Linking plant hydraulics and the fast-slow continuum to understand resilience to drought in tropical ecosystems

Seasonal Shifts in Tree Water Use and Non-Structural Carbohydrate Storage in a Tropical Dry Forest

Predictions of increased drought frequency and intensity have the potential to threaten to forest globally. The key to trees response to drought is an understanding of tree water use and carbohydrate storage. Our objective was to evaluate sap velocity and dynamics of non-structural carbohydrates (NSC) in native trees of a dry tropical forest, during rainy and drought periods. We evaluated six key species of the Caatinga: three deciduous species with low wood density (WD), two deciduous species with high WD and one evergreen species during the rainy and dry periods. We measured sap velocity, xylem water potential, stomatal conductance, phenology and NSC. We found that the evergreen specie had higher sap velocity and frequent NSC production. While the low deciduous WD species showed low sap velocity, store water and NSC mainly in the stem and roots, and have leaf sprouting and flowering at the end of the dry period. The deciduous high WD also showed low sap velocity, however, with low stored NSC. These results suggest that under longer dry seasons and an irregular rainy seasons, species with low WD that use part of the stored NSC to resprout still during dry season may be the most affected.

Resource Segmentation: A New Dimension of the Segmentation Hypothesis in Drought Adaptive Strategies and Its Links to Tree Growth Performance

The segmentation hypothesis, a framework for understanding plant drought adaptive strategy, has long been based on hydraulic resistance and vulnerability. Storage of water and carbohydrate resources is another critical function and shapes plant drought adaption and fitness together with hydraulic efficiency and vulnerability. However, patterns and implications of the interdependency of stored water and carbohydrate resources in the context of the segmentation hypothesis are poorly understood. We measured resource pools (relative water content [RWC] soluble sugar [SS] and starch [S]) and anatomical features of leaves and supporting twigs for 36 trees in a subtropical population during the dry season when the Budyko's aridity index was 0.362. For each tree, we rank-transformed the RWC (RWCrank), SS (SSrank), and S (Srank) and characterised the resource segmentation within organs using Ln(RWCrank/SSrank) and Ln(RWCrank/Srank). We also assessed the resource segmentation between organs using the difference in resource pools between leaves and twigs (RWCleaf-twig, SSleaf-twig, and Sleaf-twig). Resource segmentation was much more effective than the organ-level resource pool alone in predicting intraspecific variation of tree growth rates. Fast-growing individuals were mainly characterised by lower leaf Ln(RWCrank/SSrank), higher twig Ln(RWCrank/SSrank), and lower SSleaf-twig. The resource segmentation strategy of fast-growing individuals was associated with anatomical attributes that facilitate phloem SS loading and unloading and thus water supply upstream. Our results highlight that resource segmentation is an important dimension of plant drought adaptive strategies and enables better prediction of tree growth vigour than resource pool attributes individually.

Vegetation vulnerability is driven by either higher drought sensitivity or lower fog exposure in tropical cloud ecosystems

Both reduced precipitation and reduced fog uplift increase drought-driven plant mortality. However, it is still unclear how plant vulnerability to drought in cloud ecosystems depends on the role of fog in relieving water stress via foliar water uptake (FWU). To investigate how plants in contrasting montane vegetation rely on fog to alleviate drought impacts, we measured 11 morpho-physiological traits in 10 phylogenetic pairs of plants in a montane grassland (~2000 m a.s.l.) and in a submontane forest (~700 m a.s.l.), both in southeast Brazil. Forest species are more sensitive to drought (i.e., lower conservative trait values, lower resistance to embolism, and lower FWU) than grassland species. Nonetheless, decreased frequency of fog events in the montane grassland may expose these species to a higher risk of dehydration, despite higher FWU capacity. Both forest and grassland vegetation are vulnerable to drought, but the vulnerability is attributable to different causes: higher sensitivity to drought in forests and lower fog exposure in grasslands. Therefore, for a more accurate description of plant responses to drought, we recommend introduction of theoretical-experimental models to assess drought vulnerability to changes in both atmospheric and soil water availability.

Functional traits explain growth response to successive hotter droughts across a wide set of common and future tree species in Europe

In many regions worldwide, forests increasingly suffer from droughts. The 'hotter drought' in Europe in 2018, and the consecutive drought years 2019 and 2020 caused large-scale growth declines and forest dieback. We investigated whether tree growth responses to the 2018-2020 drought can be explained by tree functional traits related to drought tolerance, growth and resource acquisition. We assessed the growth response, that is, growth during drought compared to pre-drought conditions of 71 planted tree species, using branch shoot increments. We used gap-filled trait data related to drought tolerance (P50, stomatal density, conductivity), resource acquisition (SLA, LNC, C:N, Amax) and wood density from the TRY database to explain growth responses, while accounting for differences in growth programmes (spring vs. full-season growing species). We found significantly reduced growth during the 2018 drought across all species. Legacy effects further reduced growth in 2019 and 2020. Gymnosperms showed decreasing growth with increasing P50 and acquisitiveness, such as high SLA, LNC, and Amax. Similar results were found for angiosperms, however, with a less clear pattern. Four distinct response types emerged: 'Sufferer', 'Late sufferer', 'Recoverer' and 'Resister', with gymnosperms predominately appearing as 'Sufferer' and 'Late sufferer'. 'Late sufferers' tended to be spring growing species. This study provides evidence for significant growth reductions and legacy effects in response to consecutive hotter droughts, which can be explained by functional traits across a wide range of tree species when accounting for fundamental growth programmes. We conclude that high drought tolerance bolsters growth reductions, while acquisitive species suffer more from drought.

Trait coordination and trade-offs constrain the diversity of water use strategies in Mediterranean woody plants

The diversity of water-use strategies among dryland plants has been the focus of extensive research, but important knowledge gaps remain. Comprehensive surveys of water-use traits encompassing multiple species growing at contrasting sites are needed to further advance current understanding of plant water use in drylands. Here we show that ecohydrological niche segregation driven by differences in water uptake depth among coexisting species is widespread across Mediterranean plant communities, as evidenced by soil and stem water isotopes measured in 62 native species growing at 10 sites with contrasting climatic conditions. Foliar carbon and oxygen isotopes revealed that leaf-level stomatal regulation stringency and water-use efficiency also differ markedly among coexisting species, and are both coordinated with water uptake depth. Larger and taller woody species use a greater proportion of deeper soil water, display more conservative water use traits at leaf level ("water-savers") and show greater investment in foliage relative to shoots. Conversely, smaller species rely mainly on shallow soil water, exhibit a more profligate water use strategy ("water-spenders") and prioritize investment in shoots over foliage. Drought stress favours coordination between above and belowground water-use traits, resulting in unavoidable trade-offs that constrain the diversity of whole-plant water use strategies in Mediterranean plant communities.

Short- and Long-Term Growth Response to Multiple Drought Episodes: Evidence of Genetic Adaptation in a Conifer Species

Drought tolerance of tree species is a concern in the context of climate change, and tree ring analyses can be used to assess past growth response(s), to drought events. In the current study, we applied this approach to 1281 individuals with known pedigree in long-term genetic test plantations of lodgepole pine in western Canada. We assessed resistance, resilience, and recovery metrics, and analyzed their causal relationships with long-term growth and susceptibility to disease through structural equation modeling. We found that trees with low short-term resilience to drought events also experienced severe reductions in long-term growth. Narrow-sense heritability of drought tolerance metrics was low for short-term responses at specific sites, while a new long-term decline index for families showed moderate heritability (h ^ 2 of 0.15 to 0.30 ± 0.03). We also detected evidence of local adaptation, with trees from lower elevation showing better drought adaptation. We conclude that the selection of genotypes for drought tolerance is possible, and that other species or populations could be screened using this method. We also note that the new long-term decline index developed in this study shows a higher degree of genetic control than other metrices, and may therefore be of broader interest in dendrochronological research.

Douglas fir - A victim of its high productivity in a warming climate? Predominantly negative growth trends in the North German Lowlands

Recent hot droughts and a rising atmospheric vapor pressure deficit are exposing Central European forests to growing stress, causing growth decline, crown damage and elevated mortality of some of the economically most important tree species. Foresters therefore advocate the planting of introduced Douglas fir as a replacement of more vulnerable tree species, but the species' drought and heat resistance is not sufficiently understood. Here, we analyze long-term basal area increment (BAI) trends and the climate sensitivity of growth of 15 mature Douglas fir stands along a precipitation gradient (940-580 mm yr-1) in the North German Lowlands on similar soil. We searched for recent growth declines and assessed the potential of acclimatization to a drier climate. After a pronounced growth increase from 1980 to 2000, BAI has shifted in the last 15 years to a negative trend in the majority of stands, with drier stands being more affected. Thirty percent of the 304 studied trees show significant negative BAI trends, another 47 % non-significant negative trends, compared to 5 % with significant and 12 % with non-significant positive trends. The strongest drivers of a negative BAI trend were climate continentality (seasonal temperature amplitude), a cold February, a negative summer climatic water balance, and low precipitation, indicating declining growth rates especially in continental climates with cold winters and dry summers. A highly significant negative relation exists between recent BAI trend direction and highest growth rate in the past, indicating that faster growth in the past led to greater recent growth decline. We conclude that Douglas fir is more vulnerable to climate change in Central Europe's warmer lowlands than previously thought, which has to be considered in silvicultural planning.

Planted Forests in China Have Higher Drought Risk Than Natural Forests

To improve the environment and mitigate climate change, China has implemented ambitious projects for natural forest protection and expanded planted forests. However, increased climate variability has led to more frequent and severe droughts, exacerbating the decline of these forests. The drought risk of planted forests is rarely assessed by considering both resistance and resilience, and comparative analyses between natural and planted forests are lacking. Here, we compared drought resistance and resilience in natural and planted forests across China using satellite observations from 2001 to 2020 to understand which forests were at higher risk of drought. The results showed that planted forests exhibited lower drought resistance and resilience compared to natural forests, particularly in subtropical broad-leaved evergreen forests and warm temperate deciduous broad-leaved forests. Moreover, drought resistance in planted forests significantly increased, while resilience decreased during 2011-2020 compared to 2001-2010, suggesting a shift in the strategies of planted forests to cope with drought stress. The higher drought risk in planted forests compared to natural forests was mainly attributed to lower forest canopy height and poorer soil nutrients, which limited resistance, and lower canopy height and severe drought characteristics (severity, duration, and frequency), which reduced resilience. These results underscore the higher potential risk of drought exposure in planted forests. To mitigate future drought impacts on planted forests under climate change, enhanced management strategies, including the preservation of natural forests and augmentation of structural diversity in planted forests, are imperative.

Contrasts in hydraulics underlie the divergent performances of Populus and native tree species in water-limited sandy land environments

Populus tree species are commonly used for creating shelter forests in vast areas of northern China, at least partially due to their fast growth. However, they are facing severe problems of decline and mortality caused by drought. In contrast, tree species native to water-limited environments usually have slow growth and are currently not commonly used in afforestation, while these species are gaining more attention in forestry for their greater resilience to drought. In Horqin Sandy Land, we conducted a comparative analysis of xylem hydraulics and associated physiological traits between six Populus tree species and six tree species native to drought-prone areas. Compared to the native species, the Populus species exhibited significantly higher stem hydraulic conductivity but lower resistance to drought-induced xylem embolism than the native tree species. The observed interspecific variations and contrasts in xylem hydraulics between the two species groups were predominantly attributed to xylem anatomical characteristics at the pit level rather than at the tissue level. In line with the divergences in hydraulics, we found significantly lower intrinsic water use efficiency (WUEi) in Populus than in the native species, suggesting that the two groups adopted relatively acquisitive and conservative water use strategies, respectively. The trade-off between hydraulic efficiency and safety, as well as that between hydraulic efficiency and WUEi, underlies the contrasts in performance between Populus species and the native tree species, that is, fast growth of Populus species but high risk of hydraulic dysfunction when facing drought, and vice versa.

Hydraulic and structural traits of trees across light gradients in the Amazon secondary forest

Amazonian species are generally unable to adapt to long drought periods, indicating a low capacity to adjust their hydraulic traits. Secondary forests account for 20% of forest cover in the Amazon, making natural regeneration species crucial under climate change scenarios. In this study, we compared the hydraulic traits of five species, including non-pioneers (Bertholletia excelsa Bonpl., Carapa guianensis Aubl., Hymenaea courbaril L.) and pioneers [Cedrela fissilis Vell., Tabebuia rosea (Bertol.) Bertero ex A.DC.], across light conditions (understory, intermediate, gap) in a 22-year-old secondary forest in Central Amazon, Brazil. Twenty-five saplings were planted and monitored in 3 plots × 5 blocks. Five years after the plantation, we assessed growth, wood density, leaf water potential at predawn and midday, xylem embolism resistance (P50), and hydraulic safety margins (HSM). The leaf water potential ranged from -2.9 to 0 MPa. The non-pioneer species C. guianensis and H. courbaril exhibited the lowest P50 (-4.06 MPa), indicating higher embolism resistance, whereas the pioneer T. rosea had the highest P50 (-1.25 MPa), indicating lower resistance. The HSM varied from -1.60 to 3.26 MPa, with lower values in gap conditions during the dry period (-1.60 MPa), especially affecting pioneer species. Wood density was influenced by both light and species type, with non-pioneers showing a generally higher density, with H. courbaril reaching 0.75 g cm-3 in the understory while the pioneer T. rosea showed the lowest density (0.27 g cm-3). These results highlight that light conditions affect hydraulic traits differently across species strategies, especially during early growth. Non-pioneer, slow-growing native species appear more resilient to light variation, making them suitable for future plantations aimed at climate adaptation in secondary forests.

Conserved responses of water use to evaporative demand in mixed forest across seasons in low subtropical China

The positive correlation between diversity and production has been extensively documented. Given the intrinsic relationship between production and plant water consumption, it was anticipated that mixed forests would exhibit different water use compared to pure forests. In this study, the responses of water use to vapour pressure deficit were analyzed by monitoring the sap flow of Schima superba in both pure and mixed forests, as well as Castanopsis chinensis in mixed forest. Additionally, the relationships among leaf and stem traits were examined by measuring specific leaf area (SLA), N and P concentration per unit leaf mass, leaf δ18O and δ13C and wood density of sapwood (WD) during both wet and dry seasons. The results showed that S. superba demonstrated a comparable regulation of water use during both wet and dry seasons in mixed forest, whereas it exhibited less strict water use regulation during the wet season in comparison to the dry season in pure forest. Regardless of whether the forests were pure or mixed, both leaf δ13C and WD remained consistent across seasons, while there was an increase in SLA during the wet season compared to the dry season for S. superba. There was a different seasonal change in leaf δ18O for S. superba in pure and mixed forests. Water use and leaf economic spectrum may determine the adaptive strategies of coexisting species, and the coexisting tree species in mixed forest exhibited a resource-use differentiation, as indicated by seasonal variations in leaf and stem traits, likely explaining the conserved responses of sap flow to evaporative demand. Our research might provide insights into the impact of tree interaction on water use strategies and the water use-based forest management under current climate change.

Hydraulic traits exert greater limitations on tree-level maximum sap flux density than photosynthetic ability: Global evidence

Transpiration is a key process that couples the land-atmosphere exchange of water and carbon, and its maximum water transport ability affects plant productivity. Functional traits significantly influence the maximum transpiration rate; however, which factor plays the dominant role remains unknown. SAPFLUXNET dataset, which includes sap flux density of diverse species worldwide, provides fundamental data to test the importance of photosynthetic and hydraulic traits on maximum tree-level sap flux density (Js_max). Here, we investigated variations in Js_max of 2194 trees across 129 species using data from the SAPFLUXNET dataset, and analysed the relationship of Js_max with photosynthetic and hydraulic traits. Our results indicated that Js_max was positively correlated with photosynthetic traits at both leaf and tree level. Regarding hydraulic traits, Js_max was positively related to xylem hydraulic conductivity (Ks), leaf-specific hydraulic conductivity (Kl), xylem pressure inducing 50 % loss of hydraulic conductivity (P50), xylem vessel diameter (Vdia), and leaf-to-sapwood area ratio (AlAs). Random forest model showed that 87 % of the variability in Js_max can be explained by functional traits, and hydraulic traits (e.g., P50 and sapwood area, As) exerted larger effects on Js_max than photosynthetic traits. Moreover, trees with a lower sapwood area or depth could increase their sap flux density to compensate for the reduced whole-tree transpiration. Js_max of the angiosperms was significantly higher than that of the gymnosperms. Mean annual total precipitation (MAP) were positively related to Js_max with a weak correlation coefficient. Furthermore, Js_max showed a significant phylogenetic signal with Blomberg's K below 0.2. Overall, tree species with acquisitive resource economics or more efficient hydraulic systems show higher water transport capacity, and the efficiency of xylem hydraulic system rather than the demand for carbon uptake predominantly determines water transport capacity.

Different dry-wet pulses favor different functional strategies: A test using tropical dry forest tree species

In many terrestrial habitats, plants experience temporal heterogeneity in water availability both at the intra and inter annual scales, creating dry-wet pulse scenarios. This variability imposes two concomitant challenges for plants: surviving droughts and efficiently utilizing water when it becomes available, whose responses are closely interconnected. To date, most studies have focused on the response to drought following static designs that do not consider consequences of repeated transitions from one state to the other. In principle, different dry-wet pulse scenarios among years may differentially affect species performance, plant strategies, and promote coexistence through temporal niche separation. We predicted that short frequent droughts would disfavor drought-avoidant species, as rapid leaf loss and production could disrupt their carbon balance, whereas tolerant species, which maintain carbon gain during droughts, should thrive in such conditions. Prolonged droughts might harm tolerant species by causing severe cavitation. We assessed the survival and growth responses of seedlings from 19 tropical dry forest tree species to simulated natural dry-wet pulse scenarios, examining their relationships with the continuum of species' functional strategies under field conditions, and used greenhouse experiments to accompany the field experiment. As expected, different dry-wet pulse scenarios favored different plant functional strategies. Contrary to predictions, the most tolerant outperformed the most avoiders under all drought scenarios, while rapid water-exploiters thrived under non-drought conditions. The superiority of tolerant over avoider species was reverted in the greenhouse, suggesting that in addition to physiology, the fate of species may depend on extrinsic factors as natural enemies. The interplay between the marked variability of dry-wet pulse scenarios across the years and the diversity of water use strategies may contribute to species coexistence in the tropical dry forests. This research is relevant in predicting changes in dominant tree species under future climate scenarios characterized by increased temporal variation in water availability.

Tree drought-mortality risk depends more on intrinsic species resistance than on stand species diversity

Increasing tree diversity is considered a key management option to adapt forests to climate change. However, the effect of species diversity on a forest's ability to cope with extreme drought remains elusive. In this study, we assessed drought tolerance (xylem vulnerability to cavitation) and water stress (water potential), and combined them into a metric of drought-mortality risk (hydraulic safety margin) during extreme 2021 or 2022 summer droughts in five European tree diversity experiments encompassing different biomes. Overall, we found that drought-mortality risk was primarily driven by species identity (56.7% of the total variability), while tree diversity had a much lower effect (8% of the total variability). This result remained valid at the local scale (i.e within experiment) and across the studied European biomes. Tree diversity effect on drought-mortality risk was mediated by changes in water stress intensity, not by changes in xylem vulnerability to cavitation. Significant diversity effects were observed in all experiments, but those effects often varied from positive to negative across mixtures for a given species. Indeed, we found that the composition of the mixtures (i.e., the identities of the species mixed), but not the species richness of the mixture per se, is a driver of tree drought-mortality risk. This calls for a better understanding of the underlying mechanisms before tree diversity can be considered an operational adaption tool to extreme drought. Forest diversification should be considered jointly with management strategies focussed on favouring drought-tolerant species.

Wind Shapes the Growth Strategies of Trees in a Tropical Forest

In tropical forests, trees strategically balance growth patterns to optimise fitness amid multiple environmental stressors. Wind poses the primary risk to a tree's mechanical stability, prompting developments such as thicker trunks to withstand the bending forces. Therefore, a trade-off in resource allocation exists between diameter growth and vertical growth to compete for light. We explore this trade-off by measuring the relative wind mortality risk for 95 trees in a tropical forest in Panama and testing how it varies with tree size, species and wind exposure. Surprisingly, local wind exposure and tree size had minimal impact on wind mortality risk; instead, species wood density emerged as the crucial factor. Low wood density species exhibited a significantly greater wind mortality risk, suggesting a prioritisation of competition for light over biomechanical stability. Our study highlights the pivotal role of wind safety in shaping the life-history strategy of trees and structuring diverse tropical forests.

Rootstocks affect the vulnerability to embolism and pit membrane thickness in Citrus scions

Embolism resistance of xylem tissue varies among species and is an important trait related to drought resistance, with anatomical attributes like pit membrane thickness playing an important role in avoiding embolism spread. Grafted Citrus trees are commonly grown in orchards, with the rootstock being able to affect the drought resistance of the whole plant. Here, we evaluated how rootstocks affect the vulnerability to embolism resistance of the scion using several rootstock/scion combinations. Scions of 'Tahiti' acid lime, 'Hamlin', 'Pera' and 'Valencia' oranges grafted on a 'Rangpur' lime rootstock exhibit similar vulnerability to embolism. In field-grown trees, measurements of leaf water potential did not suggest significant embolism formation during the dry season, while stomata of Citrus trees presented an isohydric response to declining water availability. When 'Valencia' orange scions were grafted on 'Rangpur' lime, 'IAC 1710' citrandarin, 'Sunki Tropical' mandarin or 'Swingle' citrumelo rootstocks, variation in intervessel pit membrane thickness of the scion was found. The 'Rangpur' lime rootstock, which is known for its drought resistance, induced thicker pit membranes in the scion, resulting in higher embolism resistance than the other rootstocks. Similarly, the rootstock 'IAC 1710' citrandarin generated increased embolism resistance of the scion, which is highly relevant for citriculture.

Residual water losses mediate the trade-off between growth and drought survival across saplings of 12 tropical rainforest tree species with contrasting hydraulic strategies

Knowledge of the physiological mechanisms underlying species vulnerability to drought is critical for better understanding patterns of tree mortality. Investigating plant adaptive strategies to drought should thus help to fill this knowledge gap, especially in tropical rainforests exhibiting high functional diversity. In a semi-controlled drought experiment using 12 rainforest tree species, we investigated the diversity in hydraulic strategies and whether they determined the ability of saplings to use stored non-structural carbohydrates during an extreme imposed drought. We further explored the importance of water- and carbon-use strategies in relation to drought survival through a modelling approach. Hydraulic strategies varied considerably across species with a continuum between dehydration tolerance and avoidance. During dehydration leading to hydraulic failure and irrespective of hydraulic strategies, species showed strong declines in whole-plant starch concentrations and maintenance, or even increases in soluble sugar concentrations, potentially favouring osmotic adjustments. Residual water losses mediated the trade-off between time to hydraulic failure and growth, indicating that dehydration avoidance is an effective drought-survival strategy linked to the 'fast-slow' continuum of plant performance at the sapling stage. Further investigations on residual water losses may be key to understanding the response of tropical rainforest tree communities to climate change.

Lianas in tropical dry seasonal forests have a high hydraulic efficiency but not always a higher embolism resistance than lianas in rainforests

BACKGROUND AND AIMS

Lianas have higher relative abundance and biomass in drier seasonal forests than in rainforests, but whether this difference is associated with their hydraulic strategies is unclear. Here, we investigate whether lianas of seasonally dry forests are safer and more efficient in water transport than rainforest lianas, explaining patterns of liana abundance.

METHODS

We measured hydraulic traits on five pairs of congeneric lianas of the tribe Bignonieae in two contrasting forest sites: the wet 'Dense Ombrophilous Forest' in Central Amazonia (~2 dry months) and the drier 'Semideciduous Seasonal Forest' in the inland Atlantic Forest (~6 dry months). We also gathered a broader database, including 197 trees and 58 liana species from different tropical forests, to compare hydraulic safety between habits and forest types.

KEY RESULTS

Bignonieae lianas from both forests had high and similar hydraulic efficiency but exhibited variability in resistance to embolism across forest types when phylogenetic relationships were taken into account. Three genera had higher hydraulic safety in the seasonal forest than in the rainforest, but species across both forests had similar positive hydraulic safety margins despite lower predawn water potential values of seasonal forest lianas. We did not find the safety-efficiency trade-off. Merging our results with previously published data revealed a high variability of resistance to embolism in both trees and lianas, independent of forest types.

CONCLUSIONS

The high hydraulic efficiency of lianas detected here probably favours their rapid growth across tropical forests, but differences in hydraulic safety highlight that some species are highly vulnerable and may rely on other mechanisms to cope with drought. Future research on the lethal dehydration threshold and the connection between hydraulic resistance strategies and liana abundance could offer further insights into tropical forest dynamics under climatic threats.

Amazon forest biogeography predicts resilience and vulnerability to drought

Amazonia contains the most extensive tropical forests on Earth, but Amazon carbon sinks of atmospheric CO2 are declining, as deforestation and climate-change-associated droughts1-4 threaten to push these forests past a tipping point towards collapse5-8. Forests exhibit complex drought responses, indicating both resilience (photosynthetic greening) and vulnerability (browning and tree mortality), that are difficult to explain by climate variation alone9-17. Here we combine remotely sensed photosynthetic indices with ground-measured tree demography to identify mechanisms underlying drought resilience/vulnerability in different intact forest ecotopes18,19 (defined by water-table depth, soil fertility and texture, and vegetation characteristics). In higher-fertility southern Amazonia, drought response was structured by water-table depth, with resilient greening in shallow-water-table forests (where greater water availability heightened response to excess sunlight), contrasting with vulnerability (browning and excess tree mortality) over deeper water tables. Notably, the resilience of shallow-water-table forest weakened as drought lengthened. By contrast, lower-fertility northern Amazonia, with slower-growing but hardier trees (or, alternatively, tall forests, with deep-rooted water access), supported more-drought-resilient forests independent of water-table depth. This functional biogeography of drought response provides a framework for conservation decisions and improved predictions of heterogeneous forest responses to future climate changes, warning that Amazonia's most productive forests are also at greatest risk, and that longer/more frequent droughts are undermining multiple ecohydrological strategies and capacities for Amazon forest resilience.

Community assembly influences plant trait economic spectra and functional trade-offs at ecosystem scales

Plant functional traits hold the potential to greatly improve the understanding and prediction of climate impacts on ecosystems and carbon cycle feedback to climate change. Traits are commonly used to place species along a global conservative-acquisitive trade-off, yet how and if functional traits and conservative-acquisitive trade-offs scale up to mediate community and ecosystem fluxes is largely unknown. Here, we combine functional trait datasets and multibiome datasets of forest water and carbon fluxes at the species, community, and ecosystem-levels to quantify the scaling of the tradeoff between maximum flux and sensitivity to vapor pressure deficit. We find a strong conservative-acquisitive trade-off at the species scale, which weakens modestly at the community scale and largely disappears at the ecosystem scale. Functional traits, particularly plant water transport (hydraulic) traits, are strongly associated with the key dimensions of the conservative-acquisitive trade-off at community and ecosystem scales, highlighting that trait composition appears to influence community and ecosystem flux dynamics. Our findings provide a foundation for improving carbon cycle models by revealing i) that plant hydraulic traits are most strongly associated with community- and ecosystem scale flux dynamics and ii) community assembly dynamics likely need to be considered explicitly, as they give rise to ecosystem-level flux dynamics that differ substantially from trade-offs identified at the species-level.

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

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

Drought- and heat-induced mortality of conifer trees is explained by leaf and growth legacies

An increased frequency and severity of droughts and heat waves have resulted in increased tree mortality and forest dieback across the world, but underlying mechanisms are poorly understood. We used a common garden experiment with 20 conifer tree species to quantify mortality after three consecutive hot, dry summers and tested whether mortality could be explained by putative underlying mechanisms, such as stem hydraulics and legacies affected by leaf life span and stem growth responses to previous droughts. Mortality varied from 0 to 79% across species and was not affected by hydraulic traits. Mortality increased with species' leaf life span probably because leaf damage caused crown dieback and contributed to carbon depletion and bark beetle damage. Mortality also increased with lower growth resilience, which may exacerbate the contribution of carbon depletion and bark beetle sensitivity to tree mortality. Our study highlights how ecological legacies at different time scales can explain tree mortality in response to hot, dry periods and climate change.

Toward a coordinated understanding of hydro-biogeochemical root functions in tropical forests for application in vegetation models

Tropical forest root characteristics and resource acquisition strategies are underrepresented in vegetation and global models, hampering the prediction of forest-climate feedbacks for these carbon-rich ecosystems. Lowland tropical forests often have globally unique combinations of high taxonomic and functional biodiversity, rainfall seasonality, and strongly weathered infertile soils, giving rise to distinct patterns in root traits and functions compared with higher latitude ecosystems. We provide a roadmap for integrating recent advances in our understanding of tropical forest belowground function into vegetation models, focusing on water and nutrient acquisition. We offer comparisons of recent advances in empirical and model understanding of root characteristics that represent important functional processes in tropical forests. We focus on: (1) fine-root strategies for soil resource exploration, (2) coupling and trade-offs in fine-root water vs nutrient acquisition, and (3) aboveground-belowground linkages in plant resource acquisition and use. We suggest avenues for representing these extremely diverse plant communities in computationally manageable and ecologically meaningful groups in models for linked aboveground-belowground hydro-nutrient functions. Tropical forests are undergoing warming, shifting rainfall regimes, and exacerbation of soil nutrient scarcity caused by elevated atmospheric CO2. The accurate model representation of tropical forest functions is crucial for understanding the interactions of this biome with the climate.

Survival, growth, and functional traits of tropical wet forest tree seedlings across an experimental soil moisture gradient in Puerto Rico

Droughts are predicted to become more frequent and intense in many tropical regions, which may cause shifts in plant community composition. Especially in diverse tropical communities, understanding how traits mediate demographic responses to drought can help provide insight into the effects of climate change on these ecosystems. To understand tropical tree responses to reduced soil moisture, we grew seedlings of eight species across an experimental soil moisture gradient at the Luquillo Experimental Forest, Puerto Rico. We quantified survival and growth over an 8-month period and characterized demographic responses in terms of tolerance to low soil moisture-defined as survival and growth rates under low soil moisture conditions-and sensitivity to variation in soil moisture-defined as more pronounced changes in demographic rates across the observed range of soil moisture. We then compared demographic responses with interspecific variation in a suite of 11 (root, stem, and leaf) functional traits, measured on individuals that survived the experiment. Lower soil moisture was associated with reduced survival and growth but traits mediated species-specific responses. Species with relatively conservative traits (e.g., high leaf mass per area), had higher survival at low soil moisture whereas species with more extensive root systems were more sensitive to soil moisture, in that they exhibited more pronounced changes in growth across the experimental soil moisture gradient. Our results suggest that increasing drought will favor species with more conservative traits that confer greater survival in low soil moisture conditions.

Plant hydraulics at the heart of plant, crops and ecosystem functions in the face of climate change

Plant hydraulics is crucial for assessing the plants' capacity to extract and transport water from the soil up to their aerial organs. Along with their capacity to exchange water between plant compartments and regulate evaporation, hydraulic properties determine plant water relations, water status and susceptibility to pathogen attacks. Consequently, any variation in the hydraulic characteristics of plants is likely to significantly impact various mechanisms and processes related to plant growth, survival and production, as well as the risk of biotic attacks and forest fire behaviour. However, the integration of hydraulic traits into disciplines such as plant pathology, entomology, fire ecology or agriculture can be significantly improved. This review examines how plant hydraulics can provide new insights into our understanding of these processes, including modelling processes of vegetation dynamics, illuminating numerous perspectives for assessing the consequences of climate change on forest and agronomic systems, and addressing unanswered questions across multiple areas of knowledge.

Woody species with higher hydraulic efficiency or lower photosynthetic capacity discriminate more against 13C at the global scale

Leaf carbon isotope composition (δ13C) provides an integrative record on the carbon and water balance of plants over long periods. Photosynthetic ability and hydraulic traits which are highly associated with stomatal behavior could affect leaf δ13C. Association between photosynthetic ability and leaf δ13C has been examined, however, how hydraulic traits influence leaf δ13C has not been fully understood. To fill this gap, we investigated the variations in leaf δ13C among 2591 woody species (547 shrub and 2044 tree species), and analyzed the link of leaf δ13C with leaf photosynthetic and xylem hydraulic traits. Our result showed that leaf δ13C was positively correlated to leaf photosynthetic ability and capacity. For hydraulic traits, leaf δ13C was negatively related to hydraulic conductivity (Ks), xylem pressure inducing 50 % loss of hydraulic conductivity (P50) and vessel diameter (Vdia). Associations of leaf δ13C with xylem hydraulic traits indicate woody species with stronger hydraulic safety discriminated less against 13C, while woody species with higher hydraulic efficiency had more negative leaf δ13C. Shrub species, which showed a lower Vdia and P50, had a significant less negative leaf δ13C than tree species. Furthermore, woody species inhabiting in dry regions discriminated less against 13C than those growing in humid regions. Moreover, leaf δ13C displayed a low phylogenetic signal based on Blomberg's K statistic. Overall, woody species with a higher leaf photosynthetic ability or stronger hydraulic safety system discriminated less against 13C and adopt the provident water use strategy.

Water-use strategies and functional traits explain divergent linkages in physiological responses to simulated precipitation change

As a part of global climate change, precipitation patterns in arid regions will change significantly, and the different responses of desert plants to these changes will lead to alterations in community composition, thereby impacting ecosystem stability. Thus, understanding the mechanism underlying the associations among physiological response variables considering changing precipitation is crucial. Here, water-use strategies, functional traits, and physiological processes (e.g., photosynthesis (An), transpiration (Tr), leaf water potential (Ψl), stomatal conductance (gs), and soil respiration (Rs)) were measured in a precipitation experiment with two coexisting desert riparian species to determine how water-use strategies and functional traits operate together in generating physiological response mechanisms. The results showed that the two species exhibited divergent response pathways of physiological processes following rainfall events, although both were identified as isohydric plants with stringent stomatal regulation. For the shallow-rooted species N. sphaerocarpa, gs was sensitive to changes in both surface soil moisture (Swc) and Ψl, and Swc was the primary factor influencing Rs. These results were supported by the preference for shallow water and predominance of functional traits associated with drought avoidance. For the deep-rooted species R. soongorica, variations in gs were decoupled from Swc and directly influenced by enhanced Ψl, An was the main factor affecting Rs, while Ψl negatively affected Rs. These correlations could be attributed to the preference for deep water and functional traits associated with drought tolerance. These findings suggest that R. soongorica had a stronger tolerance to environmental water deficits and may expand extensively under drier climatic conditions in the future.

When do plant hydraulics matter in terrestrial biosphere modelling?

The ascent of water from the soil to the leaves of vascular plants, described by the study of plant hydraulics, regulates ecosystem responses to environmental forcing and recovery from stress periods. Several approaches to model plant hydraulics have been proposed. In this study, we introduce four different versions of plant hydraulics representations in the terrestrial biosphere model T&C to understand the significance of plant hydraulics to ecosystem functioning. We tested representations of plant hydraulics, investigating plant water capacitance, and long-term xylem damages following drought. The four models we tested were a combination of representations including or neglecting capacitance and including or neglecting xylem damage legacies. Using the models at six case studies spanning semiarid to tropical ecosystems, we quantify how plant xylem flow, plant water storage and long-term xylem damage can modulate overall water and carbon dynamics across multiple time scales. We show that as drought develops, models with plant hydraulics predict a slower onset of plant water stress, and a diurnal variability of water and carbon fluxes closer to observations. Plant water storage was found to be particularly important for the diurnal dynamics of water and carbon fluxes, with models that include plant water capacitance yielding better results. Models including permanent damage to conducting plant tissues show an additional significant drought legacy effect, limiting plant productivity during the recovery phase following major droughts. However, when considering ecosystem responses to the observed climate variability, plant hydraulic modules alone cannot significantly improve the overall model performance, even though they reproduce more realistic water and carbon dynamics. This opens new avenues for model development, explicitly linking plant hydraulics with additional ecosystem processes, such as plant phenology and improved carbon allocation algorithms.

The strengthened impact of water availability at interannual and decadal time scales on vegetation GPP

Water availability (WA) is a key factor influencing the carbon cycle of terrestrial ecosystems under climate warming, but its effects on gross primary production (EWA-GPP ) at multiple time scales are poorly understood. We used ensemble empirical mode decomposition (EEMD) and partial correlation analysis to assess the WA-GPP relationship (RWA-GPP ) at different time scales, and geographically weighted regression (GWR) to analyze their temporal dynamics from 1982 to 2018 with multiple GPP datasets, including near-infrared radiance of vegetation GPP, FLUXCOM GPP, and eddy covariance-light-use efficiency GPP. We found that the 3- and 7-year time scales dominated global WA variability (61.18% and 11.95%), followed by the 17- and 40-year time scales (7.28% and 8.23%). The long-term trend also influenced 10.83% of the regions, mainly in humid areas. We found consistent spatiotemporal patterns of the EWA-GPP and RWA-GPP with different source products: In high-latitude regions, RWA-GPP changed from negative to positive as the time scale increased, while the opposite occurred in mid-low latitudes. Forests had weak RWA-GPP at all time scales, shrublands showed negative RWA-GPP at long time scales, and grassland (GL) showed a positive RWA-GPP at short time scales. Globally, the EWA-GPP , whether positive or negative, enhanced significantly at 3-, 7-, and 17-year time scales. For arid and humid zones, the semi-arid and sub-humid zones experienced a faster increase in the positive EWA-GPP , whereas the humid zones experienced a faster increase in the negative EWA-GPP . At the ecosystem types, the positive EWA-GPP at a 3-year time scale increased faster in GL, deciduous broadleaf forest, and savanna (SA), whereas the negative EWA-GPP at other time scales increased faster in evergreen needleleaf forest, woody savannas, and SA. Our study reveals the complex and dynamic EWA-GPP at multiple time scales, which provides a new perspective for understanding the responses of terrestrial ecosystems to climate change.

Quantifying the effects of precipitation exclusion and groundwater drawdown on functional traits of Haloxylon ammodendron - How does this xeric shrub survive the drought?

The increasing frequency of drought and decline in groundwater levels are causing ecophysiological changes in woody plants, particularly in desert ecosystems in arid regions. However, the combined effects of meteorological and hydrological droughts on perennial desert plants, especially phreatophytes, remain poorly understood. To address this knowledge gap, we conducted a 5-year precipitation exclusion experiment at two sites with contrasting groundwater depths in the Gurbantunggut Desert located in northwest China. Our study aimed to investigate the impacts of precipitation exclusion and groundwater depth decline on multiple traits of H. ammodendron. We found that long-term precipitation exclusion enhanced midday leaf water potential, stomatal conductance, chlorophyll content, root nonstructural carbohydrates concentration, leaf starch concentration, but decreased water use efficiency. Groundwater drawdown decreased predawn and midday leaf water potentials, maximum net photosynthetic rate, stomatal conductance, Huber value, stem water δ18O, but enhanced water use efficiency and branch nonstructural carbohydrates concentration. A combination of precipitation exclusion and groundwater depth decline reduced Huber value, but did not show exacerbated effects. The findings demonstrate that hydrological drought induced by groundwater depth decline poses a greater threat to the survival of H. ammodendron than future changes in precipitation.

Wood-density has no effect on stomatal control of leaf-level water use efficiency in an Amazonian forest

Forest disturbances increase the proportion of fast-growing tree species compared to slow-growing ones. To understand their relative capacity for carbon uptake and their vulnerability to climate change, and to represent those differences in Earth system models, it is necessary to characterise the physiological differences in their leaf-level control of water use efficiency and carbon assimilation. We used wood density as a proxy for the fast-slow growth spectrum and tested the assumption that trees with a low wood density (LWD) have a lower water-use efficiency than trees with a high wood density (HWD). We selected 5 LWD tree species and 5 HWD tree species growing in the same location in an Amazonian tropical forest and measured in situ steady-state gas exchange on top-of-canopy leaves with parallel sampling and measurement of leaf mass area and leaf nitrogen content. We found that LWD species invested more nitrogen in photosynthetic capacity than HWD species, had higher photosynthetic rates and higher stomatal conductance. However, contrary to expectations, we showed that the stomatal control of the balance between transpiration and carbon assimilation was similar in LWD and HWD species and that they had the same dark respiration rates.

Vapour pressure deficit modulates hydraulic function and structure of tropical rainforests under nonlimiting soil water supply

Atmospheric conditions are expected to become warmer and drier in the future, but little is known about how evaporative demand influences forest structure and function independently from soil moisture availability, and how fast-response variables (such as canopy water potential and stomatal conductance) may mediate longer-term changes in forest structure and function in response to climate change. We used two tropical rainforest sites with different temperatures and vapour pressure deficits (VPD), but nonlimiting soil water supply, to assess the impact of evaporative demand on ecophysiological function and forest structure. Common species between sites allowed us to test the extent to which species composition, relative abundance and intraspecific variability contributed to site-level differences. The highest VPD site had lower midday canopy water potentials, canopy conductance (gc ), annual transpiration, forest stature, and biomass, while the transpiration rate was less sensitive to changes in VPD; it also had different height-diameter allometry (accounting for 51% of the difference in biomass between sites) and higher plot-level wood density. Our findings suggest that increases in VPD, even in the absence of soil water limitation, influence fast-response variables, such as canopy water potentials and gc , potentially leading to longer-term changes in forest stature resulting in reductions in biomass.

Hydraulic variability of tropical forests is largely independent of water availability

Tropical rainforest woody plants have been thought to have uniformly low resistance to hydraulic failure and to function near the edge of their hydraulic safety margin (HSM), making these ecosystems vulnerable to drought; however, this may not be the case. Using data collected at 30 tropical forest sites for three key traits associated with drought tolerance, we show that site-level hydraulic diversity of leaf turgor loss point, resistance to embolism (P50 ), and HSMs is high across tropical forests and largely independent of water availability. Species with high HSMs (>1 MPa) and low P50 values (< -2 MPa) are common across the wet and dry tropics. This high site-level hydraulic diversity, largely decoupled from water stress, could influence which species are favoured and become dominant under a drying climate. High hydraulic diversity could also make these ecosystems more resilient to variable rainfall regimes.

Accounting for trait variability and coordination in predictions of drought-induced range shifts in woody plants

Functional traits offer a promising avenue to improve predictions of species range shifts under climate change, which will entail warmer and often drier conditions. Although the conceptual foundation linking traits with plant performance and range shifts appears solid, the predictive ability of individual traits remains generally low. In this review, we address this apparent paradox, emphasizing examples of woody plants and traits associated with drought responses at the species' rear edge. Low predictive ability reflects the fact not only that range dynamics tend to be complex and multifactorial, as well as uncertainty in the identification of relevant traits and limited data availability, but also that trait effects are scale- and context-dependent. The latter results from the complex interactions among traits (e.g. compensatory effects) and between them and the environment (e.g. exposure), which ultimately determine persistence and colonization capacity. To confront this complexity, a more balanced coverage of the main functional dimensions involved (stress tolerance, resource use, regeneration and dispersal) is needed, and modelling approaches must be developed that explicitly account for: trait coordination in a hierarchical context; trait variability in space and time and its relationship with exposure; and the effect of biotic interactions in an ecological community context.

Ecophysiological controls on water use of tropical cloud forest trees in response to experimental drought

Tropical montane cloud forests (TMCFs) are expected to experience more frequent and prolonged droughts over the coming century, yet understanding of TCMF tree responses to moisture stress remains weak compared with the lowland tropics. We simulated a severe drought in a throughfall reduction experiment (TFR) for 2 years in a Peruvian TCMF and evaluated the physiological responses of several dominant species (Clusia flaviflora Engl., Weinmannia bangii (Rusby) Engl., Weinmannia crassifolia Ruiz & Pav. and Prunus integrifolia (C. Presl) Walp). Measurements were taken of (i) sap flow; (ii) diurnal cycles of stem shrinkage, stem moisture variation and water-use; and (iii) intrinsic water-use efficiency (iWUE) estimated from foliar δ13C. In W. bangii, we used dendrometers and volumetric water content (VWC) sensors to quantify daily cycles of stem water storage. In 2 years of sap flow (Js) data, we found a threshold response of water use to vapor pressure deficit vapor pressure deficit (VPD) > 1.07 kPa independent of treatment, though control trees used more soil water than the treatment trees. The daily decline in water use in the TFR trees was associated with a strong reduction in both morning and afternoon Js rates at a given VPD. Soil moisture also affected the hysteresis strength between Js and VPD. Reduced hysteresis under moisture stress implies that TMCFs are strongly dependent on shallow soil water. Additionally, we suggest that hysteresis can serve as a sensitive indicator of environmental constraints on plant function. Finally, 6 months into the experiment, the TFR treatment significantly increased iWUE in all study species. Our results highlight the conservative behavior of TMCF tree water use under severe soil drought and elucidate physiological thresholds related to VPD and its interaction with soil moisture. The observed strongly isohydric response likely incurs a cost to the carbon balance of the tree and reduces overall ecosystem carbon uptake.

Functional traits and water-transport strategies of woody species in an insular environment in a tropical forest

PREMISE

Plants survive in habitats with limited resource availability and contrasting environments by responding to variation in environmental factors through morphophysiological traits related to species performance in different ecosystems. However, how different plant strategies influence the megadiversity of tropical species has remained a knowledge gap.

METHODS

We analyzed variations in 27 morphophysiological traits of leaves and secondary xylem in Erythroxylum pulchrum and Tapirira guianensis, which have the highest absolute dominance in these physiognomies and occur together in areas of restinga and dense ombrophilous forest to infer water-transport strategies of Atlantic Forest woody plants.

RESULTS

The two species presented different sets of morphophysiological traits, strategies to avoid embolism and ensure water transport, in different phytophysiognomies. Tapirira guianensis showed possible adaptations influenced by phytophysiognomy, while E. pulchrum showed less variation in the set of characteristics between different phytophysiognomies.

CONCLUSIONS

Our results provide essential tools to understand how the environment can modulate morphofunctional traits and how each species adjusts differently to adapt to different phytophysiognomies. In this sense, the results for these species reveal new species-specific responses in the tropical forest. Such knowledge is a prerequisite to predict future development of the most vulnerable forests as climate changes.

Declining tree growth resilience mediates subsequent forest mortality in the US Mountain West

Climate change-triggered forest die-off is an increasing threat to global forests and carbon sequestration but remains extremely challenging to predict. Tree growth resilience metrics have been proposed as measurable proxies of tree susceptibility to mortality. However, it remains unclear whether tree growth resilience can improve predictions of stand-level mortality. Here, we use an extensive tree-ring dataset collected at ~3000 permanent forest inventory plots, spanning 13 dominant species across the US Mountain West, where forests have experienced strong drought and extensive die-off has been observed in the past two decades, to test the hypothesis that tree growth resilience to drought can explain and improve predictions of observed stand-level mortality. We found substantial increases in growth variability and temporal autocorrelation as well declining drought resistance and resilience for a number of species over the second half of the 20th century. Declining resilience and low tree growth were strongly associated with cross- and within-species patterns of mortality. Resilience metrics had similar explicative power compared to climate and stand structure, but the covariance structure among predictors implied that the effect of tree resilience on mortality could partially be explained by stand and climate variables. We conclude that tree growth resilience offers highly valuable insights on tree physiology by integrating the effect of stressors on forest mortality but may have only moderate potential to improve large-scale projections of forest die-off under climate change.

Optimal balancing of xylem efficiency and safety explains plant vulnerability to drought

In vast areas of the world, forests and vegetation are water limited and plant survival depends on the ability to avoid catastrophic hydraulic failure. Therefore, it is remarkable that plants take hydraulic risks by operating at water potentials (ψ) that induce partial failure of the water conduits (xylem). Here we present an eco-evolutionary optimality principle for xylem conduit design that explains this phenomenon based on the hypothesis that conductive efficiency and safety are optimally co-adapted to the environment. The model explains the relationship between the tolerance to negative water potential (ψ50 ) and the environmentally dependent minimum ψ (ψmin ) across a large number of species, and along the xylem pathway within individuals of two species studied. The wider hydraulic safety margin in gymnosperms compared to angiosperms can be explained as an adaptation to a higher susceptibility to accumulation of embolism. The model provides a novel optimality-based perspective on the relationship between xylem safety and efficiency.

Long-term changes in forest response to extreme atmospheric dryness

Atmospheric dryness, as indicated by vapor pressure deficit (VPD), has a strong influence on forest greenhouse gas exchange with the atmosphere. In this study, we used long-term (10-30 years) net ecosystem productivity (NEP) measurements from 60 forest sites across the world (1003 site-years) to quantify long-term changes in forest NEP resistance and NEP recovery in response to extreme atmospheric dryness. We tested two hypotheses: first, across sites differences in NEP resistance and NEP recovery of forests will depend on both the biophysical characteristics (i.e., leaf area index [LAI] and forest type) of the forest as well as on the local meteorological conditions of the site (i.e., mean VPD of the site), and second, forests experiencing an increasing trend in frequency and intensity of extreme dryness will show an increasing trend in NEP resistance and NEP recovery over time due to emergence of long-term ecological stress memory. We used a data-driven statistical learning approach to quantify NEP resistance and NEP recovery over multiple years. Our results showed that forest types, LAI, and median local VPD conditions explained over 50% of variance in both NEP resistance and NEP recovery, with drier sites showing higher NEP resistance and NEP recovery compared to sites with less atmospheric dryness. The impact of extreme atmospheric dryness events on NEP lasted for up to 3 days following most severe extreme events in most forests, indicated by an NEP recovery of less than 100%. We rejected our second hypothesis as we found no consistent relationship between trends of extreme VPD with trends in NEP resistance and NEP recovery across different forest sites, thus an increase in atmospheric dryness as it is predicted might not increase the resistance or recovery of forests in terms of NEP.

Plant water use theory should incorporate hypotheses about extreme environments, population ecology, and community ecology

Plant water use theory has largely been developed within a plant-performance paradigm that conceptualizes water use in terms of value for carbon gain and that sits within a neoclassical economic framework. This theory works very well in many contexts but does not consider other values of water to plants that could impact their fitness. Here, we survey a range of alternative hypotheses for drivers of water use and stomatal regulation. These hypotheses are organized around relevance to extreme environments, population ecology, and community ecology. Most of these hypotheses are not yet empirically tested and some are controversial (e.g. requiring more agency and behavior than is commonly believed possible for plants). Some hypotheses, especially those focused around using water to avoid thermal stress, using water to promote reproduction instead of growth, and using water to hoard it, may be useful to incorporate into theory or to implement in Earth System Models.

The two sides of resistance-resilience relationship in both aboveground and belowground communities in the Eurasian steppe

The ongoing nitrogen (N) deposition has led to profound changes in aboveground and belowground ecosystems. However, the stability of plant and soil microbial community toward N addition in terms of resistance and resilience is less understood. We established a long-running field trial (2008-2018) in a series of N applications in combination with a mowing and fencing (unmown) treatment in a semiarid steppe. We assessed the resistance via ongoing N treatment of one subplot and the resilience via discontinuing N treatment in another to promote natural recovery since 2014. Plant resistance was negatively correlated with N application rate, while microbial resistance was independent of N rate. Mowing significantly reduced plant resistance and resilience, reduced soil microbial resistance but improved its resilience. Generally, plants are more resilient but less resistant to N than soil microbes. The two sides of resistance-resilience relationship were revealed: trade-offs exist between resistance and resilience for both plants and microbes at the community level; and trade-offs between resistance and resilience cannot be scaled down to species/group level. This study provided an important theoretical basis for the recovery and conservation of semiarid steppe and new insight into resistance-resilience relationship.

Basin-wide variation in tree hydraulic safety margins predicts the carbon balance of Amazon forests

Tropical forests face increasing climate risk1,2, yet our ability to predict their response to climate change is limited by poor understanding of their resistance to water stress. Although xylem embolism resistance thresholds (for example, [Formula: see text]50) and hydraulic safety margins (for example, HSM50) are important predictors of drought-induced mortality risk3-5, little is known about how these vary across Earth's largest tropical forest. Here, we present a pan-Amazon, fully standardized hydraulic traits dataset and use it to assess regional variation in drought sensitivity and hydraulic trait ability to predict species distributions and long-term forest biomass accumulation. Parameters [Formula: see text]50 and HSM50 vary markedly across the Amazon and are related to average long-term rainfall characteristics. Both [Formula: see text]50 and HSM50 influence the biogeographical distribution of Amazon tree species. However, HSM50 was the only significant predictor of observed decadal-scale changes in forest biomass. Old-growth forests with wide HSM50 are gaining more biomass than are low HSM50 forests. We propose that this may be associated with a growth-mortality trade-off whereby trees in forests consisting of fast-growing species take greater hydraulic risks and face greater mortality risk. Moreover, in regions of more pronounced climatic change, we find evidence that forests are losing biomass, suggesting that species in these regions may be operating beyond their hydraulic limits. Continued climate change is likely to further reduce HSM50 in the Amazon6,7, with strong implications for the Amazon carbon sink.

Leaf hydraulics coordinated with leaf economics and leaf size in mangrove species along a salinity gradient

Independence among leaf economics, leaf hydraulics and leaf size confers plants great capability in adapting to heterogeneous environments. However, it remains unclear whether the independence of the leaf traits revealed across species still holds within species, especially under stressed conditions. Here, a suite of traits in these dimensions were measured in leaves and roots of a typical mangrove species, Ceriops tagal, which grows in habitats with a similar sunny and hot environment but different soil salinity in southern China. Compared with C. tagal under low soil salinity, C. tagal under high soil salinity had lower photosynthetic capacity, as indicated directly by a lower leaf nitrogen concentration and higher water use efficiency, and indirectly by a higher investment in defense function and thinner palisade tissue; had lower water transport capacity, as evidenced by thinner leaf minor veins and thinner root vessels; and also had much smaller single leaf area. Leaf economics, hydraulics and leaf size of the mangrove species appear to be coordinated as one trait dimension, which likely stemmed from co-variation of soil water and nutrient availability along the salinity gradient. The intraspecific leaf trait relationship under a stressful environment is insightful for our understanding of plant adaption to the multifarious environments.

Physiological response and photosynthetic recovery to an extreme drought: Evidence from plants in a dry-hot valley savanna of Southwest China

The frequency of extreme drought events has been rising worldwide, but due to its unpredictability, how plants will respond remains poorly understood. Here, we aimed to characterize how the hydraulics and photosynthesis of savanna plants respond to extreme drought, and tested whether they can subsequently recover photosynthesis after drought. There was an extreme drought in 2019 in Southwest (SW) China. We investigated photosynthetic gas exchange, leaf-, stem-, and whole-shoot hydraulic conductance of 18 plant species with diverse leaf habits (deciduous, semi-deciduous and evergreen) and growth forms (tree and shrub) from a dry-hot valley savanna in SW China for three rainy seasons from 2019 to 2021. We also compared photosynthetic gas exchange to those of a regular year (2014). We found that leaf stomatal and hydraulic conductance and maximum photosynthetic rate were significantly lower during the drought in 2019 than in the wetter years. In 2019, all studied plants maintained stomatal conductance at their minimum level observed, which could be related to high vapor pressure deficits (VPD, >2 kPa). However, no significant difference in stem and shoot hydraulic conductance was detected across years. The reductions in leaf hydraulic conductance and stomatal regulation under extreme drought might help keep the stem hydraulic function. Stomatal conductance and photosynthesis after drought (2020 and 2021) showed comparable or even higher values compared to that of 2014, suggesting high recovery of photosynthetic gas exchange. In addition, the response of hydraulic and photosynthetic traits to extreme drought was convergent across leaf habits and growth forms. Our results will help better understand the physiological mechanism underlying the response of savanna ecosystems to climate change.

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

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

Is a seasonally reduced growth potential a convergent strategy to survive drought and frost in plants?

BACKGROUND

Plants have adapted to survive seasonal life-threatening frost and drought. However, the timing and frequency of such events are impacted by climate change, jeopardizing plant survival. Understanding better the strategies of survival to dehydration stress is therefore timely and can be enhanced by the cross-fertilization of research between disciplines (ecology, physiology), models (woody, herbaceous species) and types of stress (drought, frost).

SCOPE

We build upon the 'growth-stress survival' trade-off, which underpins the identification of global plant strategies across environments along a 'fast-slow' economics spectrum. Although phenological adaptations such as dormancy are crucial to survive stress, plant global strategies along the fast-slow economic spectrum rarely integrate growth variations across seasons. We argue that the growth-stress survival trade-off can be a useful framework to identify convergent plant ecophysiological strategies to survive both frost and drought. We review evidence that reduced physiological activity, embolism resistance and dehydration tolerance of meristematic tissues are interdependent strategies that determine thresholds of mortality among plants under severe frost and drought. We show that complete dormancy, i.e. programmed growth cessation, before stress occurrence, minimizes water flows and maximizes dehydration tolerance during seasonal life-threatening stresses. We propose that incomplete dormancy, i.e. the programmed reduction of growth potential during the harshest seasons, could be an overlooked but major adaptation across plants. Quantifying stress survival in a range of non-dormant versus winter- or summer-dormant plants, should reveal to what extent incomplete to complete dormancy could represent a proxy for dehydration tolerance and stress survival.

CONCLUSIONS

Our review of the strategies involved in dehydration stress survival suggests that winter and summer dormancy are insufficiently acknowledged as plant ecological strategies. Incorporating a seasonal fast-slow economics spectrum into global plant strategies improves our understanding of plant resilience to seasonal stress and refines our prevision of plant adaptation to extreme climatic events.

Leaf habit determines the hydraulic and resource-use strategies in tree saplings from the Sonoran Desert

The drought susceptibility of woody saplings may explain their low survival in arid environments. Therefore, it is critical to determine which morphological and physiological traits are more responsive to drought among young plants. This study tested whether plant responses to experimental drought differ between two plant functional groups: the deciduous and evergreen species. We predicted that deciduous species would present a tighter stomatal control under drought, coupled with fast carbon fixation under no stress, tending toward isohydry and faster growth rates than the evergreen species. Using 1-year-old saplings from three evergreen and four deciduous Sonoran Desert tree species, we evaluated their hydraulic and gas exchange traits under three experimental irrigation conditions: high, intermediate and low water availability. We measured CO2 assimilation rates (A), stomatal conductance (gs), the level of iso-anisohydry (as the plant's ability to maintain constant their water potential) and seven morphological and growth-related traits throughout 2 months. Under high water availability, saplings reached their maximum values of A and gs, which were significantly higher for deciduous than evergreen species. Correlations among hydroscape area (HA) and leaf traits positioned species along the iso/anisohydric continuum. Deciduous species presented isohydric characteristics, including low HA, high gs, A and Huber values (HVs), and traits indicative of a faster use of resources, such as low stem-specific density (SSD) and low leaf mass per area (LMA). By contrast, evergreen species showed traits that indicate slow resource use and anisohydric behavior, such as high HA, SSD and LMA, and low gs, A and HVs. Deciduous species drastically reduced gas exchange rates in response to drought, while evergreen maintained low rates independently of drought intensity. Overall, desert saplings showed strategies concordant with the iso-anisohydric continuum and the fast-slow use of resources.

The evolution and expression of stomatal regulators in C3 and C4 crops: Implications on the divergent drought tolerance

Drought stress is a major environmental hazard. Stomatal development is highly responsive to abiotic stress and has been used as a cellular marker for drought-tolerant crop selection. C3 and C4 crops have evolved into different photosynthetic systems and physiological responses to water deficits. The genome sequences of maize, sorghum, and sugarcane make it possible to explore the association of the stomatal response to drought stress with the evolution of the key stomatal regulators. In this study, phylogenic analysis, gene expression analysis and stomatal assay under drought stress were used to investigate the drought tolerance of C3 and C4 plants. Our data shows that C3 and C4 plants exhibit different drought responses at the cellular level. Drought represses the growth and stomatal development of C3 crops but has little effect on that of C4 plants. In addition, stomatal development is unresponsive to drought in drought-tolerant C3 crops but is repressed in drought-tolerant C4 plants. The different developmental responses to drought in C3 and C4 plants might be associated with the divergent expression of their SPEECHLESS genes. In particular, C4 crops have evolved to generate multiple SPEECHLESS homologs with different genetic structure and expression levels. Our research provides not only molecular evidence that supports the evolutionary history of C4 from C3 plants but also a possible molecular model that controls the cellular response to abiotic stress in C3 and C4 crops.

The other side of tropical forest drought: do shallow water table regions of Amazonia act as large-scale hydrological refugia from drought?

Tropical forest function is of global significance to climate change responses, and critically determined by water availability patterns. Groundwater is tightly related to soil water through the water table depth (WT), but historically neglected in ecological studies. Shallow WT forests (WT < 5 m) are underrepresented in forest research networks and absent in eddy flux measurements, although they represent c. 50% of the Amazon and are expected to respond differently to global-change-related droughts. We review WT patterns and consequences for plants, emerging results, and advance a conceptual model integrating environment and trait distributions to predict climate change effects. Shallow WT forests have a distinct species composition, with more resource-acquisitive and hydrologically vulnerable trees, shorter canopies and lower biomass than deep WT forests. During 'normal' climatic years, shallow WT forests have higher mortality and lower productivity than deep WT forests, but during moderate droughts mortality is buffered and productivity increases. However, during severe drought, shallow WT forests may be more sensitive due to shallow roots and drought-intolerant traits. Our evidence supports the hypothesis of neglected shallow WT forests being resilient to moderate drought, challenging the prevailing view of widespread negative effects of climate change on Amazonian forests that ignores WT gradients, but predicts they could collapse under very strong droughts.