Variation In Hydraulic Traits Research Articles

Hydraulic traits are not robust predictors of tree species stem growth during a severe drought in a wet tropical forest

Abstract Severe droughts have led to lower plant growth and high mortality in many ecosystems worldwide, including tropical forests. Drought vulnerability differs among species, but there is limited consensus on the nature and degree of this variation in tropical forest communities. Understanding species‐level vulnerability to drought requires examination of hydraulic traits since these reflect the different strategies species employ for surviving drought. Here, we examined hydraulic traits and growth reductions during a severe drought for 12 common woody species in a wet tropical forest community in Puerto Rico to ask: Q1. To what extent can hydraulic traits predict growth declines during drought? We expected that species with more hydraulically vulnerable xylem and narrower safety margins (SMP50) would grow less during drought. Q2. How does species successional association relate to the levels of vulnerability to drought and hydraulic strategies? We predicted that early‐ and mid‐successional species would exhibit more acquisitive strategies, making them more susceptible to drought than shade‐tolerant species. Q3. What are the different hydraulic strategies employed by species and are there trade‐offs between drought avoidance and drought tolerance? We anticipated that species with greater water storage capacity would have leaves that lose turgor at higher xylem water potential and be less resistant to embolism forming in their xylem (P50). We found a large range of variation in hydraulic traits across species; however, they did not closely capture the magnitude of growth declines during drought. Among larger trees (≥10 cm diameter at breast height—DBH), some tree species with high xylem embolism vulnerability (P50) and risk of hydraulic failure (SMP50) experienced substantial growth declines during drought, but this pattern was not consistent across species. We found a trade‐off among species between drought avoidance (capacitance) and drought tolerating (P50) in this tropical forest community. Hydraulic strategies did not align with successional associations. Instead, some of the more drought‐vulnerable species were shade‐tolerant dominants in the community, suggesting that a drying climate could lead to shifts in long‐term forest composition and function in Puerto Rico and the Caribbean. Read the free Plain Language Summary for this article on the Journal blog.

Plant hydraulics provide guidance for irrigation management in mature polar plantation

Irrigation is essential for sustaining the productivity of plantation forests in arid, semi-arid, and seasonal drought regions. Current irrigation management is commonly scheduled based on time or soil water status, which often fails to capture plant physiological demand. Here we report on the variation of traits related to plant water use for 7-year-old Populus tomentosa plantations under different irrigation treatments in North China during two growth seasons with contrasting environmental dryness. Plant hydraulics were assessed by stomatal regulation strategy as well as hydraulic traits conferring water transport efficiency and safety, including branch hydraulic conductivity (Ks), thresholds for xylem embolism (Px) and percentage loss of leaf rehydration capacity (PLRC), leaf water potential at turgor loss point (Ψtlp) as well as hydraulic capacitance before and after turgor loss (Cbulk-pre and Cbulk-post respectively). Our results showed that both seasonal increases in rainfall and irrigation could modify stomatal regulation strategy, resulting in a more anisohydric stomatal behavior, but through different underlying mechanisms. Specifically, seasonal variation in stomatal regulation was primarily manipulated by the daily fluctuation of leaf water potential, Ψtlp, and possibly the water supply-demand relationship. At the same time, we speculated that irrigation-dependent alternation was presumably associated with shifted belowground traits. Seasonal variation in hydraulic traits was found for Ks and Ψtlp, with trees in the dry season displaying higher Ks and less negative Ψtlp regardless of irrigation, while other traits governing hydraulic functioning were essentially invariable across seasons or irrigation treatments. Tree growth rate was lower in the wet season and was independent of irrigation, which was likely explained by the occurrence of hydraulic impairment due to less stringent stomatal control. Overall, our results suggest that limited precipitation and high atmospheric vapor pressure deficit in the dry season would not compromise tree growth because of the tighter stomatal control, while hydraulic dysfunction could occur in the wet season due to excessive water use strategy, subsequently perhaps contributing to the reduction in growth rate. The findings of this study underscore the usefulness of physiological traits in guiding irrigation management and advocate reconsidering plantation water management strategy based on reliable indicators.

Beyond leaf habit: generalities in plant function across 97 tropical dry forest tree species.

Leaf habit has been hypothesized to define a linkage between the slow-fast plant economic spectrum and the drought resistance-avoidance trade-off in tropical forests ('slow-safe vs fast-risky'). However, variation in hydraulic traits as a function of leaf habit has rarely been explored for a large number of species. We sampled leaf and branch functional traits of 97 tropical dry forest tree species from four sites to investigate whether patterns of trait variation varied consistently in relation to leaf habit along the 'slow-safe vs fast-risky' trade-off. Leaf habit explained from 0% to 43.69% of individual trait variation. We found that evergreen and semi-deciduous species differed in their location along the multivariate trait ordination when compared to deciduous species. While deciduous species showed consistent trait values, evergreen species trait values varied as a function of the site. Last, trait values varied in relation to the proportion of deciduous species in the plant community. We found that leaf habit describes the strategies that define drought avoidance and plant economics in tropical trees. However, leaf habit alone does not explain patterns of trait variation, which suggests quantifying site-specific or species-specific uncertainty in trait variation as the way forward.

Variation in Xylem Hydraulic Structure and Function of Two Mangrove Species across a Latitudinal Gradient in Eastern Australia

Mangroves growing in tropical and subtropical intertidal zones face challenges from warming and altered rainfall patterns associated with global climate change. Intraspecific variation in hydraulic traits may allow a mangrove species to acclimate to novel climatic conditions, yet little is known regarding the potential for adaptive plasticity in these traits. In this study, we aimed to quantify the variation in plant hydraulic traits of two widespread mangrove species growing across a latitudinal gradient. We investigated the xylem hydraulic structure and function of Avicennia marina and Aegiceras corniculatum, across three sites spanning a latitudinal gradient of 17.45° in eastern Australia. We found that both species were highly resistant to xylem embolism and that there was significant intraspecific variation in hydraulic traits between sites. The highest embolism resistance and sapwood-specific hydraulic conductivity (KS) were found at the lowest latitude site that had the highest mean annual temperature and precipitation. A. marina showed no differences in vessel size and density among sites. It has other special features such as successive cambia enhancing its ability to adapt to a large environmental gradient. In contrast, A. corniculatum showed higher vessel densities at lower latitudes. There was a significant and positive correlation (R2 = 0.72, p < 0.05) between KS and embolism resistance across species and sites, suggesting the absence of a tradeoff between hydraulic efficiency and safety. Both embolism resistance and KS were negatively correlated with wood density but positively with vessel wall reinforcement. This study reveals that these two widespread mangrove species were adapted to warmer climates by enhancing both hydraulic efficiency and safety.

Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, U.S.A

In this study, we examined the inter- and intra-specific variation of hydraulic traits of three conifers of the Northern Rockies:Pinus ponderosa, Picea engelmannii, and Pseudotsuga menziesiito understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant xylem water potential (ψx), xylem tissue vulnerability to cavitation (P50, or ψ causing 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem vulnerability to cavitation with rooting depth. We found that xylem vulnerability to cavitation did not correspond to where tree species were found in the landscape. For example,P. ponderosagrew in more xeric locations, whileP. engelmannii were largely confined to more mesic locations, yet P. engelmanniihad more negative P50values.P. menziesiihad the lowest P50value, but displayed little variation in vulnerability to cavitation across the hydroclimatic gradient. These patterns were also reflected in the safety margins;P. menziesiihad the widest safety margin,P. engelmanniiwas intermediate, andP. ponderosadisplayed the narrowest safety margin. All three species were also using water sources deeper than 30cm in depth, allowing them to persist throughout the mid-summer drought. Overall, species-specific hydraulic traits did not necessarily follow a predictable response to the environment; instead, a combination of physiological and morphological traits likely allow trees to persist across a broader hydroclimatic gradient than would be otherwise expected from hydraulic trait measurements alone.

Responses of functional traits to seven-year nitrogen addition in two tree species: coordination of hydraulics, gas exchange and carbon reserves.

Atmospheric nitrogen (N) deposition has been observed to impact plant structure and functional traits in terrestrial ecosystems. Although the effect of N deposition on plant water use has been well-evaluated in laboratories and in experimental forests, the linkages between water and carbon relations under N deposition are unclear. Here, we report on hydraulics, gas exchange and carbon reserves of two broad-leaved tree species (Quercus mongolica and Fraxinus mandshurica) in mature temperate forests after a seven-year experiment with different levels of N addition (control (CK), low (23kgNha-1yr-1), medium (46kgNha-1yr-1) and high (69kgNha-1yr-1)). We investigated variation in hydraulic traits (xylem-specific hydraulic conductivity (Ks), native percentage loss of conductivity (PLC) and leaf water potential), xylem anatomy (vessel diameter and density), gas exchange (maximum net photosynthesis rate and stomatal conductance) and carbon reserves (soluble sugars, starch and total nonstructural carbohydrates (NSC)) with different N addition levels. We found that medium N addition significantly increased Ks and vessel diameter compared to control, but accompanied increasing PLC and decreasing leaf water potential, suggesting that N addition results in a greater hydraulic efficiency and higher risk of embolism. N addition promoted photosynthetic capacity via increasing foliar N concentration but did not change stomatal conductance. In addition, we found increase in foliar soluble sugar concentration and decrease in starch concentration with N addition, and positive correlations between hydraulic traits (vessel diameter and PLC) and soluble sugars. These coupled responses of tree hydraulics and carbon metabolism are consistent with a regulatory role of carbohydrates in maintaining hydraulic integrity. Our study provides an important insight into the relationship of plant water transport and carbon dynamics under increasing N deposition.

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 2000ma.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.

Use of hydraulic traits for modeling genotype‐specific acclimation in cotton under drought

Summary Understanding the genetic and physiological basis of abiotic stress tolerance under field conditions is key to varietal crop improvement in the face of climate variability. Here, we investigate dynamic physiological responses to water stress in silico and their relationships to genotypic variation in hydraulic traits of cotton (Gossypium hirsutum), an economically important species for renewable textile fiber production.In conjunction with an ecophysiological process‐based model, heterogeneous data (plant hydraulic traits, spatially‐distributed soil texture, soil water content and canopy temperature) were used to examine hydraulic characteristics of cotton, evaluate their consequences on whole plant performance under drought, and explore potential genotype × environment effects.Cotton was found to have R‐shaped hydraulic vulnerability curves (VCs), which were consistent under drought stress initiated at flowering. Stem VCs, expressed as percent loss of conductivity, differed across genotypes, whereas root VCs did not. Simulation results demonstrated how plant physiological stress can depend on the interaction between soil properties and irrigation management, which in turn affect genotypic rankings of transpiration in a time‐dependent manner.Our study shows how a process‐based modeling framework can be used to link genotypic variation in hydraulic traits to differential acclimating behaviors under drought.

Allometric constraints and competition enable the simulation of size structure and carbon fluxes in a dynamic vegetation model of tropical forests (LM3PPA‐TV)

Tropical forests are a key determinant of the functioning of the Earth system, but remain a major source of uncertainty in carbon cycle models and climate change projections. In this study, we present an updated land model (LM3PPA-TV) to improve the representation of tropical forest structure and dynamics in Earth system models (ESMs). The development and parameterization of LM3PPA-TV drew on extensive datasets on tropical tree traits and long-term field censuses from Barro Colorado Island (BCI), Panama. The model defines a new plant functional type (PFT) based on the characteristics of shade-tolerant, tropical tree species, implements a new growth allocation scheme based on realistic tree allometries, incorporates hydraulic constraints on biomass accumulation, and features a new compartment for tree branches and branch fall dynamics. Simulation experiments reproduced observed diurnal and seasonal patterns in stand-level carbon and water fluxes, as well as mean canopy and understory tree growth rates, tree size distributions, and stand-level biomass on BCI. Simulations at multiple sites captured considerable variation in biomass and size structure across the tropical forest biome, including observed responses to precipitation and temperature. Model experiments suggested a major role of water limitation in controlling geographic variation forest biomass and structure. However, the failure to simulate tropical forests under extreme conditions and the systematic underestimation of forest biomass in Paleotropical locations highlighted the need to incorporate variation in hydraulic traits and multiple PFTs that capture the distinct floristic composition across tropical domains. The continued pressure on tropical forests from global change demands models which are able to simulate alternative successional pathways and their pace to recovery. LM3PPA-TV provides a tool to investigate geographic variation in tropical forests and a benchmark to continue improving the representation of tropical forests dynamics and their carbon storage potential in ESMs.

Hydraulic traits vary as the result of tip-to-base conduit widening in vascular plants.

Plant hydraulic traits are essential metrics for characterizing variation in plant function, but they vary markedly with plant size and position in a plant. We explore the potential effect of conduit widening on variation in hydraulic traits along the stem. We examined three species that differ in conduit diameter at the stem base for a given height (Moringa oleifera, Casimiroa edulis, and Pinus ayacahuite). We made anatomical and hydraulic measurements at different distances from the stem tip, constructed vulnerability curves, and examined the safety-efficiency trade-off with height-standardized data. Our results showed that segment-specific hydraulic resistance varied predictably along the stem, paralleling changes in mean conduit diameter and total number of conduits. The Huber value and leaf specific conductivity also varied depending on the sampling point. Vulnerability curves were markedly less noisy with height standardization, making the vulnerability-efficiency trade-off clearer. Because conduits widen predictably along the stem, taking height and distance from the tip into account provides a way of enhancing comparability and interpretation of hydraulic traits. Our results suggest the need for rethinking hydraulic sampling for comparing plant functional differences and strategies across individuals.

Traits and trade-offs of wood anatomy between trunks and branches in tropical dry forest species

Differences in wood traits are related to contrasting forces acting over trunks and branches. The wood economic spectrum is one of the most important groups of traits for plant performance due to the multiple functions in mechanical support, water conductivity and water and nutrient storage. Owing to the multiple functions, there are conflicting demands on wood anatomy depending upon environmental and architectural forces that change according to the structure (trunk/branch) in which functional traits are estimated. In this context, we explored how the mean values, variability and correlation patterns of wood anatomy traits varied between trunks and branches. We measured seven wood traits related to hydraulic efficiency and safety and mechanical support in 19 tree species that are widely distributed in tropical dry forests in Colombia. We found higher mechanical support and hydraulic efficiency in trunks than in branches and higher variation in hydraulic traits when compared to mechanical wood traits in both trunks and branches. We also detected higher traits coupling in branches when compared to trunks. Our results showed that contrasting forces acting over trunks and branches result in substantial intra-individual variability. These results are an important contribution when addressing ecological questions relating to the identification of functional strategies, species’ response capacity to changing environmental conditions and aboveground biomass estimations.

Drought tolerance traits do not vary across sites differing in water availability in Banksia serrata (Proteaceae).

Interspecific variation in plant hydraulic traits plays a major role in shaping species distributions across climates, yet variation within species is poorly understood. Here we report on intraspecific variation of hydraulic traits in Banksia serrata (L.f.) sampled from three sites characterised by contrasting climates (warm-wet, warm-dry and cool-wet). Hydraulic characteristics including vulnerability to embolism, hydraulic conductance, pressure-volume traits and key morphological traits were measured. Vulnerability to embolism in leaf and stem, defined by the water potential inducing 50 and 88% loss of hydraulic conductivity (P50 and P88 respectively), did not differ across sites. However, plants from the warm-dry environment exhibited higher stem conductivity (Ks) than the cool-wet environment. Leaf turgor loss point (TLP) did not vary among sites, but warm-dry site plants showed lower leaf capacitance (C*FT) and higher modulus of elasticity (ε) than the other two sites. Plants from the cool-wet site had lower specific leaf area (SLA) and plants from the warm-dry site had lower sapwood density (WD). Overall, key hydraulic traits were generally conserved across populations despite differences in mean site water availability, and the safety-efficiency trade-off was absent in this species. These results suggest that B. serrata has limited ability to adjust hydraulic architecture in response to environmental change and thus may be susceptible to climate change-type drought stress.

Water transport through tall trees: A vertically explicit, analytical model of xylem hydraulic conductance in stems

Trees grow by vertically extending their stems, so accurate stem hydraulic models are fundamental to understanding the hydraulic challenges faced by tall trees. Using a literature survey, we showed that many tree species exhibit continuous vertical variation in hydraulic traits. To examine the effects of this variation on hydraulic function, we developed a spatially explicit, analytical water transport model for stems. Our model allows Huber ratio, stem-saturated conductivity, pressure at 50% loss of conductivity, leaf area, and transpiration rate to vary continuously along the hydraulic path. Predictions from our model differ from a matric flux potential model parameterized with uniform traits. Analyses show that cavitation is a whole-stem emergent property resulting from non-linear pressure-conductivity feedbacks that, with gravity, cause impaired water transport to accumulate along the path. Because of the compounding effects of vertical trait variation on hydraulic function, growing proportionally more sapwood and building tapered xylem with height, as well as reducing xylem vulnerability only at branch tips while maintaining transport capacity at the stem base, can compensate for these effects. We therefore conclude that the adaptive significance of vertical variation in stem hydraulic traits is to allow trees to grow tall and tolerate operating near their hydraulic limits.

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.

Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v.1-Hydro)

Abstract. Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the diversity of hydraulic traits (including variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of individual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among individuals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (Amax), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.

Leaf size serves as a proxy for xylem vulnerability to cavitation in plantation trees.

Hybrid poplars are an important renewable forest resource known for their high productivity. At the same time, they are highly vulnerable to water stress. Identifying traits that can serve as indicators for growth performance remains an important task, particularly under field conditions. Understanding which trait combinations translate to improved productivity is key in order to satisfy the demand for poplar wood in an uncertain future climate. In this study, we compared hydraulic and leaf traits among five hybrid poplar clones at 10 plantations in central Alberta. We also assessed the variation of these traits between 2- to 3-year-old branches from the lower to mid-crown and current-year long shoots from the mid to upper crown. Our results showed that (1) hybrid poplars differed in key hydraulic parameters between branch type, (2) variation of hydraulic traits among clones was relatively large for some clones and less for others, and (3) strong relationships between measured hydraulic traits, such as vessel diameter, cavitation resistance, xylem-specific and leaf-specific conductivity and leaf area, were observed. Our results suggest that leaf size could serve as an additional screening tool when selecting for drought-tolerant genotypes in forest management and tree improvement programmes.

Fine-scale niche structure of Neotropical forests reflects a legacy of the Great American Biotic Interchange

The tendency of species to retain their ancestral niches may link processes that determine community assembly with biogeographic histories that span geological time scales. Biogeographic history is likely to have had a particularly strong impact on Neotropical forests because of the influence of the Great American Biotic Interchange, which followed emergence of a land connection between North and South America ~3 Ma. Here we examine the community structure, ancestral niches and ancestral distributions of the related, hyperdiverse woody plant genera Psychotria and Palicourea (Rubiaceae) in Panama. We find that 49% of the variation in hydraulic traits, a strong determinant of community structure, is explained by species' origins in climatically distinct biogeographic regions. Niche evolution models for a regional sample of 152 species indicate that ancestral climatic niches are associated with species' habitat distributions, and hence local community structure and composition, even millions of years after dispersal into new geographic regions.

Vulnerability to cavitation, hydraulic efficiency, growth and survival in an insular pine (Pinus canariensis)

It is widely accepted that hydraulic failure due to xylem embolism is a key factor contributing to drought-induced mortality in trees. In the present study, an attempt is made to disentangle phenotypic plasticity from genetic variation in hydraulic traits across the entire distribution area of a tree species to detect adaptation to local environments. A series of traits related to hydraulics (vulnerability to cavitation and hydraulic conductivity in branches), growth performance and leaf mass per area were assessed in eight Pinus canariensis populations growing in two common gardens under contrasting environments. In addition, the neutral genetic variability (FST) and the genetic differentiation of phenotypic variation (QST) were compared in order to identify the evolutionary forces acting on these traits. The variability for hydraulic traits was largely due to phenotypic plasticity. Nevertheless, the vulnerability to cavitation displayed a significant genetic variability (approx. 5 % of the explained variation), and a significant genetic × environment interaction (between 5 and 19 % of the explained variation). The strong correlation between vulnerability to cavitation and survival in the xeric common garden (r = -0·81; P < 0·05) suggests a role for the former in the adaptation to xeric environments. Populations from drier sites and higher temperature seasonality were less vulnerable to cavitation than those growing at mesic sites. No trade-off between xylem safety and efficiency was detected. QST of parameters of the vulnerability curve (0·365 for P50 and the slope of the vulnerability curve and 0·452 for P88) differed substantially from FST (0·091), indicating divergent selection. In contrast, genetic drift alone was found to be sufficient to explain patterns of differentiation for xylem efficiency and growth. The ability of P. canariensis to inhabit a wide range of ecosystems seemed to be associated with high phenotypic plasticity and some degree of local adaptations of xylem and leaf traits. Resistance to cavitation conferred adaptive potential for this species to adapt successfully to xeric conditions.

The blind men and the elephant: the impact of context and scale in evaluating conflicts between plant hydraulic safety and efficiency

Given the fundamental importance of xylem safety and efficiency for plant survival and fitness, it is not surprising that these are among the most commonly studied features of hydraulic architecture. However, much remains to be learned about the nature and universality of conflicts between hydraulic safety and efficiency. Although selection for suites of hydraulic traits that confer adequate plant fitness under given conditions is likely to occur at the organismal level, most studies of hydraulic architecture have been confined to scales smaller than the whole plant, such as small-diameter branches and roots. Here we discuss the impact of the spatial and temporal contexts in which hydraulic traits are studied on the interpretation of their role in maintaining plant hydraulic function. We argue that further advances in understanding the ecological implications of different suites of plant hydraulic traits will be enhanced by adopting an integrated approach that considers variation in hydraulic traits throughout the entire plant, dynamic behavior of water transport, xylem tension and water transport efficiency in intact plants, alternate mechanisms that modulate hydraulic safety and efficiency, and alternate measures of hydraulic safety and safety margins.

Topographical and seasonal trends in transpiration by two co-occurring Eucalyptus species during two contrasting years in a low rainfall environment

Understanding the strategies that confer resilience on natural woodlands in drought prone environments is important for the conservation of these and similar ecosystems. Our main aim in this 2-year study was to assess traits (sapwood area, sapwood density and leaf area index) that control transpiration in Eucalyptus camaldulensis and E. microcarpa in a natural forest in which topographical variation created surface soils of sandy clay in a depression (clay-zone) and of loamy sand underlain by a dense profile on the terraces (sand-zone). The clay-zone had a wetter profile due to extra water supply through subsurface lateral flow from the adjoining, topographically higher, sand-zone. In the clay-zone, the differences between the two tree species in their hydraulic attributes were large and rates of water use were widely divergent. Rates of transpiration per unit land area ( E c) and canopy conductance of E. camaldulensis that was dominant in the clay-zone were about 50% lower than those for E. microcarpa in the same zone. This was in marked contrast to the behavior of trees growing in the sand-zone where water availability was persistently low and variations in sapwood density, sapwood area and canopy conductance were narrow. This resulted in almost identical rates of water use for the two species in the sand-zone, despite E. microcarpa dominating the stand. Contrary to many previous studies, sapwood density was positively correlated with E c in these eucalypt species, while the proportion of trunk area assigned to sapwood declined with sapwood density. Consequently in this low rainfall environment, with prolonged dry seasons, dense sapwood safeguards against turgor loss, and possibly xylem embolism, thereby allowing E c to be sustained under extremely low soil-water availability. We concluded that variation in hydraulic traits is less likely where trees are under persistent water-stress than where the stress is short and relatively mild. We developed single functions for predicting E c for the two species by integrating their responses to micrometeorological and soil-water conditions.