Ashley M. Matheny

Water potential gradients drive water flow within and between soils and plants, and the internal plant water potential controls a wide range of physiological processes including photosynthesis, growth, and mortality. Notwithstanding this clear relevance for many critical aspects of ecosystem function, water potential data have historically been relatively inaccessible and unnetworked. The absence of a centralized repository for plant water potential time series limits our ability to integrate a wealth of ecophysiological information from other networks and from remote sensing. Closing this gap is necessary to address unresolved questions about plant responses to drought and heat stress, and to make confident predictions about plant and ecosystem function in a warming world. Here, we introduce the PSInet database -- a global collection of plant water potential time series from 285 datasets representing 523 species. We present the workflow that guided database development and evaluate its key features. Through a series of preliminary analyses, we then highlight the potential of the PSInet database for applications including: a) advancing plant water use strategy frameworks; b) disentangling the impacts of soil versus atmospheric drought stress; c) assessing the long-held assumption of pre-dawn equilibration of ecosystem water potential; d) understanding the risk of drought-driven mortality; and e) benchmarking remote-sensing data products and land-surface models.

Mangroves are halophytes that thrive across salinity gradients along tropical and subtropical coastlines. Salinity affects photosynthesis and plant water use, influencing carbon and energy fluxes and ultimately blue carbon storage. However, many plant hydrodynamic models do not account for osmotic potential or osmoregulation, limiting our ability to represent mangrove responses to salinity. Here, we present a modification to the advanced plant hydrodynamic model FETCH4 that incorporates soil osmotic potential, xylem osmoregulation, and salinity‐dependent plant traits. The updated model, FETCH4‐Osmo, couples plant hydraulics with the Farquhar photosynthesis model, allowing plant water potential to regulate stomatal conductance and photosynthesis. The model was parameterized using species‐specific hydraulic and photosynthetic traits of black and white mangroves (Avicennia germinans and A. marina) and evaluated with sap flux and photosynthesis measurements from two field sites: a low to moderate salinity site in Louisiana, USA, and a hypersaline site near Exmouth, Australia. FETCH4‐Osmo reproduced transpiration well ( R ² = 0.80, normalized mean absolute error (NMAE) = 0.19) and improved simulations compared to the original FETCH4 model ( R ² = 0.77, NMAE = 0.37). Under hypersaline conditions, inclusion of osmoregulation substantially improved performance (NMAE = 0.20 vs. 3.78). Comparisons with leaf‐level photosynthesis and stomatal conductance suggest that the model captures salinity effects on carbon fluxes. FETCH4‐Osmo improves our ability to simulate salinity driven controls on water and carbon fluxes in coastal mangrove ecosystems.

Summary: Wood density is central for estimating vegetation carbon storage and a plant functional trait of great ecological and evolutionary importance. However, the global extent of wood density variation is unclear, especially at the intraspecific level. We assembled the most comprehensive wood density collection to date, including 109 626 records from 16 829 plant species across woody life forms and biomes (GWDD v.2, available here: doi: 10.5281/zenodo.16919509 ). Using the GWDD v.2, we explored the sources of wood density variation within individuals, within species and across environmental gradients. Intraspecific variation accounted for c . 15% of overall wood density variation (SD = 0.068 g cm ⁻³ ). Variance was 50% smaller in sapwood than heartwood, and 30% smaller in branchwood than trunkwood. Individuals in extreme environments (dry, hot and acidic soils) had higher wood density than conspecifics elsewhere (+0.02 g cm ⁻³ , c . 4% of the mean). Intraspecific environmental effects strongly tracked interspecific patterns ( r = 0.83) but were 70–80% smaller and varied considerably among taxa. Individual plant wood density was difficult to predict (root mean square error > 0.08 g cm ⁻³ ; single‐measurement R ² = 0.59). We recommend: (1) systematic sampling of multiple individuals and tissues for local applications; and (2) expanded taxonomic coverage combined with integrative models for robust estimates across ecological scales.

Wood density is central for estimating vegetation carbon storage and a plant functional trait of great ecological and evolutionary importance. However, the global extent of wood density variation is unclear, especially at the intraspecific level. We assembled the most comprehensive wood density collection to date (GWDD v.2), including 109,626 records from 16,829 plant species across woody life forms and biomes. Using the GWDD v.2, we explored the sources of variation in wood density within individuals, within species, and across environmental gradients. Intraspecific variation accounted for up to 15% of overall wood density variation (sd = 0.068 g cm ^-3 ). Sapwood densities varied 50% less than heartwood densities, and branchwood densities varied 30% less than trunkwood densities. Individuals in extreme environments (dry, hot, acidic soils) had higher wood density than conspecifics elsewhere (+0.02 g cm ^-3 , ∼4% of the mean). Intraspecific environmental effects strongly tracked interspecific patterns (r = 0.83) but were only 20–30% as large and varied considerably among taxa. Individual plant wood density was difficult to predict (RMSE > 0.08 g cm ^-3 ; single-measurement R ² = 0.59). We recommend (i) systematic within-species sampling for local applications, and (ii) expanded taxonomic coverage combined with integrative models for robust estimates across ecological scales.

Wood density influences how quickly woody plants grow, how long they live and how much carbon they store, yet its global variation remains poorly mapped. Here we combined 109,626 wood density measurements from 16,829 species with 300,949 vegetation plots to produce a km-scale map of community-weighted wood density for every woody biome. Our model led to a prediction accuracy 32–51 % higher than previous global products, and a 1.8–3.7-fold wider wood density range (0.28–1.00 g cm ⁻³ ; global mean: 0.57 g cm ⁻³ ) than previously assumed. Spatial cross-validation showed low bias (±2.5 % of the mean), and uncertainties decreased from 20% in poorly sampled drylands and boreal regions to 5% in data-rich temperate forests. Mean annual temperature was the best predictor of community-weighted mean wood density, increasing by 0.01 g cm ⁻³ for every 1°C change. We deliver a low-bias, high-resolution wood density layer for Earth system models, together with spatially explicit error maps. This study represents a major step forward for carbon accounting and trait-based forecasts of vegetation change.

Mangrove forests are expanding poleward due to ongoing climate change. Near the range limits of mangrove expansion, stressors such as salinity and vapour pressure deficit play a critical role in shaping ecosystem carbon and water fluxes. These stressors, which often compound, are expected to become more severe with ongoing climate change. Here, we analysed the independent and coupled impacts of salinity and VPD stresses on plant hydraulics and photosynthesis in Avicennia germinans in a greenhouse experiment. We exposed A. germinans grown in low‐salinity (10 parts per thousand [ppt]), midrange‐salinity (20 ppt) and high‐salinity (40 ppt) regimes to a 30‐ppt NaCl treatment and to high‐ and low‐VPD conditions. Plants experiencing high osmotic stress had a stronger relationship between ψ _s and VPD than plants experiencing lower osmotic stress, highlighting the impact of compounding stressors on plant hydraulics. Under osmotic stress and non‐limiting VPD conditions, root traits regulated gas exchange and water movement. Under high VPD, the most dominant water‐regulating traits shifted from roots to leaves, with increased stomatal closure acting to conserve water at the cost of reduced photosynthetic uptake. Isohydricity in A. germinans was revealed to be dynamic. Under increased atmospheric and osmotic stress, plants become more isohydric. While under low stress, they behaved more anisohydrically. Plants maximized carbon gain when chances of embolism were low and minimized water loss at the expense of carbon gain under high‐stress scenarios. Dynamic shifts acted as a resilience mechanism against cavitation, allowing plants to survive under a wide range of conditions. Our results highlight the plasticity of A. germinans ' hydraulic strategy and its ability to cope with combined salinity and VPD stresses.

Species‐specific hydraulic traits play an important role in ecosystem response to water stress; however, representation of biodiverse forest canopies remains a challenge in land surface models. We introduce FETCH4, a multispecies, canopy‐level, hydrodynamic model, which builds upon previous versions of the finite‐difference ecosystem‐scale tree crown hydrodynamics model (FETCH). FETCH4 simulates water transport through the soil, roots, and stem as porous media flow. Stomatal conductance is controlled by xylem water potential, which is resolved along the vertical dimension. A key feature of FETCH4 is a multispecies canopy formulation, which uses crown and stem dimensional characteristics to allow the model to produce both tree‐level and plot‐level outputs and improves the representation of hydraulic traits and their variation among trees and species. We demonstrate the model's performance in a mixed temperate forest in Michigan with species of contrasting hydraulic strategies. We optimize species‐specific hydraulic parameters using a Bayesian optimization framework incorporating sapflow measurements. FETCH4 performed well in simulating sapflow of species with contrasting hydraulic strategies under conditions of water stress. In addition, the model was able to capture higher‐level emergent traits, such as drought sensitivity. Using FETCH4 in combination with available observations can provide unique insights about difficult to measure hydraulic traits and plant hydrodynamics.

Vegetation provides a critical nexus between the subsurface, biosphere, and atmosphere through the mediation of the exchange of water, carbon, and energy. Plants respond dynamically to local microclimates at both short and long timescales via mechanisms ranging from physiological behaviors, such as stomatal closure, to acclimation and adaptation. These responses influence land-atmosphere fluxes directly and are therefore crucial to understanding and predicting Earth system responses to a changing climate. As our community progresses towards increasingly physically-realistic models of vegetation responses to the environment, we face several new challenges such as understanding how whole-plant hydraulic strategies can best be represented using limited parameters, connecting non-linearly related observations such as water content and water potential within different organs, and representing responses to simultaneous stressor such as high temperatures and low water supply or high salinities and high evaporative demands. We use vignettes from two long-term tree and ecosystem level studies to demonstrate both progress and pitfalls towards overcoming each of these hurdles in terms of observational understanding of plant function and individual tree-based simulations of new observational data and the accompanying uncertainties. As we progress towards further incorporation of such vegetation ecohydrology modules within climate and weather models, it is becoming increasingly critical to represent well, and with limited parameters the manners in which plants manifest stress responses across time scales in order to better predict the complex feedbacks to carbon, water, and energy fluxes that subsequently develop.

Given the pressing challenges posed by climate change, it is crucial to develop a deeper understanding of the impacts of escalating drought and heat stress on terrestrial ecosystems and the vital services they offer. Soil and plant water potential play a pivotal role in governing the dynamics of water within ecosystems and exert direct control over plant function and mortality risk during periods of ecological stress. However, existing observations of water potential suffer from significant limitations, including their sporadic and discontinuous nature, inconsistent representation of relevant spatio-temporal scales and numerous methodological challenges. These limitations hinder the comprehensive and synthetic research needed to enhance our conceptual understanding and predictive models of plant function and survival under limited moisture availability. In this article, we present PSInet (PSI—for the Greek letter Ψ used to denote water potential), a novel collaborative network of researchers and data, designed to bridge the current critical information gap in water potential data. The primary objectives of PSInet are as follows. (i) Establishing the first openly accessible global database for time series of plant and soil water potential measurements, while providing important linkages with other relevant observation networks. (ii) Fostering an inclusive and diverse collaborative environment for all scientists studying water potential in various stages of their careers. (iii) Standardizing methodologies, processing and interpretation of water potential data through the engagement of a global community of scientists, facilitated by the dissemination of standardized protocols, best practices and early career training opportunities. (iv) Facilitating the use of the PSInet database for synthesizing knowledge and addressing prominent gaps in our understanding of plants’ physiological responses to various environmental stressors. The PSInet initiative is integral to meeting the fundamental research challenge of discerning which plant species will thrive and which will be vulnerable in a world undergoing rapid warming and increasing aridification.