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We measured the sap flux density of eighteen ash trees (Fraxinus spp.) of varying health and canopy conditions across four urban parks in the City of St. Paul, MN, USA in summer 2023 with a low-cost, compact data logger system we designed in-house. Although many ash trees in the city have either been killed or removed to control the spread of Emerald Ash Borer, chemical insecticide treatments are available for trees that are in early stages infestation. The trees selected for the research have all been receiving insecticide treatment for a few years, but their health and canopy conditions vary. We also have collocated temperature, soil moisture, and precipitation measurements at the same site for summer 2023. 

Summary statement We recommend that stomatal slope parameters (g₁) be inferred by inversion so that variations ing₁may be attributed to variations physiological and environmental conditions. Understandingg₁will advance predictions of plant gas exchange and performance under global climate. 

ABSTRACT In snow‐dominated areas, runoff from winter precipitation can comprise up to 80% of the annual water budget. Warming winters are shifting snow precipitation to rain, shortening the snow accumulation and melt seasons, and increasing midwinter melt events and flooding during rain‐on‐snow events. At the same time, forests are changing in species composition and geographical extent, and forest‐dominated catchments can mediate the effects of increased winter temperatures on snow dynamics. Here, we combine climatic data and high‐resolution forest and snow observations to investigate the complex relationships between forest canopy structure and below‐canopy snow depth in two low‐relief Mississippi headwater catchments. To do so, we use a two‐dimensional canopy cover metric (i.e., leaf area index) with forest canopy and understory surveys and catchment terrain (e.g., slope, aspect) to examine their joint influences on snow depth. Results show that (1) co‐dominant tree density is a better predictor of peak snow depth over leaf area index, due to representation of both canopy overlap and interception and (2) canopy structural diversity increases peak snow depth. These results suggest that maintaining forest structural diversity not only contributes to forest health but also allows for a deeper snowpack, thereby increasing the potential for water storage in snow‐dominated low‐relief watersheds. 

Abstract Under increasingly variable rainfall, trends toward more intense and less frequent daily‐scale precipitation have been identified using regional and global averages. However, it has not been explicitly demonstrated whether and where these trends are co‐located, which is important given their potential impacts on land surface processes. Here, using global observation and model‐based data sets, we find that trends toward fewer, larger daily precipitation events are common and relatively distributed across terrestrial ecosystems; they are approximately as common as trends toward more, larger daily precipitation events (which underpin increases in annual precipitation totals). Therefore, widespread precipitation intensification is not consistently increasing annual precipitation totals partly because precipitation events, especially of small‐to‐moderate depths (<10 mm/day), are simultaneously becoming less frequent. Independent of the consequences of changes in mean annual precipitation, these daily‐scale precipitation alterations can substantially impact water resource availability, floods, land‐atmosphere interactions, crop yields, wildfire fuel loads, and carbon sequestration. 

Abstract Plant responses to water stress is a major uncertainty to predicting terrestrial ecosystem sensitivity to drought. Different approaches have been developed to represent plant water stress. Empirical approaches (the empirical soil water stress (or Beta) function and the supply‐demand balance scheme) have been widely used for many decades; more mechanistic based approaches, that is, plant hydraulic models (PHMs), were increasingly adopted in the past decade. However, the relationships between them—and their underlying connections to physical processes—are not sufficiently understood. This limited understanding hinders informed decisions on the necessary complexities needed for different applications, with empirical approaches being mechanistically insufficient, and PHMs often being too complex to constrain. Here we introduce a unified framework for modeling transpiration responses to water stress, within which we demonstrate that empirical approaches are special cases of the full PHM, when the plant hydraulic parameters satisfy certain conditions. We further evaluate their response differences and identify the associated physical processes. Finally, we propose a methodology for assessing the necessity of added complexities of the PHM under various climatic conditions and ecosystem types, with case studies in three typical ecosystems: a humid Midwestern cropland, a semi‐arid evergreen needleleaf forest, and an arid grassland. Notably, Beta function overestimates transpiration when VPD is high due to its lack of constraints from hydraulic transport and is therefore insufficient in high VPD environments. With the unified framework, we envision researchers can better understand the mechanistic bases of and the relationships between different approaches and make more informed choices. 

ABSTRACT Stomata control plant water loss and photosynthetic carbon gain. Developing more generalized and accurate stomatal models is essential for earth system models and predicting responses under novel environmental conditions associated with global change. Plant optimality theories offer one promising approach, but most such theories assume that stomatal conductance maximizes photosynthetic net carbon assimilation subject to some cost orconstraintof water. We move beyond this approach by developing a new, generalized optimality theory of stomatal conductance, optimizing any non‐foliar proxy that requires water and carbon reserves, like growth, survival, and reproduction. We overcome two prior limitations. First, we reconcile the computational efficiency ofinstantaneousoptimization with a more biologically meaningfuldynamic feedbackoptimization over plant lifespans. Second, we incorporatenon‐steady‐statephysics in the optimization to account for the temporal changes in the water, carbon, and energy storage within a plant and its environment that occur over the timescales that stomata act, contrary to previous theories. Our optimal stomatal conductance compares well to observations from seedlings, saplings, and mature trees from field and greenhouse experiments. Our model predicts predispositions to mortality during the 2018 European drought and captures realistic responses to environmental cues, including the partial alleviation of heat stress by evaporative cooling and the negative effect of accumulating foliar soluble carbohydrates, promoting closure under elevated CO₂. We advance stomatal optimality theory by incorporating generalized evolutionary fitness proxies and enhance its utility without compromising its realism, offering promise for future models to more realistically and accurately predict global carbon and water fluxes.