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Summary Shifts in the age or turnover time of non‐structural carbohydrates (NSC) may underlie changes in tree growth under long‐term increases in drought stress associated with climate change. But NSC responses to drought are challenging to quantify, due in part to large NSC stores in trees and subsequently long response times of NSC to climate variation.We measured NSC age (Δ¹⁴C) along with a suite of ecophysiological metrics inPinus edulistrees experiencing either extreme short‐term drought (−90% ambient precipitation plot, 2020–2021) or a decade of severe drought (−45% plot, 2010–2021). We tested the hypothesis that carbon starvation – consumption exceeding synthesis and storage – increases the age of sapwood NSC.One year of extreme drought had no impact on NSC pool size or age, despite significant reductions in predawn water potential, photosynthetic rates/capacity, and twig and needle growth. By contrast, long‐term drought halved the age of the sapwood NSC pool, coupled with reductions in sapwood starch concentrations (−75%), basal area increment (−39%), and bole respiration rates (−28%).Our results suggest carbon starvation takes time, as tree carbon reserves appear resilient to extreme disturbance in the short term. However, after a decade of drought, trees apparently consumed old stored NSC to support metabolism. 

Abstract Climate‐driven woody vegetation mortality is a defining feature of semiarid biomes that drives fundamental changes in ecosystem structure. However, the observed impacts of woody mortality on ecosystem‐scale energy and water budgets and the responses of surviving vegetation are highly variable among studies in water‐limited environments. A previous girdling manipulation experiment in a piñon‐juniper woodland suggested that although ecosystem‐scale evapotranspiration was not altered by large‐scale piñon mortality, soil water content decreased and the surviving juniper experienced greater water stress than juniper in an undisturbed woodland. Here we experimentally explored to what extent mortality‐induced changes in energy balance components can explain these results. We compared energy fluxes measured above two adjacent piñon‐juniper woodlands where piñon girdling was implemented at one site and the other subsequently experienced large‐scale natural piñon mortality. We found that the mortality‐induced decrease in canopy area was not sufficient to alter surface reflectance, roughness, and partitioning between energy budget components at both sites. A radiative transfer model estimated that because of the sparse premortality canopy, surface reflectance is more sensitive to a large increase in understory leaf area than further loss of crown area. Increased water stress in the remaining juniper following both mortality events can be explained by an increase in radiation on the ground that promoted higher soil temperature and evaporation. We found similar responses of ecosystem and tree‐level functions to both girdling and natural mortality. This suggests that girdling is an appropriate approach to explore the impact of tree mortality on ecosystem structure, function, and energy balance. 

Abstract Non‐forest ecosystems, dominated by shrubs, grasses and herbaceous plants, provide ecosystem services including carbon sequestration and forage for grazing, and are highly sensitive to climatic changes. Yet these ecosystems are poorly represented in remotely sensed biomass products and are undersampled by in situ monitoring. Current global change threats emphasize the need for new tools to capture biomass change in non‐forest ecosystems at appropriate scales. Here we developed and deployed a new protocol for photogrammetric height using unoccupied aerial vehicle (UAV) images to test its capability for delivering standardized measurements of biomass across a globally distributed field experiment. We assessed whether canopy height inferred from UAV photogrammetry allows the prediction of aboveground biomass (AGB) across low‐stature plant species by conducting 38 photogrammetric surveys over 741 harvested plots to sample 50 species. We found mean canopy height was strongly predictive of AGB across species, with a median adjustedR²of 0.87 (ranging from 0.46 to 0.99) and median prediction error from leave‐one‐out cross‐validation of 3.9%. Biomass per‐unit‐of‐height was similarwithinbut differentamong,plant functional types. We found that photogrammetric reconstructions of canopy height were sensitive to wind speed but not sun elevation during surveys. We demonstrated that our photogrammetric approach produced generalizable measurements across growth forms and environmental settings and yielded accuracies as good as those obtained from in situ approaches. We demonstrate that using a standardized approach for UAV photogrammetry can deliver accurate AGB estimates across a wide range of dynamic and heterogeneous ecosystems. Many academic and land management institutions have the technical capacity to deploy these approaches over extents of 1–10 ha⁻¹. Photogrammetric approaches could provide much‐needed information required to calibrate and validate the vegetation models and satellite‐derived biomass products that are essential to understand vulnerable and understudied non‐forested ecosystems around the globe.