The determinants of leaf turgor loss point and prediction of drought tolerance of species and biomes: a global meta-analysis

Water Adsorption to Leaves of Tall Cryptomeria japonica Tree Analyzed by Infrared Spectroscopy under Relative Humidity Control

Leaf water storage is a complex interaction between live tissue properties (anatomy and physiology) and physicochemical properties of biomolecules and water. How leaves adsorb water molecules based on interactions between biomolecules and water, including hydrogen bonding, challenges our understanding of hydraulic acclimation in tall trees where leaves are exposed to more water stress. Here, we used infrared (IR) microspectroscopy with changing relative humidity (RH) on leaves of tall Cryptomeria japonica trees. OH band areas correlating with water content were larger for treetop (52 m) than for lower-crown (19 m) leaves, regardless of relative humidity (RH). This high water adsorption in treetop leaves was not explained by polysaccharides such as Ca-bridged pectin, but could be attributed to the greater cross-sectional area of the transfusion tissue. In both treetop and lower-crown leaves, the band areas of long (free water: around 3550 cm⁻¹) and short (bound water: around 3200 cm⁻¹) hydrogen bonding OH components showed similar increases with increasing RH, while the band area of free water was larger at the treetop leaves regardless of RH. Free water molecules with longer H bonds were considered to be adsorbed loosely to hydrophobic CH surfaces of polysaccharides in the leaf-cross sections.

The physiology of drought stress in grapevine: towards an integrative definition of drought tolerance

Water availability is arguably the most important environmental factor limiting crop growth and productivity. Erratic precipitation patterns and increased temperatures resulting from climate change will likely make drought events more frequent in many regions, increasing the demand on freshwater resources and creating major challenges for agriculture. Addressing these challenges through increased irrigation is not always a sustainable solution so there is a growing need to identify and/or breed drought-tolerant crop varieties in order to maintain sustainability in the context of climate change. Grapevine (Vitis vinifera), a major fruit crop of economic importance, has emerged as a model perennial fruit crop for the study of drought tolerance. This review synthesizes the most recent results on grapevine drought responses, the impact of water deficit on fruit yield and composition, and the identification of drought-tolerant varieties. Given the existing gaps in our knowledge of the mechanisms underlying grapevine drought responses, we aim to answer the following question: how can we move towards a more integrative definition of grapevine drought tolerance?

Drought and frost resistance vary between evergreen and deciduous Atlantic Forest canopy trees

Frost and drought are key stress factors limiting the growth and distribution of tree species. Resistance to stress involves energy costs that may result in trade-offs between different functional traits. Structures or mechanisms that can help to withstand stress imply differences in the carbon economy of the species. Although adaptive responses to frost and drought resistance are usually of a similar nature, they are rarely assessed simultaneously. We investigated these resistance mechanisms in 10 canopy tree species coexisting in the semi-deciduous subtropical forests of northern Argentina. We measured leaf lifespan, anatomical, photosynthetic and water relations traits and performed a thermal analysis in leaves to determined ice nucleation and tissue damage temperatures. Our results showed that evergreen and deciduous species have different adaptive responses to cope with freezing temperatures and water deficits. Evergreen species exhibited cold tolerance, while deciduous species were more resistant to hydraulic dysfunction and showed greater water transport efficiency. Further research is needed to elucidate resistance strategies to stress factors at the whole tree- and stand level, and possible links with hydraulic safety and efficiency among different phenological groups. This will allow us to predict the responses of subtropical forest species to changes in environmental conditions under climate change scenarios.

Linking Forest Flammability and Plant Vulnerability to Drought

Globally, fire regimes are being altered by changing climatic conditions. New fire regimes have the potential to drive species extinctions and cause ecosystem state changes, with a range of consequences for ecosystem services. Despite the co-occurrence of forest fires with drought, current approaches to modelling flammability largely overlook the large body of research into plant vulnerability to drought. Here, we outline the mechanisms through which plant responses to drought may affect forest flammability, specifically fuel moisture and the ratio of dead to live fuels. We present a framework for modelling live fuel moisture content (moisture content of foliage and twigs) from soil water content and plant traits, including rooting patterns and leaf traits such as the turgor loss point, osmotic potential, elasticity and leaf mass per area. We also present evidence that physiological drought stress may contribute to previously observed fuel moisture thresholds in south-eastern Australia. Of particular relevance is leaf cavitation and subsequent shedding, which transforms live fuels into dead fuels, which are drier, and thus easier to ignite. We suggest that capitalising on drought research to inform wildfire research presents a major opportunity to develop new insights into wildfires, and new predictive models of seasonal fuel dynamics.

The sequence and thresholds of leaf hydraulic traits underlying grapevine varietal differences in drought tolerance

Adapting agriculture to climate change is driving the need for the selection and breeding of drought-tolerant crops. The aim of this study was to identify key drought tolerance traits and determine the sequence of their water potential thresholds across three grapevine cultivars with contrasting water use behaviors, Grenache, Syrah, and Semillon. We quantified differences in water use between cultivars and combined this with the determination of other leaf-level traits (e.g. leaf turgor loss point, π TLP), leaf vulnerability to embolism (P50), and the hydraulic safety margin (HSM P50). Semillon exhibited the highest maximum transpiration (Emax), and lowest sensitivity of canopy stomatal conductance (Gc) to vapor pressure deficit (VPD), followed by Syrah and Grenache. Increasing Emax was correlated with more negative water potential at which stomata close (Pgs90), π TLP, and P50, suggesting that increasing water use is associated with hydraulic traits allowing gas exchange under more negative water potentials. Nevertheless, all the cultivars closed their stomata prior to leaf embolism formation. Modeling simulations demonstrated that despite a narrower HSM, Grenache takes longer to reach thresholds of hydraulic failure due to its conservative water use. This study demonstrates that the relationships between leaf hydraulic traits are complex and interactive, stressing the importance of integrating multiple traits in characterizing drought tolerance.

Non-structural carbohydrate dynamics associated with antecedent stem water potential and air temperature in a dominant desert shrub

Non-structural carbohydrates (NSCs) are necessary for plant growth and affected by plant water status, but the temporal dynamics of water stress impacts on NSC are not well understood. We evaluated how seasonal NSC concentrations varied with plant water status (predawn xylem water potential, Ψ) and air temperature (T) in the evergreen desert shrub Larrea tridentata. Aboveground sugar and starch concentrations were measured weekly or monthly for ~1.5 years on 6-12 shrubs simultaneously instrumented with automated stem psychrometers; leaf photosynthesis (A_net ) was measured monthly for 1 year. Leaf sugar increased during the dry, premonsoon period, associated with lower Ψ (greater water stress) and high T. Leaf sugar accumulation coincided with declines in leaf starch and stem sugar, suggesting the prioritization of leaf sugar during low photosynthetic uptake. Leaf starch was strongly correlated with A_net and peaked during the spring and monsoon seasons, while stem starch remained relatively constant except for depletion during the monsoon. Recent photosynthate appeared sufficient to support spring growth, while monsoon growth required the remobilization of stem starch reserves. The coordinated responses of different NSC fractions to water status, photosynthesis, and growth demands suggest that NSCs serve multiple functions under extreme environmental conditions, including severe drought.

Importance of Physiological Traits Vulnerability in Determine Halophytes Tolerance to Salinity Excess: A Comparative Assessment in Atriplex halimus

Many halophytic physiological traits related to the tolerance of plants to salinity excess have been extensively studied, with a focus on biomass and/or gas exchange parameters. To gain a more complete understanding of whether salinity excess affects the physiological performance of halophytes, an experiment was performed using the halophyte Atriplex halimus L. as a model. A. halimus plants were subjected to two salinity treatments (171 and 513 mM NaCl) over 60 days in a controlled environment. After this period, dry biomass, specific stem conductivity, water potential at turgor loss point, osmotic potential, gas exchange parameters, and the fluorescence of chlorophyll a derived parameters were assessed in order to obtain knowledge about the differences in vulnerability that these parameters can show when subjected to salinity stress. Our results showed a decrease in belowground and aboveground biomass. The decrement in biomass seen at 513 mM NaCl was related to photosynthetic limitations and specific stem conductivity. Turgor loss point did not vary significantly with the increment of salinity. Therefore, the parameter that showed less vulnerability to saline stress was the turgor loss point, with only a 5% decrease, and the more vulnerable trait was the stem conductivity, with a reduction of nearly 50%.

Climatic limits of temperate rainforest tree species are explained by xylem embolism resistance among angiosperms but not among conifers

Hydraulic failure explains much of the increased rates of drought-induced tree mortality around the world, underlining the importance of understanding how species distributions are shaped by their vulnerability to embolism. Here we determined which physiological traits explain species climatic limits among temperate rainforest trees in a region where chronic water limitation is uncommon. We quantified the variation in stem embolism vulnerability and leaf turgor loss point among 55 temperate rainforest tree species in New Zealand and tested which traits were most strongly related to species climatic limits. Leaf turgor loss point and stem P50 (tension at which hydraulic conductance is at 50% of maximum) were uncorrelated. Stem P50 and hydraulic safety margin were the most strongly related physiological traits to climatic limits among angiosperms, but not among conifers. Morphological traits such as wood density and leaf dry matter content did not explain species climatic limits. Stem embolism resistance and leaf turgor loss point appear to have evolved independently. Embolism resistance is the most useful predictor of the climatic limits of angiosperm trees. High embolism resistance in the curiously overbuilt New Zealand conifers suggests that their xylem properties may be more closely related to growing slowly under nutrient limitation and to resistance to microbial decomposition.

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought

Drought extent and severity have increased and are predicted to continue to increase in many parts of the world. Understanding tree vulnerability to drought at both individual and species levels is key to ongoing forest management and preparation for future transitions in community composition. The influence of subsurface hydrologic processes is particularly important in water-limited ecosystems, and is an under-studied aspect of tree drought vulnerability. With California's 2013-2016 extraordinary drought as a natural experiment, we studied four co-occurring woodland tree species, blue oak (Quercus douglasii), valley oak (Quercus lobata), gray pine (Pinus sabiniana), and California juniper (Juniperus californica), examining drought vulnerability as a function of climate, lithology and hydrology using regional aerial dieback surveys and site-scale field surveys. We found that in addition to climatic drought severity (i.e., rainfall), subsurface processes explained variation in drought vulnerability within and across species at both scales. Regionally for blue oak, severity of dieback was related to the bedrock lithology, with higher mortality on igneous and metamorphic substrates, and to regional reductions in groundwater. At the site scale, access to deep subsurface water, evidenced by stem water stable isotope composition, was related to canopy condition across all species. Along hillslope gradients, channel locations supported similar environments in terms of water stress across a wide climatic gradient, indicating that subsurface hydrology mediates species' experience of drought, and that areas associated with persistent access to subsurface hydrologic resources may provide important refugia at species' xeric range edges. Despite this persistent overall influence of the subsurface environment, individual species showed markedly different response patterns. We argue that hydrologic niche segregation can be a useful lens through which to interpret these differences in vulnerability to climatic drought and climate change.

A catastrophic tropical drought kills hydraulically vulnerable tree species

Drought-related tree mortality is now a widespread phenomenon predicted to increase in magnitude with climate change. However, the patterns of which species and trees are most vulnerable to drought, and the underlying mechanisms have remained elusive, in part due to the lack of relevant data and difficulty of predicting the location of catastrophic drought years in advance. We used long-term demographic records and extensive databases of functional traits and distribution patterns to understand the responses of 20-53 species to an extreme drought in a seasonally dry tropical forest in Costa Rica, which occurred during the 2015 El Niño Southern Oscillation event. Overall, species-specific mortality rates during the drought ranged from 0% to 34%, and varied little as a function of tree size. By contrast, hydraulic safety margins correlated well with probability of mortality among species, while morphological or leaf economics spectrum traits did not. This firmly suggests hydraulic traits as targets for future research.

Phenotypic plasticity of two M. oleifera ecotypes from different climatic zones under water stress and re-watering

Moringa oleifera is a fast-growing hygrophilic tree native to a humid sub-tropical region of India, now widely planted in many regions of the Southern Hemisphere characterized by low soil water availability. The widespread cultivation of this plant worldwide may have led to populations with different physiological and biochemical traits. In this work, the impact of water stress on the physiology and biochemistry of two M. oleifera populations, one from Chaco Paraguayo (PY) and one from Indian Andhra Pradesh (IA) region, was studied in a screenhouse experiment where the water stress treatment was followed by re-watering. Through transcriptome sequencing, 2201 potential genic simple sequence repeats were identified and used to confirm the genetic differentiation of the two populations. Both populations of M. oleifera reduced photosynthesis, water potential, relative water content and growth under drought, compared to control well-watered plants. A complete recovery of photosynthesis after re-watering was observed in both populations, but growth parameters recovered better in PY than in IA plants. During water stress, PY plants accumulated more secondary metabolites, especially β-carotene and phenylpropanoids, than IA plants, but IA plants invested more into xanthophylls and showed a higher de-epoxidation state of xanthophylls cycle that contributed to protect the photosynthetic apparatus. M. oleifera demonstrated a high genetic variability and phenotypic plasticity, which are key factors for adaptation to dry environments. A higher plasticity (e.g. in PY plants adapted to wet environments) will be a useful trait to endure recurrent but brief water stress episodes, whereas long-term investment of resources into secondary metabolism (e.g. in IA plants adapted to drier environments) will be a successful strategy to cope with prolonged periods of drought. This makes M. oleifera an important resource for agro-forestry in a climate change scenario.

Functional Divergence Drives Invasibility of Plant Communities at the Edges of a Resource Availability Gradient

Invasive Alien Species (IAS) are a serious threat to biodiversity, severely affecting natural habitats and species assemblages. However, no consistent empirical evidence emerged on which functional traits or trait combination may foster community invasibility. Novel insights on the functional features promoting community invasibility may arise from the use of mechanistic traits, like those associated with drought resistance, which have been seldom included in trait-based studies. Here, we tested for the functional strategies of native and invasive assemblage (i.e., environmental filtering hypothesis vs. niche divergence), and we assessed how the functional space determined by native species could influence community invasibility at the edges of a resource availability gradient. Our results showed that invasive species pools need to have a certain degree of differentiation in order to persist in highly invaded communities, suggesting that functional niche divergence may foster community invasibility. In addition, resident native communities more susceptible to invasion are those which, on average, have higher resource acquisition capacity, and lower drought resistance coupled with an apparently reduced water-use efficiency. We advocate the use of a mechanistic perspective in future research to comprehensively understand invasion dynamics, providing also new insights on the factors underlying community invasibility in different ecosystems.

Prediction of leaf water potential and relative water content using terahertz radiation spectroscopy

Increases in the frequency and severity of droughts across many regions worldwide necessitate an improved capacity to determine the water status of plants at organ, whole plant, canopy, and regional scales. Noninvasive methods have most potential for simultaneously improving basic water relations research and ground-, flight-, and space-based sensing of water status, with applications in sustainability, food security, and conservation. The most frequently used methods to measure the most salient proxies of plant water status, that is, water mass per leaf area (WMA), relative water content (RWC), and leaf water potential (Ψleaf), require the excision of tissues and laboratory analysis, and have thus been limited to relatively low throughput and small study scales. Applications using electromagnetic radiation in the visible, infrared, and terahertz ranges can resolve the water status of canopies, yet heretofore have typically focused on statistical approaches to estimating RWC for leaves before and after severe dehydration, and few have predicted Ψleaf. Terahertz radiation has great promise to estimate leaf water status across the range of leaf dehydration important for the control of gas exchange and leaf survival. We demonstrate a refined method and physical model to predict WMA, RWC, and Ψleaf from terahertz transmission across a wide range of levels of dehydration for given leaves of three species, as well as across leaves of given species and across multiple species. These findings highlight the powerful potential and the outstanding challenges in applying in vivo terahertz spectrometry as a remote sensor of water status for a range of applications.

Differential drought resistance strategies of co-existing woodland species enduring the long rainless Eastern Mediterranean summer

In anticipation of a drier climate and to project future changes in forest dynamics, it is imperative to understand species-specific differences in drought resistance. The objectives of this study were to form a comprehensive understanding of the drought resistance strategies adopted by Eastern Mediterranean woodland species, and to elaborate specific ecophysiological traits that can explain the observed variation in survival among these species. We examined leaf water potential (𝛹), gas exchange and stem hydraulics during 2-3 years in mature individuals of the key woody species Phillyrea latifolia L., Pistacia lentiscus L. and Quercus calliprinos Webb that co-exist in a dry woodland experiencing ~ 6 rainless summer months. As compared with the other two similarly functioning species, Phillyrea displayed considerably lower 𝛹 (minimum 𝛹 of -8.7 MPa in Phillyrea vs -4.2 MPa in Pistacia and Quercus), lower 𝛹 at stomatal closure and lower leaf turgor loss point (𝛹TLP ), but reduced hydraulic vulnerability and wider safety margins. Notably, Phillyrea allowed 𝛹 to drop below 𝛹TLP under severe drought, whereas the other two species maintained positive turgor. These results indicate that Phillyrea adopted a more anisohydric drought resistance strategy, while Pistacia and Quercus exhibited a more isohydric strategy and probably relied on deeper water reserves. Unlike the two relatively isohydric species, Phillyrea reached complete stomatal closure at the end of the dry summer. Despite assessing a large number of physiological traits, none of them could be directly related to tree mortality. Higher mortality was observed for Quercus than for the other two species, which may result from higher water consumption due to its 2.5-10 times larger crown volume. The observed patterns suggest that similar levels of drought resistance in terms of survival can be achieved via different drought resistance strategies. Conversely, similar resistance strategies in terms of isohydricity can lead to different levels of vulnerability to extreme drought.

Dung beetles response to livestock management in three different regional contexts

The response of biological communities to human disturbances depends on factors acting at local and regional scale and on the interaction between them. We compared the response of native forest dung beetle communities to cattle grazing under regional contexts differing on precipitation patterns (Atlantic forest and humid and dry Chaco). Through multivariate and GLMM analyses we contrasted richness and composition across regions and land uses and explored the role of local and regional variables accounting for those changes. We captured a total of 44101 individuals of 109 species. The interaction between local and regional variables influenced the response to livestock management. In the two wet regions (humid Chaco and Atlantic forest) diversity was similar in the native forest regardless of cattle presence but differs strongly in open pastures. In contrast, in the dry Chaco, differences between native forest and land use were not evident. Vegetation structure was a major determinant of species richness, whereas regional climate determined differences in species composition. We concluded that the response of dung beetles to livestock management cannot be generalized for all biomes. In dry ecosystems, dung beetles are probably pre-adapted to environmental conditions imposed by cattle ranching whereas in wet ecosystems the impact of cattle ranching is more significant.

Non-structural carbohydrate pools not linked to hydraulic strategies or carbon supply in tree saplings during severe drought and subsequent recovery

Non-structural carbohydrate (NSC) pools fluctuate based on the interplay between photosynthesis, demand from various carbon (C) sinks and tree hydraulic status. Thus, it has been hypothesized that tree species with isohydric stomatal control (i.e., trees that close stomata rapidly in response to drought) rely heavily on NSC pools to sustain metabolism, which can lead to negative physiological consequences such as C depletion. Here, we seek to use a species' degree of isohydry or anisohydry as a conceptual framework for understanding the interrelations between photosynthetic C supply, hydraulic damage and fluctuations in NSC pools. We conducted a 6-week experimental drought, followed by a 6-week recovery period, in a greenhouse on seven tree species that span the spectrum from isohydric to anisohydric. Throughout the experiment, we measured photosynthesis, hydraulic damage and NSC pools. Non-structural carbohydrate pools were remarkably stable across species and tissues-even highly isohydric species that drastically reduced C assimilation were able to maintain stored C. Despite these static NSC pools, we still inferred an important role for stored C during drought, as most species converted starches into sugars during water stress (and back again post-drought). Finally, we did not observe any linkages between C supply, hydraulic damage and NSC pools, indicating that NSC was maintained independent of variation in photosynthesis and hydraulic function. Our results advance the idea that C depletion is a rare phenomenon due to either active maintenance of NSC pools or sink limitation, and thus question the hypothesis that reductions in C assimilation necessarily lead to C depletion.

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

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

A Leaf Selfie: Using a Smartphone to Quantify Leaf Vulnerability to Hydraulic Dysfunction

Accurate predictions of species distribution under current and future climate conditions require modeling efforts based on clear mechanistic relationships between climate variables and plant physiological functions. Vulnerability of leaves to xylem embolism is a key mechanistic trait that might be included in these modeling efforts. Here, we propose a simple set-up to measure leaf vulnerability to embolism on the basis of the optical method using a smartphone, a light source, and a notebook. Our data show that this proposed set-up can adequately quantify the vulnerability to xylem embolism of leaf major veins in Populus nigra and Ostrya carpinifolia, producing values consistent with those obtained in temperate tree species with other methods, allowing virtually any laboratory to quantify species-specific drought tolerance on the basis of a sound mechanistic trait.

Linking water relations and hydraulics with photosynthesis

For land plants, water is the principal governor of growth. Photosynthetic performance is highly dependent on the stable and suitable water status of leaves, which is balanced by the water transport capacity, the water loss rate as well as the water capacitance of the plant. This review discusses the links between leaf water status and photosynthesis, specifically focussing on the coordination of CO₂ and water transport within leaves, and the potential role of leaf capacitance and elasticity on CO₂ and water transport.

Identification of suites of traits that explains drought resistance and phenological patterns of plants in a semi-arid grassland community

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (K_sat), resistance to hydraulic failure (P₅₀), leaf gas exchange, plant and soil water stable isotope ratios (δ¹⁸O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P₅₀ and δ¹⁸O of xylem water explained 67% of differences in phenology, with lower P₅₀ values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P₅₀ values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Leaf ecophysiological and metabolic response in Quercus pyrenaica Willd seedlings to moderate drought under enriched CO2 atmosphere

Impact of drought under enriched CO₂ atmosphere on ecophysiological and leaf metabolic response of the sub-mediterranean Q. pyrenaica oak was studied. Seedlings growing in climate chamber were submitted to moderate drought (WS) and well-watered (WW) under ambient ([CO₂]_amb =400 ppm) or CO₂ enriched atmosphere ([CO₂]_enr =800 ppm). The moderate drought endured by seedlings brought about a decrease in leaf gas exchange. However, net photosynthesis (A_net) was highly stimulated for plants at [CO₂]_enr. There was a decrease of the stomatal conductance to water vapour (g_wv) in response to drought, and a subtle trend to be lower under [CO₂]_enr. The consequence of these changes was an important increase in the intrinsic leaf water use efficiency (WUEi). The electron transport rate (ETR) was almost a 20 percent higher in plants at [CO₂]_enr regardless drought endured by seedlings. The ETR/A_net was lower under [CO₂]_enr, pointing to a high capacity to maintain sinks for the uptake of extra carbon in the atmosphere. Impact of drought on the leaf metabolome, as a whole, was more evident than that from [CO₂] enrichment of the atmosphere. Changes in pool of non-structural carbohydrates were observed mainly as a consequence of water deficit including increases of fructose, glucose, and proto-quercitol. Most of the metabolites affected by drought back up to levels of non-stressed seedlings after rewetting (recovery phase). It can be concluded that carbon uptake was stimulated by [CO₂]_enr, even under the stomatal closure that accompanied moderate drought. In the last, there was a positive effect in intrinsic water use efficiency (WUEi), which was much more improved under [CO₂]_enr. Leaf metabolome was little responsible and some few metabolites changed mainly in response to drought, with little differences between [CO₂] growth conditions.

Cell wall components regulate photosynthesis and leaf water relations of Vitis vinifera cv. Grenache acclimated to contrasting environmental conditions

Environmental conditions determine plants performance as they shape - among other key factors - leaf features and physiology. However, little is known regarding to the changes occurring in leaf cell wall composition during the acclimation to an environmental stress and, specially, if these changes have an impact on other leaf physiology aspects. In order to induce changes in photosynthesis, leaf water relations and cell wall main components (i.e., cellulose, hemicelluloses and pectins) and see how they co-vary, Vitis vinifera cv. Grenache was tested under four different conditions: (i) non-stress conditions (i.e., control, with high summer temperature and irradiance), (ii) growth chamber conditions, (iii) growth chamber under water stress and (iv) cold growth chamber. Plants developed in growth chambers decreased net CO₂ assimilation (A_N) and mesophyll conductance (g_m) compared to control. Although cold did not change the bulk modulus of elasticity (ε), it decreased in growth chamber conditions and water stress. Control treatment showed the highest values for photosynthetic parameters and ε as well as for leaf structural traits such as leaf mass area (LMA) and leaf density (LD). Whereas cellulose content correlated with photosynthetic parameters, particularly A_N and g_m, pectins and the amount of alcohol insoluble residue (AIR) - an approximation of the isolated cell wall fraction - correlated with leaf water parameters, specifically, ε. Although preliminary, our results suggest that cell wall modifications due to environmental acclimations can play a significant role in leaf physiology by affecting distinctly photosynthesis and water relations in a manner that might depend on environmental conditions.

Temporal shifts in iso/anisohydry revealed from daily observations of plant water potential in a dominant desert shrub

Plant species are characterized along a spectrum of isohydry to anisohydry depending on their regulation of water potential (Ψ), but the plasticity of hydraulic strategies is largely unknown. The role of environmental drivers was evaluated in the hydraulic behavior of Larrea tridentata, a drought-tolerant desert shrub that withstands a wide range of environmental conditions. With a 1.5 yr time-series of 2324 in situ measurements of daily predawn and midday Ψ, the temporal variability of hydraulic behavior was explored in relation to soil water supply, atmospheric demand and temperature. Hydraulic behavior in Larrea was highly dynamic, ranging from partial isohydry to extreme anisohydry. Larrea exhibited extreme anisohydry under wet soil conditions corresponding to periods of high productivity, whereas partial isohydry was exhibited after prolonged dry or cold conditions, when productivity was low. Environmental conditions can strongly influence plant hydraulic behavior at relatively fast timescales, which enhances our understanding of plant drought responses. Although species may exhibit a dominant hydraulic behavior, variable environmental conditions can prompt plasticity in Ψ regulation, particularly for species in seasonally dry climates.

Molecular responses to freshwater limitation in the mangrove tree Avicennia germinans (Acanthaceae)

Environmental variation along the geographical space can shape populations by natural selection. In the context of global warming and changing precipitation regimes, it is crucial to understand the role of environmental heterogeneity in tropical trees adaptation, given their disproportional contribution to water and carbon biogeochemical cycles. Here, we investigated how heterogeneity in freshwater availability along tropical wetlands has influenced molecular variations of the black mangrove (Avicennia germinans). A total of 57 trees were sampled at seven sites differing markedly in precipitation regime and riverine freshwater inputs. Using 2,297 genome-wide single nucleotide polymorphic markers, we found signatures of natural selection by the association between variations in allele frequencies and environmental variables, including the precipitation of the warmest quarter and the annual precipitation. Additionally, we found candidate loci for selection based on statistical deviations from neutral expectations of interpopulation differentiation. Most candidate loci within transcribed sequences were functionally associated with central aspects of drought tolerance or plant response to drought. Moreover, our results suggest the occurrence of the rapid evolution of a population, probably in response to sudden and persistent limitations in plant access to soil water, following a road construction in 1974. Observations supporting rapid evolution included the reduction in tree size and changes in allele frequencies and in transcript expression associated with increased drought tolerance through the accumulation of osmoprotectants and antioxidants, biosynthesis of cuticles, protection against protein degradation, stomatal closure, photorespiration and photosynthesis. We describe a major role of spatial heterogeneity in freshwater availability in the specialization of this typically tropical tree.

Leaf Physiological Responses to Drought Stress and Community Assembly in an Asian Savanna

Deciduous dipterocarp forest (DDF) is the most extensive forest type in continental Southeast Asia, but across much of its range is functionally more similar to tropical savannas than tropical forests. We investigated water relations and drought responses of the four dominant tree species (two Shorea and two Dipterocarpus species) of the DDF in central Vietnam to determine how they responded to prolonged periods of drought stress. We quantified leaf water relations in nursery- and field-grown seedlings of the four species and conducted a dry-down experiment on 258 seedlings to study leaf water potential and morphological responses of the seedlings following the drought stress. The two Shorea and two Dipterocarpus species differed significantly in leaf water potential at turgor loss point and osmotic potential at full turgor, but they showed similar responses to drought stress. All species shed leaves and suffered from stem loss when exposed to water potentials lower than their turgor loss point (approximately −1.7 MPa for Dipterocarpus and −2.6 MPa for Shorea species). Upon rewatering, all species resprouted vigorously regardless of the degree of leaf or stem loss, resulting in only 2% whole-plant mortality rate. Our results suggest that none of the four deciduous dipterocarp species is drought tolerant in terms of their water relations; instead, they employ drought-adaptive strategies such as leaf shedding and vigorous resprouting. Given that all species showed similar drought avoidance and drought-adaptive strategies, it is unlikely that seasonal drought directly influences the patterns of species assembly in the DDF of Southeast Asia.

Non-structural carbohydrate and hydraulic dynamics during drought and recovery in Fraxinus ornus and Ostrya carpinifolia saplings

The maintenance of hydraulic function during and after a drought event is crucial for tree survival, but the importance of non-structural carbohydrates (NSCs) in the recovery phase is still debated. We tested whether higher NSC availability facilitates post-drought hydraulic recovery, by applying a short-term drought (S_dr) and a long-term drought combined with shading (L_dr+sh) in Fraxinus ornus and Ostrya carpinifolia. Plants were then re-irrigated and recovery was checked 24 h later, by measuring water potential, stem percentage loss of hydraulic conductance (PLC) and NSC content. The relative magnitude of hydraulic and carbon constraints was also assessed in desiccated plants. During drought, PLC increased only in F. ornus, while it was maintained almost constant in O. carpinifolia due to tighter stomatal control of xylem pressure (i.e. more isohydric). In F. ornus, only S_dr plants maintained high NSC contents at the end of drought and, when re-irrigated, recovered PLC to control values. Whereas hydraulic failure was ubiquitous, only F. ornus depleted NSC reserves at mortality. Our results suggest that preserving higher NSC content at the end of a drought can be important for the hydraulic resilience of trees.

Leveraging plant hydraulics to yield predictive and dynamic plant leaf allocation in vegetation models with climate change

Plant functional traits provide a link in process-based vegetation models between plant-level physiology and ecosystem-level responses. Recent advances in physiological understanding and computational efficiency have allowed for the incorporation of plant hydraulic processes in large-scale vegetation models. However, a more mechanistic representation of water limitation that determines ecosystem responses to plant water stress necessitates a re-evaluation of trait-based constraints for plant carbon allocation, particularly allocation to leaf area. In this review, we examine model representations of plant allocation to leaves, which is often empirically set by plant functional type-specific allometric relationships. We analyze the evolution of the representation of leaf allocation in models of different scales and complexities. We show the impacts of leaf allocation strategy on plant carbon uptake in the context of recent advancements in modeling hydraulic processes. Finally, we posit that deriving allometry from first principles using mechanistic hydraulic processes is possible and should become standard practice, rather than using prescribed allometries. The representation of allocation as an emergent property of scarce resource constraints is likely to be critical to representing how global change processes impact future ecosystem dynamics and carbon fluxes and may reduce the number of poorly constrained parameters in vegetation models.

Seedling response to water stress in valley oak (Quercus lobata) is shaped by different gene networks across populations

Drought is a major stress for plants, creating a strong selection pressure for traits that enable plant growth and survival in dry environments. Many drought responses are conserved species-wide responses, while others vary among populations distributed across heterogeneous environments. We tested how six populations of the widely distributed California valley oak (Quercus lobata) sampled from contrasting climates would differ in their response to soil drying relative to well-watered controls in a common environment by measuring ecophysiological traits in 93 individuals and gene expression (RNA-seq) in 42 individuals. Populations did not differ in their adjustment of turgor loss point during soil drying, suggesting a generalized species-wide response. Differential expression analysis identified 689 genes with a common response to treatment across populations and 470 genes with population-specific responses. Weighted gene co-expression network analysis (WGCNA) identified groups of genes with similar expression patterns that may be regulated together (gene modules). Several gene modules responded differently to water stress among populations, suggesting regional differences in gene network regulation. Populations from sites with a high mean annual temperature responded to the imposed water stress with significantly greater changes in gene module expression, indicating that these populations may be locally adapted to respond to drought. We propose that this variation among valley oak populations provides a mechanism for differential tolerance to the increasingly frequent and severe droughts in California.

Legacies of more frequent drought in ponderosa pine across the western United States

Despite widespread interest in drought legacies-multiyear impacts of drought on tree growth-the key implication of reported drought legacies remains unaddressed: as impaired growth and slow recovery associated with drought legacies are pervasive across forest ecosystems, what is the impact of more frequent drought conditions? We investigated the assumption that either multiple drought years occurring during a short period (multiyear droughts), or droughts occurring during the recovery period from previous drought (compounded droughts), are detrimental to subsequent growth. There is evidence that drought responses may vary among populations of widespread species, leading us to examine regional differences in responses of the conifer Pinus ponderosa to historic drought frequency in the western United States. More frequent drought conditions incurred additional growth declines and shifts in growth-climate sensitivities in the years following drought relative to single-drought events, with 'triple-droughts' being worse than 'double-droughts'. Notably, prediction skill was not strongly reduced when ignoring compounded droughts, a consequence of the temporally comprehensive formulation of our stochastic antecedent model that accounts for the climatic memory of tree growth. We argue that incorporating drought-induced temporal variability in tree growth sensitivities can aid inference gained from statistical models, where more simplistic models could overestimate the severity of drought legacies. We also found regional differences in response to repeated drought, and suggest plastic post-drought sensitivities and climatic memory may represent beneficial physiological adjustments in interior regions. Within-species variability may thus mediate forest responses to increasing drought frequency under future climate change, but experimental approaches using more species are necessary to improve our understanding of the mechanisms that underlie drought legacy effects on tree growth.

How do leaf wetting events affect gas exchange and leaf lifespan of plants from seasonally dry tropical vegetation?

Foliar uptake of dew is likely an important mechanism of water acquisition for plants from tropical dry environments. However, there is still limited experimental evidence describing the anatomical pathways involved in this process and the effects of this water subsidy on the maintenance of gas exchange and leaf lifespan of species from seasonally dry tropical vegetation such as the Brazilian caatinga. We performed scanning electron, bright-field and confocal microscopic analyses and used apoplastic tracers to examine the foliar water uptake (FWU) routes in four woody species with different foliar phenology and widely distributed in the caatinga. Leaves of plants subjected to water stress were exposed to dew simulation to evaluate the effects of the FWU on leaf water potentials, gas exchange and leaf lifespan. All species absorbed water through their leaf cuticles and/or peltate trichomes but FWU capacity differed among species. Leaf wetting by dew increased leaf lifespan duration up to 36 days compared to plants in the drought treatment. A positive effect on leaf gas exchange and new leaf production was only observed in the anisohydric and evergreen species. We showed that leaf wetting by dew is relevant for the physiology and leaf lifespan of plants from seasonally dry tropical vegetation, especially for evergreen species.

Sustainable bioenergy for climate mitigation: developing drought-tolerant trees and grasses

BACKGROUND AND AIMS

Bioenergy crops are central to climate mitigation strategies that utilize biogenic carbon, such as BECCS (bioenergy with carbon capture and storage), alongside the use of biomass for heat, power, liquid fuels and, in the future, biorefining to chemicals. Several promising lignocellulosic crops are emerging that have no food role - fast-growing trees and grasses - but are well suited as bioenergy feedstocks, including Populus, Salix, Arundo, Miscanthus, Panicum and Sorghum.

SCOPE

These promising crops remain largely undomesticated and, until recently, have had limited germplasm resources. In order to avoid competition with food crops for land and nature conservation, it is likely that future bioenergy crops will be grown on marginal land that is not needed for food production and is of poor quality and subject to drought stress. Thus, here we define an ideotype for drought tolerance that will enable biomass production to be maintained in the face of moderate drought stress. This includes traits that can readily be measured in wide populations of several hundred unique genotypes for genome-wide association studies, alongside traits that are informative but can only easily be assessed in limited numbers or training populations that may be more suitable for genomic selection. Phenotyping, not genotyping, is now the major bottleneck for progress, since in all lignocellulosic crops studied extensive use has been made of next-generation sequencing such that several thousand markers are now available and populations are emerging that will enable rapid progress for drought-tolerance breeding. The emergence of novel technologies for targeted genotyping by sequencing are particularly welcome. Genome editing has already been demonstrated for Populus and offers significant potential for rapid deployment of drought-tolerant crops through manipulation of ABA receptors, as demonstrated in Arabidopsis, with other gene targets yet to be tested.

CONCLUSIONS

Bioenergy is predicted to be the fastest-developing renewable energy over the coming decade and significant investment over the past decade has been made in developing genomic resources and in collecting wild germplasm from within the natural ranges of several tree and grass crops. Harnessing these resources for climate-resilient crops for the future remains a challenge but one that is likely to be successful.

Linking coordinated hydraulic traits to drought and recovery responses in a tropical montane cloud forest

PREMISE

Understanding plant hydraulic functioning and water balance during drought has become key in predicting species survival and recovery. However, there are few insightful studies that couple physiological and morphological attributes for many ecosystems, such as the vulnerable Tropical Montane Cloud Forests (TMCF). In this study, we evaluated drought resistance and recovery for saplings for five tree species spanning deciduous to evergreen habits from a Mexican TMCF.

METHODS

In drought simulations, water was withheld until plants reached species-specific P₅₀ or P₈₈ values (pressures required to induce a 50 or 88% loss in hydraulic conductivity), then they were rewatered. Drought resistance was considered within the isohydric-anisohydric framework and compared to leaf gas exchange, water status, pressure-volume curves, specific leaf area, and stomatal density.

RESULTS

The TMCF species closed stomata well before significant losses in hydraulic conductivity (isohydric). Yet, despite the coordination of these traits, the traits were not useful for predicting the time needed for the species to reach critical hydraulic thresholds. Instead, maximum photosynthetic rates explained these times, reinforcing the linkage between hydraulic and carbon dynamics. Despite their varying hydraulic conductivities, stomatal responses, and times to hydraulic thresholds, 58 of the 60 study plants recovered after the rewatering. The recovery of photosynthesis and stomatal conductance can be explained by the P₅₀ values and isohydry.

CONCLUSIONS

This study raises new questions surrounding drought management strategies, recovery processes, and how lethal thresholds are defined. Further studies need to consider the role of water and carbon balance in allowing for both survival and recovery from drought.

Desiccation Tolerance in Chlorophyllous Fern Spores: Are Ecophysiological Features Related to Environmental Conditions?

Fern spores of most species are desiccation tolerant (DT) and, in some cases, are photosynthetic at maturation, the so-called chlorophyllous spores (CS). The lifespan of CS in the dry state is very variable among species. The physiological, biochemical, and biophysical mechanisms underpinning this variability remain understudied and their interpretation from an ecophysiological approach virtually unexplored. In this study, we aimed at fulfilling this gap by assessing photochemical, hydric, and biophysical properties of CS from three temperate species with contrasting biological strategies and longevity in the dry state: Equisetum telmateia (spore maturation and release in spring, ultrashort lifespan), Osmunda regalis (spore maturation and release in summer, medium lifespan), Matteuccia struthiopteris (spore maturation and release in winter, medium-long lifespan). After subjection of CS to controlled drying treatments, results showed that the three species displayed different extents of DT. CS of E. telmateia rapidly lost viability after desiccation, while the other two withstood several dehydration-rehydration cycles without compromising viability. The extent of DT was in concordance with water availability in the sporulation season of each species. CS of O. regalis and M. struthiopteris carried out the characteristic quenching of chlorophyll fluorescence, widely displayed by other DT cryptogams during drying, and had higher tocopherol and proline contents. The turgor loss point of CS is also related to the extent of DT and to the sporulation season: lowest values were found in CS of M. struthiopteris and O. regalis. The hydrophobicity of spores in these two species was higher and probably related to the prevention of water absorption under unfavorable conditions. Molecular mobility, estimated by dynamic mechanical thermal analysis, confirmed an unstable glassy state in the spores of E. telmateia, directly related to the low DT, while the DT species entered in a stable glassy state when dried. Overall, our data revealed a DT syndrome related to the season of sporulation that was characterized by higher photoprotective potential, specific hydric properties, and lower molecular mobility in the dry state. Being unicellular haploid structures, CS represent not only a challenge for germplasm preservation (e.g., as these spores are prone to photooxidation) but also an excellent opportunity for studying mechanisms of DT in photosynthetic cells.

Large branch and leaf hydraulic safety margins in subtropical evergreen broadleaved forest

As a global biodiversity hotspot, the subtropical evergreen broadleaved forest (SEBF) in southern China is strongly influenced by the humid monsoon climate, with distinct hot-wet and cool-dry seasons. However, the hydraulic strategies of this forest are not well understood. Branch and leaf hydraulic safety margins (HSMbranch and HSMleaf, respectively), as well as seasonal changes in predawn and midday leaf water potential (Ψpd and Ψmd), stomatal conductance (Gs), leaf to sapwood area ratio (AL/AS) and turgor loss point (Ψtlp), were examined for woody species in a mature SEBF. For comparison, we compiled these traits of tropical dry forests (TDFs) and Mediterranean-type woodlands (MWs) from the literature because they experience a hot-dry season. We found that on average, SEBF showed larger HSMbranch and HSMleaf than TDF and MW. During the dry season, TDF and MW species displayed a significant decrease in Ψpd and Ψmd. However, SEBF species showed a slight decrease in Ψpd but an increase in Ψmd. Similar to TDF and MW species, Gs was substantially lower in the dry season for SEBF species, but this might be primarily because of the low atmospheric temperature (low vapor pressure deficit). On the other hand, AL/AS and Ψtlp were not significant different between seasons for any SEBF species. Most SEBF species had leaves that were more resistant to cavitation than branches. Additionally, species with stronger leaf-to-branch vulnerability segmentation tended to have smaller HSMleaf but larger HSMbranch. Our results suggest that SEBF is at low hydraulic risk under the current climate.

Large variation in branch and branch-tip hydraulic functional traits in Douglas-fir (Pseudotsuga menziesii) approaching lower treeline

Few studies have quantified intraspecific variation of hydraulic functional traits in conifers across elevation gradients that include range boundaries. In the Intermountain West, USA, the lower elevational limit of forests (lower treeline) is generally assumed to be caused by water limitations to growth and water relations, yet few studies directly show this. To test this assumption, we measured changes in a suite of traits that characterize drought tolerance such as drought-induced hydraulic vulnerability, hydraulic transport capacity and morphological traits in branch tips and branches of Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) along a 400-m elevation gradient in southeastern Idaho that included lower treeline. As elevation decreased, vulnerability to hydraulic dysfunction and maximum conductivity both decreased in branches; some hydraulic safety-efficiency trade-offs were evident. In branch tips, the water potential at the turgor loss point decreased, while maximum conductance increased with decreasing elevation, highlighting that branch-tip-level responses to less moisture availability accompanied by warmer temperatures might not be coordinated with branch responses. As the range boundary was approached, we did not observe non-linear changes in parameters among sites or increased variance within sites, which current ecological hypotheses on range limits suggest. Our results indicate that there is substantial plasticity in hydraulic functional traits in branch tips and branches of Douglas-fir, although the direction of the trends along the elevation gradient sometimes differed between organs. Such plasticity may mitigate the negative impacts of future drought on Douglas-fir productivity, slowing shifts in its range that are expected to occur with climate change.

Rainfall exclusion and thinning can alter the relationships between forest functioning and drought

Increasing drought caused by the ongoing climate change, and forest management by thinning that aims at mitigating its impact, may modify the current relationships between forest functions and drought intensity and preclude our ability to forecast future ecosystem responses. We used 15 yr of data from an experimental rainfall exclusion (-27% of rainfall) combined with thinning (-30% stand basal area) to investigate differences in the drought-function relationships for each component of above-ground net primary productivity (ANPP) and stand transpiration in a Mediterranean Quercus ilex stand. Rainfall exclusion reduced stand ANPP by 10%, mainly because of lowered leaf and acorn production, whereas wood production remained unaffected. These responses were consistent with the temporal sensitivity to drought among tree organs but revealed an increased allocation to wood. Thinning increased wood and acorn production and reduced the sensitivity of standing wood biomass change to drought. Rainfall exclusion and thinning lowered the intercept of the transpiration-drought relationship as a result of the structural constraints exerted by lower leaf and sapwood area. The results suggest that historical drought-function relationships can be used to infer future drought impacts on stand ANPP but not on water fluxes. Thinning can mitigate drought effects and reduce forest sensitivity to drought.

Macro to micro: microwave remote sensing of plant water content for physiology and ecology

Although primarily valued for their suitability for oceanographic applications and soil moisture estimation, microwave remote sensing observations are also sensitive to plant water content (M_w ). Since M_w depends on both plant water status and biomass, these observations have the potential to be useful for a range of plant drought response studies. In this paper, we introduce the principles behind microwave remote sensing observations to illustrate how they are sensitive to plant water content and discuss the relationship between landscape-scale M_w and common stand-scale metrics, including plant-scale relative water content, live fuel moisture content and leaf water potential. Lastly, we discuss how various sensor types can be leveraged for specific applications depending on the spatio-temporal resolution needed.

Greater focus on water pools may improve our ability to understand and anticipate drought-induced mortality in plants

Drought-induced tree mortality has major impacts on ecosystem carbon and water cycles, and is expected to increase in forests across the globe with climate change. A large body of research in the past decade has advanced our understanding of plant water and carbon relations under drought. However, despite intense research, we still lack generalizable, cross-scale indicators of mortality risk. In this Viewpoint, we propose that a more explicit consideration of water pools could improve our ability to monitor and anticipate mortality risk. Specifically, we focus on the relative water content (RWC), a classic metric in plant water relations, as a potential indicator of mortality risk that is physiologically relevant and integrates different aspects related to hydraulics, stomatal responses and carbon economy under drought. Measures of plant water content are likely to have a strong mechanistic link with mortality and to be integrative, threshold-prone and relatively easy to measure and monitor at large spatial scales, and may complement current mortality metrics based on water potential, loss of hydraulic conductivity and nonstructural carbohydrates. We discuss some of the potential advantages and limitations of these metrics to improve our capacity to monitor and predict drought-induced tree mortality.

Thresholds for leaf damage due to dehydration: declines of hydraulic function, stomatal conductance and cellular integrity precede those for photochemistry

Given increasing water deficits across numerous ecosystems world-wide, it is urgent to understand the sequence of failure of leaf function during dehydration. We assessed dehydration-induced losses of rehydration capacity and maximum quantum yield of the photosystem II (F_v /F_m ) in the leaves of 10 diverse angiosperm species, and tested when these occurred relative to turgor loss, declines of stomatal conductance g_s , and hydraulic conductance K_leaf , including xylem and outside xylem pathways for the same study plants. We resolved the sequences of relative water content and leaf water potential Ψ_leaf thresholds of functional impairment. On average, losses of leaf rehydration capacity occurred at dehydration beyond 50% declines of g_s , K_leaf and turgor loss point. Losses of F_v /F_m occurred after much stronger dehydration and were not recovered with leaf rehydration. Across species, tissue dehydration thresholds were intercorrelated, suggesting trait co-selection. Thresholds for each type of functional decline were much less variable across species in terms of relative water content than Ψ_leaf . The stomatal and leaf hydraulic systems show early functional declines before cell integrity is lost. Substantial damage to the photochemical apparatus occurs at extreme dehydration, after complete stomatal closure, and seems to be irreversible.

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

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

Drought response strategies and hydraulic traits contribute to mechanistic understanding of plant dry-down to hydraulic failure

Drought-induced tree mortality alters forest structure and function, yet our ability to predict when and how different species die during drought remains limited. Here, we explore how stomatal control and drought tolerance traits influence the duration of drought stress leading to critical levels of hydraulic failure. We examined the growth and physiological responses of four woody plant species (three angiosperms and one conifer) representing a range of water-use and drought tolerance traits over the course of two controlled drought-recovery cycles followed by an extended dry-down. At the end of the final dry-down phase, we measured changes in biomass ratios and leaf carbohydrates. During the first and second drought phases, plants of all species closed their stomata in response to decreasing water potential, but only the conifer species avoided water potentials associated with xylem embolism as a result of early stomatal closure relative to thresholds of hydraulic dysfunction. The time it took plants to reach critical levels of water stress during the final dry-down was similar among the angiosperms (ranging from 39 to 57 days to stemP88) and longer in the conifer (156 days to stemP50). Plant dry-down time was influenced by a number of factors including species stomatal-hydraulic safety margin (gsP90 - stemP50), as well as leaf succulence and minimum stomatal conductance. Leaf carbohydrate reserves (starch) were not depleted at the end of the final dry-down in any species, irrespective of the duration of drought. These findings highlight the need to consider multiple structural and functional traits when predicting the timing of hydraulic failure in plants.

Is plant survival on green roofs related to their drought response, water use or climate of origin?

Green roofs are novel urban ecosystems with shallow substrate depths and low water availability. Hence, it is critical to select green roof plants that can survive water-deficits, particularly in climates with hot and dry summers. Shrubs are perennial plants which can be drought resistant and may be suitable for green roofs. However, studies about survival and health of shrubs are limited. The aim of this study was to determine whether plant climate of origin aridity, drought response and water use strategies could be used to select shrubs which can survive on green roofs that experience water-deficit. We selected 15 shrub species from a range of climates (dry, mesic and wet) and planted them together in 20 replicate green roof modules with 130 mm deep substrate. We monitored substrate water contents, plant minimum water potentials (ψ_min), health (visual score), percentage survival and related survival with their turgor loss point (ψ_tlp) and water use strategies (evapotranspiration rates in a related glasshouse experiment). We also determined whether plants could recover after dry periods by rewatering after the summer. Mean gravimetric soil water content decreased to approximately 5% after summer drought, which resulted in mortality. Overall, survival ranged between 10% and 100% for the 15 species. However, survival was not related to their ψ_tlp or water use strategies. While shrubs from more arid climates had lower ψ_min in response to dry substrates, this did not result in greater survival and health. Following rewatering, only four shrub species resprouted. Hence, as plant drought response, water use strategy and climate of origin were not strongly related to survival, we suggest survival on green roofs is likely to be determined by a combination of physiological traits. Emergency irrigation for shrubs growing on green roofs in hot and dry climates is recommended during summer to keep them alive.

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.

A simplified framework for fast and reliable measurement of leaf turgor loss point

Drought tolerance shapes the distribution of plant species, and it is mainly determined by the osmotic potential at full turgor (π₀) and the water potential at turgor loss point (Ψ_tlp). We provide a simplified framework for π₀ and Ψ_tlp measurements based on osmometer determination of π₀ (π_{0_osm}). Specifically, we ran regression models to i) improve the predictive power of the estimation of π₀ from π_{0_osm} and morpho-anatomical traits; ii) obtain the most accurate model to predict Ψ_tlp on the basis of the global relationship between π₀ and Ψ_tlp. The inclusion of the leaf dry matter content (LDMC), an easy-to-measure trait, in the regression model improved the predictive power of the estimation of π₀ from π_{0_osm}. When π_{0_osm} was used as a simple predictor of Ψ_tlp, discrepancies arose in comparison with global relationship between π₀ and Ψ_tlp. Ψ_tlp values calculated as a function of the π₀ derived from π_{0_osm} and LDMC (π_{0_fit}) were consistent with the global relationship between π₀ and Ψ_tlp. The simplified framework provided here could encourage the inclusion of mechanistically sound drought tolerance traits in ecological studies.

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s ) and leaf hydraulic conductance (K_Leaf ) across species. All species had similar maximum photosynthetic rates (A_max ). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.