. Interestingly, some relationships between traits changed (from significant correlation to non-correlation) in common species across two regions (from LP to QM), but not in endemic species. The change of these seven pairs of relationships might be a reason for common species' wide occurrence in the two Q. wutaishanica forests with different soil water availability. In drought or humid conditions, congeneric species developed different types of adaptation mechanisms. The study helps to understand the environmental adaptive strategies of plant species, and the results improve our understanding of the role of both hydraulic and economic traits during community assembly.

Directional Selection on Tree Seedling Traits Driven by Experimental Drought Differs Between Mesic and Dry Populations

We evaluated population differences and drought-induced phenotypic selection on four seedling traits of the Australian forest tree Eucalyptus pauciflora using a glasshouse dry-down experiment. We compared dry and mesic populations and tested for directional selection on lamina length (reflecting leaf size), leaf shape, the node of ontogenetic transition to the petiolate leaf (reflecting the loss of vegetative juvenility), and lignotuber size (reflecting a recovery trait). On average, the dry population had smaller and broader leaves, greater retention of the juvenile leaf state and larger lignotubers than the mesic population, but the populations did not differ in seedling survival. While there was statistical support for directional selection acting on the focal traits in one or other population, and for differences between populations in selection gradient estimates for two traits, only one trait—lamina length—exhibited a pattern of directional selection consistent with the observed population differences being a result of past adaptation to reduce seedling susceptibility to acute drought. The observed directional selection for lamina length in the mesic population suggests that future increases in drought risk in the wild will shift the mean of the mesic population toward that of the dry population. Further, we provide evidence suggesting an early age trade-off between drought damage and recovery traits, with phenotypes which develop larger lignotubers early being more susceptible to drought death. Such trade-offs could have contributed to the absence of population mean differences in survival, despite marked differentiation in seedling traits.

Contrasting life-history traits of black spruce and jack pine influence their physiological response to drought and growth recovery in northeastern boreal Canada

An increase in frequency, intensity and duration of drought events affects forested ecosystems. Trees react to these changes by adjusting stomatal conductance to maximize the trade-off between carbon gains and water losses. A better understanding of the consequences of these drought-induced physiological adjustments for tree growth could help inferring future productivity potentials of boreal forests. Here, we used samples from a forest inventory network in Canada where a decline in growth rates of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) occurred in 1988-1992, an exceptionally dry period, to verify if this growth decline resulted from physiological adjustments of trees to drought. We measured carbon and oxygen isotope ratios in growth rings of 95 spruces and 49 pines spanning 1985-1993. We used ¹³C discrimination (Δ¹³C) and ¹⁸O enrichment (Δ¹⁸O) as proxies for intrinsic water use efficiency and stomatal conductance, respectively. We studied how inter-annual variability in isotopic signals was linked to climate moisture index, vapor pressure deficit and annual snowfall amount. We found significantly lower Δ¹³C values over 1988-1990, and significantly higher Δ¹⁸O values in 1988-1989 and 1991 compared to the 1985-1993 averages. We also observed that a low climatic water balance and a high vapor pressure deficit were linked with low Δ¹³C and high Δ¹⁸O in the two study species, in parallel with low growth rates. The latter effect persisted into the year following drought for black spruce, but not for jack pine. These findings highlight that small differences in physiological parameters between species could translate into large differences in post-drought recovery. The stronger and longer lasting impact on black spruce compared to jack pine suggests a less efficient carbon use and a lower acclimation potential to future warmer and drier climate conditions.

Annual precipitation explains variability in dryland vegetation greenness globally but not locally

Dryland vegetation productivity is strongly modulated by water availability. As precipitation patterns and variability are altered by climate change, there is a pressing need to better understand vegetation responses to precipitation variability in these ecologically fragile regions. Here we present a global analysis of dryland sensitivity to annual precipitation variations using long-term records of normalized difference vegetation index (NDVI). We show that while precipitation explains 66% of spatial gradients in NDVI across dryland regions, precipitation only accounts for <26% of temporal NDVI variability over most (>75%) dryland regions. We observed this weaker temporal relative to spatial relationship between NDVI and precipitation across all global drylands. We confirmed this result using three alternative water availability metrics that account for water loss to evaporation, and growing season and precipitation timing. This suggests that predicting vegetation responses to future rainfall using space-for-time substitution will strongly overestimate precipitation control on interannual variability in aboveground growth. We explore multiple mechanisms to explain the discrepancy between spatial and temporal responses and find contributions from multiple factors including local-scale vegetation characteristics, climate and soil properties. Earth system models (ESMs) from the latest Coupled Model Intercomparison Project overestimate the observed vegetation sensitivity to precipitation variability up to threefold, particularly during dry years. Given projections of increasing meteorological drought, ESMs are likely to overestimate the impacts of future drought on dryland vegetation with observations suggesting that dryland vegetation is more resistant to annual precipitation variations than ESMs project.

Living on the edge: A continental-scale assessment of forest vulnerability to drought

Globally, forests are facing an increasing risk of mass tree mortality events associated with extreme droughts and higher temperatures. Hydraulic dysfunction is considered a key mechanism of drought-triggered dieback. By leveraging the climate breadth of the Australian landscape and a national network of research sites (Terrestrial Ecosystem Research Network), we conducted a continental-scale study of physiological and hydraulic traits of 33 native tree species from contrasting environments to disentangle the complexities of plant response to drought across communities. We found strong relationships between key plant hydraulic traits and site aridity. Leaf turgor loss point and xylem embolism resistance were correlated with minimum water potential experienced by each species. Across the data set, there was a strong coordination between hydraulic traits, including those linked to hydraulic safety, stomatal regulation and the cost of carbon investment into woody tissue. These results illustrate that aridity has acted as a strong selective pressure, shaping hydraulic traits of tree species across the Australian landscape. Hydraulic safety margins were constrained across sites, with species from wetter sites tending to have smaller safety margin compared with species at drier sites, suggesting trees are operating close to their hydraulic thresholds and forest biomes across the spectrum may be susceptible to shifts in climate that result in the intensification of drought.

Vulnerability to xylem cavitation of Hakea species (Proteaceae) from a range of biomes and life histories predicted by climatic niche

BACKGROUND AND AIMS

Extreme drought conditions across the globe are impacting biodiversity, with serious implications for the persistence of native species. However, quantitative data on physiological tolerance are not available for diverse flora to inform conservation management. We quantified physiological resistance to cavitation in the diverse Hakea genus (Proteaceae) to test predictions based on climatic origin, life history and functional traits.

METHODS

We sampled terminal branches of replicate plants of 16 species in a common garden. Xylem cavitation was induced in branches under varying water potentials (tension) in a centrifuge, and the tension generating 50 % loss of conductivity (stem P50) was characterized as a metric for cavitation resistance. The same branches were used to estimate plant functional traits, including wood density, specific leaf area and Huber value (sap flow area to leaf area ratio).

KEY RESULTS

There was significant variation in stem P50 among species, which was negatively associated with the species climate origin (rainfall and aridity). Cavitation resistance did not differ among life histories; however, a drought avoidance strategy with terete leaf form and greater Huber value may be important for species to colonize and persist in the arid biome.

CONCLUSIONS

This study highlights climate (rainfall and aridity), rather than life history and functional traits, as the key predictor of variation in cavitation resistance (stem P50). Rainfall for species origin was the best predictor of cavitation resistance, explaining variation in stem P50, which appears to be a major determinant of species distribution. This study also indicates that stem P50 is an adaptive trait, genetically determined, and hence reliable and robust for predicting species vulnerability to climate change. Our findings will contribute to future prediction of species vulnerability to drought and adaptive management under climate change.

Hydroscapes: A Useful Metric for Distinguishing Iso-/Anisohydric Behavior in Almond Cultivars

As a consequence of climate change, water scarcity has increased the use of the iso-/anisohydric concept with the aim of identifying anisohydric or drought-tolerant genotypes. Recently, Meinzer and colleagues developed a metric for discriminating between iso- and anisohydric behavior called the hydroscape, which describes a range in which stomata control leaf water potential (Ψ) with decreasing water availability, and it is linked to several water-regulation and drought-tolerance traits. Thus, our objective was to test the usefulness of the hydroscape in discriminating between iso- and anisohydric Prunus dulcis cultivars, a species that is widely cultivated in Mediterranean central Chile due to its ability to withstand water stress. Through a pot desiccation experiment, we determined that the hydroscape was able to discriminate between two contrasting Prunus cultivars; the more anisohydric cultivar had a hydroscape 4.5 times greater than that of the other cultivar, and the hydroscape correlated with other metrics of plant water-use strategies, such as the maximum range of daily Ψ variation and the Ψ at stomatal closure. Moreover, the photosynthesis rates were also differently affected between cultivars. The more isohydric cultivar, which had a smaller hydroscape, displayed a steeper photosynthesis reduction at progressively lower midday Ψ. This methodology could be further used to identify drought-tolerant anisohydric Prunus cultivars.

Mesophyll photosynthetic sensitivity to leaf water potential in Eucalyptus: a new dimension of plant adaptation to native moisture supply

Photosynthetic sensitivity to drought is a fundamental constraint on land-plant evolution and ecosystem function. However, little is known about how the sensitivity of photosynthesis to nonstomatal limitations varies among species in the context of phylogenetic relationships. Using saplings of 10 Eucalyptus species, we measured maximum CO₂ -saturated photosynthesis using A-c_i curves at several different leaf water potentials (ψ_leaf ) to quantify mesophyll photosynthetic sensitivity to ψ_leaf (MPS), a measure of how rapidly nonstomatal limitations to carbon uptake increase with declining ψ_leaf . MPS was compared to the macroclimatic moisture availability of the species' native habitats, while accounting for phylogenetic relationships. We found that species native to mesic habitats have greater MPS but higher maximum photosynthetic rates during non-water-stressed conditions, revealing a trade-off between maximum photosynthesis and drought sensitivity. Species with lower turgor loss points have lower MPS, indicating coordination among photosynthetic and water-relations traits. By accounting for phylogenetic relationships among closely related species, we provide the first compelling evidence that MPS in Eucalyptus evolved in an adaptive fashion with climatically determined moisture availability, opening the way for further study of this poorly explored dimension of plant adaptation to drought.

El Niño-Southern Oscillation affects the water relations of tree species in the Yucatan Peninsula, Mexico

We evaluated the effect of ENSO 2015/16 on the water relations of eight tree species in seasonally dry tropical forests of the Yucatan Peninsula, Mexico. The functional traits: wood density, relative water content in wood, xylem water potential and specific leaf area were recorded during the rainy season and compared in three consecutive years: 2015 (pre-ENSO conditions), 2016 (ENSO conditions) and 2017 (post-ENSO conditions). We analyzed tree size on the capacity to respond to water deficit, considering young and mature trees, and if this response is distinctive in species with different leaf patterns in seasonally dry tropical forests distributed along a precipitation gradient (700–1200 mm year⁻¹). These traits showed a strong decrease in all species in response to water stress in 2016, mainly in the driest site. Deciduous species had lower wood density, higher predawn water potential and higher specific leaf area than evergreen species. In all cases, mature trees were more tolerant to drought. In the driest site, there was a significant reduction in water status, regardless of their leaf phenology, indicating that seasonally dry tropical forests are highly vulnerable to ENSO. Vulnerability of deciduous species is intensified in the driest areas and in the youngest trees.

Hydraulic failure and tree size linked with canopy die-back in eucalypt forest during extreme drought

Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die-back in eucalypt forests. The role of hydraulic failure and tree size on canopy die-back in three eucalypt tree species during this drought was examined. We measured pre-dawn and midday leaf water potential (Ψleaf ), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought-induced xylem embolism. Tree size and tree health was also surveyed. Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78-100%). This is in contrast to trees with partial canopy die-back (30-70% canopy die-back: 72-78% native embolism), or relatively healthy trees (little evidence of canopy die-back: 25-31% native embolism). Midday Ψleaf was significantly more negative in trees exhibiting partial canopy die-back (-2.7 to -6.3 MPa), compared with relatively healthy trees (-2.1 to -4.5 MPa). In two of the species the majority of individuals showing complete canopy die-back were in the small size classes. Our results indicate that hydraulic failure is strongly associated with canopy die-back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk.

Interannual variability of ecosystem iso/anisohydry is regulated by environmental dryness

●Plants are characterized by the iso/anisohydry continuum depending on how they regulate leaf water potential (Ψ_L ). However, how iso/anisohydry changes over time in response to year-to-year variations in environmental dryness and how such responses vary across different regions remains poorly characterized. ●We investigated how dryness, represented by aridity index, affects the interannual variability of ecosystem iso/anisohydry at the regional scale, estimated using satellite microwave vegetation optical depth (VOD) observations. This ecosystem-level analysis was further complemented with published field observations of species-level Ψ_L . ●We found different behaviors in the directionality and sensitivity of isohydricity (σ) with respect to the interannual variation of dryness in different ecosystems. These behaviors can largely be differentiated by the average dryness of the ecosystem itself: in mesic ecosystems, σ decreases in drier years with a higher sensitivity to dryness; in xeric ecosystems, σ increases in drier years with a lower sensitivity to dryness. These results were supported by the species-level synthesis. ●Our study suggests that how plants adjust their water use across years - as revealed by their interannual variability in isohydricity - depends on the dryness of plants' living environment. This finding advances our understanding of plant responses to drought at regional scales.

Weak tradeoff between xylem hydraulic efficiency and safety: climatic seasonality matters

A classic theory proposes that plant xylem cannot be both highly efficient in water transport and resistant to embolism, and therefore a hydraulic efficiency-safety trade-off should exist. However, the trade-off is weak, and many species exhibit both low efficiency and low safety, falling outside of the expected trade-off space. It remains unclear under what climatic conditions these species could maintain competitive fitness. We compiled hydraulic efficiency and safety traits for 682 observations of 499 woody species from 178 sites world-wide and measured the position of each observation within the proposed trade-off space. For both angiosperms and gymnosperms, observations from sites with high climatic seasonality, especially precipitation seasonality, tended to have higher hydraulic safety and efficiency than observations from sites with low seasonality. Specifically, high vapour pressure deficit, high solar radiation, and low precipitation during the wet season were driving factors. Strong climatic seasonality and drought in both dry and wet seasons appear to be ecological filters that select for species with co-optimized safety and efficiency, whereas the opposite environmental conditions may allow the existence of plants with low efficiency and safety.

Aridity drives coordinated trait shifts but not decreased trait variance across the geographic range of eight Australian trees

Large intraspecific functional trait variation strongly impacts many aspects of communities and ecosystems, and is the medium upon which evolution works. Yet intraspecific trait variation is inconsistent and hard to predict across traits, species and locations. We measured within-species variation in leaf mass per area (LMA), leaf dry matter content (LDMC), branch wood density (WD), and allocation to stem area vs leaf area in branches (branch Huber value (HV)) across the aridity range of seven Australian eucalypts and a co-occurring Acacia species to explore how traits and their variances change with aridity. Within species, we found consistent increases in LMA, LDMC and WD and HV with increasing aridity, resulting in consistent trait coordination across leaves and branches. However, this coordination only emerged across sites with large climate differences. Unlike trait means, patterns of trait variance with aridity were mixed across populations and species. Only LDMC showed constrained trait variation in more xeric species and drier populations that could indicate limits to plasticity or heritable trait variation. Our results highlight that climate can drive consistent within-species trait patterns, but that patterns might often be obscured by the complex nature of morphological traits, sampling incomplete species ranges or sampling confounded stress gradients.

Relating the climate envelopes of urban tree species to their drought and thermal tolerance

Analysing the climate envelope of plant species has been suggested as a tool to predict the vulnerability of tree species in future urban climates. However, there is little evidence that the climate envelope of a plant species directly relates to the drought and thermal tolerance of that species, at least not at the resolution required to identify or rank species vulnerability. Here, we attempted to predict drought and thermal tolerance of commonly used urban tree species using climate variables derived exclusively from open-source global occurrence data. We quantified three drought and thermal tolerance traits for 43 urban tree species in a common garden experiment: stomatal sensitivity to vapour pressure deficit, leaf water potential at the turgor loss point, and leaf thermal tolerance. We then attempted to predict each tolerance trait from variables derived from the climate envelope of each species, using occurrence data from the Global Biodiversity Information Facility. We found no strong relationships between drought and thermal tolerance traits and climatic variables. Across wide environmental gradients, plant tolerance and climate are inherently linked. But our results suggest that climate envelopes determined from species occurrence data alone may not predict drought or thermal tolerance at the resolution required to select tree species for future urban forests. We should focus on identifying the most relevant strategies and traits required to describe tolerance which in combination with climate envelope analysis should ultimately predict growth and mortality of trees in urban landscapes.

Strong Environmental Filtering Based on Hydraulic Traits Occurring in the Lower Water Availability of Temperate Forest Communities

The trait-based approaches have made progress in understanding the community assembly process. Here, we explore the key traits that may shape community assembly patterns of the same community type but within different water availabilities. Natural Quercus wutaishanica forests were chosen as a suitable study system to test the difference between economic and hydraulic traits across water availability on the Loess Plateau (LP, drought region) and Qinling Mountains (QL, humid region) of China. A total of 75 plots were established separately in two sites, and 12 functional traits (seven hydraulic traits and five economic traits) of 167 species were studied. Community-weighted mean trait values and functional diversity indices were compared between the two sites. Canonical component analysis was performed to infer whether the changes of community traits and their relationships are driven by intraspecific variation or species turnover. Evidence for likely community assembly processes was tested using the null model to determine whether functional structure among seven hydraulic traits and five economic traits was dominated by different ecological processes between two sites. We found that forests in the Loess Plateau and Qinling Mountains showed different hydraulic and economic traits. Hydraulic and economic traits coupled at the community level were driven by species turnover. Hydraulic traits showed more significant convergent patterns on LP than that in QL. Our results suggest a strong environmental filtering process occurred in hydraulic-based community assembly in the temperate forest with low water availability. Reveal the relationship of hydraulic and economic traits at the community level. Emphasize the critical role of multi-dimensional traits selecting like hydraulic traits in community ecology.

Juniperus communis populations exhibit low variability in hydraulic safety and efficiency

The performance and distribution of woody species strongly depend on their adjustment to environmental conditions based on genotypic and phenotypic properties. Since more intense and frequent drought events are expected due to climate change, xylem hydraulic traits will play a key role under future conditions, and thus, knowledge of hydraulic variability is of key importance. In this study, we aimed to investigate the variability in hydraulic safety and efficiency of the conifer shrub Juniperus communis based on analyses along an elevational transect and a common garden approach. We studied (i) juniper plants growing between 700 and 2000 m a.s.l. Innsbruck, Austria, and (ii) plants grown in the Innsbruck botanical garden (Austria) from seeds collected at different sites across Europe (France, Austria, Ireland, Germany and Sweden). Due to contrasting environmental conditions at different elevation and provenance sites and the wide geographical study area, pronounced variation in xylem hydraulics was expected. Vulnerability to drought-induced embolisms (hydraulic safety) was assessed via the Cavitron and ultrasonic acoustic emission techniques, and the specific hydraulic conductivity (hydraulic efficiency) via flow measurements. Contrary to our hypothesis, relevant variability in hydraulic safety and efficiency was neither observed across elevations, indicating a low phenotypic variation, nor between provenances, despite expected genotypic differences. Interestingly, the provenance from the most humid and warmest site (Ireland) and the northernmost provenance (Sweden) showed the highest and the lowest embolism resistance, respectively. The hydraulic conductivity was correlated with plant height, which indicates that observed variation in hydraulic traits was mainly related to morphological differences between plants. We encourage future studies to underlie anatomical traits and the role of hydraulics for the broad ecological amplitude of J. communis.

Wood day capacitance is related to water content, wood density, and anatomy across 30 temperate tree species

Water released from wood during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twigs of 30 temperate angiosperm tree species, we measured day capacitance (between predawn and midday), water content, wood density, and anatomical traits, that is, vessel dimensions, tissue fractions, and vessel-tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, wood density (WD) and predawn lumen volumetric water content (VWC_L-pd ) together were the strongest predictors of day capacitance (r²_adj = .44). Vessel-tissue contact fractions explained an additional ~10% of the variation in day capacitance. Regression models were not improved by including tissue lumen fractions. Among diffuse-porous species, VWC_L-pd and vessel-ray contact fraction together were the best predictors of day capacitance, whereas among semi/ring-porous species, VWC_L-pd , WD and vessel-fibre contact fraction were the best predictors. At predawn, wood was less than fully saturated for all species (lumen relative water content = 0.52 ± 0.17). Our findings imply that day capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue.

Visual and hydraulic techniques produce similar estimates of cavitation resistance in woody species

Hydraulic failure of the plant vascular system is a principal cause of forest die-off under drought. Accurate quantification of this process is essential to our understanding of the physiological mechanisms underpinning plant mortality. Imaging techniques increasingly are applied to estimate xylem cavitation resistance. These techniques allow for in situ measurement of embolism formation in real time, although the benefits and trade-offs associated with different techniques have not been evaluated in detail. Here we compare two imaging methods, microcomputed tomography (microCT) and optical vulnerability (OV), to standard hydraulic methods for measurement of cavitation resistance in seven woody species representing a diversity of major phylogenetic and xylem anatomical groups. Across the seven species, there was strong agreement between cavitation resistance values (P₅₀ ) estimated from visualization techniques (microCT and OV) and between visual techniques and hydraulic techniques. The results indicate that visual techniques provide accurate estimates of cavitation resistance and the degree to which xylem hydraulic function is impacted by embolism. Results are discussed in the context of trade-offs associated with each technique and possible causes of discrepancy between estimates of cavitation resistance provided by visual and hydraulic techniques.