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  1. Summary

    Non‐structural carbohydrates (NSCs), as the labile fraction and dominant carbon currency, are essential mediators of plant adaptation to environments. However, whether and how NSC coordinates with plant economic strategy frameworks, particularly the well‐recognized leaf economics spectrums (LES) and root economics space (RES), remains unclear.

    We examined the relationships between NSC and key plant economics traits in leaves and fine roots across 90 alpine coniferous populations on the Tibetan Plateau, China.

    We observed contrasting coordination of NSC with economics traits in leaves and roots. Leaf total NSC and soluble sugar aligned with the leaf economic spectrum, conveying a trade‐off between growth and storage in leaves. However, NSC in roots was independent of the root economic spectrum, but highly coordinated with root foraging, with more starch and less sugar in forage‐efficient, thinner roots. Further, NSC‐trait coordination in leaves and roots was, respectively, driven by local temperature and precipitation.

    These findings highlight distinct roles of NSC in shaping the above‐ and belowground multidimensional economics trait space, and NSC‐based carbon economics provides a mechanistic understanding of how plants adapt to heterogeneous habitats and respond to environmental changes.

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  2. Summary

    Frequent observations of higher mortality in larger trees than in smaller ones during droughts have sparked an increasing interest in size‐dependent drought‐induced mortality. However, the underlying physiological mechanisms are not well understood, with height‐associated hydraulic constraints often being implied as the potential mechanism driving increased drought vulnerability. We performed a quantitative synthesis on how key traits that drive plant water and carbon economy change with tree height within species and assessed the implications that the different constraints and compensations may have on the interacting mechanisms (hydraulic failure, carbon starvation and/or biotic‐agent attacks) affecting tree vulnerability to drought. While xylem tension increases with tree height, taller trees present a range of structural and functional adjustments, including more efficient water use and transport and greater water uptake and storage capacity, that mitigate the path‐length‐associated drop in water potential. These adaptations allow taller trees to withstand episodic water stress. Conclusive evidence for height‐dependent increased vulnerability to hydraulic failure and carbon starvation, and their coupling to defence mechanisms and pest and pathogen dynamics, is still lacking. Further research is needed, particularly at the intraspecific level, to ascertain the specific conditions and thresholds above which height hinders tree survival under drought.

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  3. Abstract

    Within vascular plants, the partitioning of hydraulic resistance along the soil‐to‐leaf continuum affects transpiration and its response to environmental conditions. In trees, the fractional contribution of leaf hydraulic resistance (Rleaf) to total soil‐to‐leaf hydraulic resistance (Rtotal), or fRleaf(=Rleaf/Rtotal), is thought to be large, but this has not been tested comprehensively. We compiled a multibiome data set of fRleafusing new and previously published measurements of pressure differences within trees in situ. Across 80 samples, fRleafaveraged 0.51 (95% confidence interval [CI] = 0.46−0.57) and it declined with tree height. We also used the allometric relationship between field‐based measurements of soil‐to‐leaf hydraulic conductance and laboratory‐based measurements of leaf hydraulic conductance to compute the average fRleaffor 19 tree samples, which was 0.40 (95% CI = 0.29−0.56). The in situ technique produces a more accurate descriptor of fRleafbecause it accounts for dynamic leaf hydraulic conductance. Both approaches demonstrate the outsized role of leaves in controlling tree hydrodynamics. A larger fRleafmay help stems from loss of hydraulic conductance. Thus, the decline in fRleafwith tree height would contribute to greater drought vulnerability in taller trees and potentially to their observed disproportionate drought mortality.

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  4. Abstract

    Heat and drought affect plant chemical defenses and thereby plant susceptibility to pests and pathogens. Monoterpenes are of particular importance for conifers as they play critical roles in defense against bark beetles. To date, work seeking to understand the impacts of heat and drought on monoterpenes has primarily focused on young potted seedlings, leaving it unclear how older age classes that are more vulnerable to bark beetles might respond to stress. Furthermore, we lack a clear picture of what carbon resources might be prioritized to support monoterpene synthesis under drought stress. To address this, we measured needle and woody tissue monoterpene concentrations and physiological variables simultaneously from mature piñon pines (Pinus edulis) from a unique temperature and drought manipulation field experiment. While heat had no effect on total monoterpene concentrations, trees under combined heat and drought stress exhibited ~ 85% and 35% increases in needle and woody tissue, respectively, over multiple years. Plant physiological variables like maximum photosynthesis each explained less than 10% of the variation in total monoterpenes for both tissue types while starch and glucose + fructose measured 1-month prior explained ~ 45% and 60% of the variation in woody tissue total monoterpene concentrations. Although total monoterpenes increased under combined stress, some key monoterpenes with known roles in bark beetle ecology decreased. These shifts may make trees more favorable for bark beetle attack rather than well defended, which one might conclude if only considering total monoterpene concentrations. Our results point to cumulative and synergistic effects of heat and drought that may reprioritize carbon allocation of specific non-structural carbohydrates toward defense.

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  6. Abstract

    An exponential rise in the atmospheric vapour pressure deficit (VPD) is among the most consequential impacts of climate change in terrestrial ecosystems. Rising VPD has negative and cascading effects on nearly all aspects of plant function including photosynthesis, water status, growth and survival. These responses are exacerbated by land–atmosphere interactions that couple VPD to soil water and govern the evolution of drought, affecting a range of ecosystem services including carbon uptake, biodiversity, the provisioning of water resources and crop yields. However, despite the global nature of this phenomenon, research on how to incorporate these impacts into resilient management regimes is largely in its infancy, due in part to the entanglement of VPD trends with those of other co‐evolving climate drivers. Here, we review the mechanistic bases of VPD impacts at a range of spatial scales, paying particular attention to the independent and interactive influence of VPD in the context of other environmental changes. We then evaluate the consequences of these impacts within key management contexts, including water resources, croplands, wildfire risk mitigation and management of natural grasslands and forests. We conclude with recommendations describing how management regimes could be altered to mitigate the otherwise highly deleterious consequences of rising VPD.

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  8. 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.

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