Forests cover roughly one-third of Earth’s land surface, shelter most of the planet’s terrestrial biodiversity, store vast quantities of carbon, and help regulate climate by moving water and energy through the atmosphere. Yet the trees that perform these services are increasingly exposed to longer and more severe droughts. A global field study now suggests that many forest trees may have far less capacity than expected to adjust their internal physiology to prolonged water stress. Rather than substantially redesigning their water-transport systems or boosting their resistance to drought damage, trees in natural forests largely maintained their existing traits while operating increasingly close to dangerous hydraulic limits.
The study, led by researchers from the South China Botanical Garden of the Chinese Academy of Sciences, is based on 40 throughfall reduction experiments carried out in forests around the world. Together, the experiments represent one of the most extensive field-based investigations of drought acclimation—the ability of an organism to adjust its functional traits in response to persistent environmental change. The findings, published in the Proceedings of the National Academy of Sciences on Aug. 24, challenge a widely held assumption that mature trees can gradually adapt to a drier climate by making their water-conducting tissues safer or by modifying their photosynthetic machinery.
The experiments were designed to reproduce drought while preserving the ecological complexity of a real forest. Researchers installed gutters and other collection systems beneath forest canopies to intercept a portion of rainfall before it reached the ground. This reduced the amount of water entering the soil, while trees remained rooted in natural soils and continued growing alongside neighboring plants, fungi, microbes, and other members of their forest communities. The approach avoided a major limitation of many earlier studies, which examined potted seedlings or compared forests located along natural rainfall gradients. Those methods can reveal important biological responses, but they may not fully represent how established trees behave under long-term drought in their natural surroundings.
Across the 40 experiments, researchers examined 24 physiological traits associated with water transport, drought resistance, and carbon acquisition. These included characteristics related to xylem function, the specialized vascular tissue that carries water from roots to leaves; embolism resistance, which describes a tree’s ability to prevent air bubbles from blocking that transport system; hydraulic efficiency; leaf nutrient concentrations; and photosynthetic capacity. Across forests experiencing different climates and levels of water stress, most of these traits changed little. The trees did not substantially increase their resistance to embolism, improve their hydraulic efficiency, or alter their leaf nutrient composition and photosynthetic machinery in ways that would indicate strong physiological acclimation.
That apparent stability may initially sound encouraging, but the study revealed a potentially serious cost. As drought intensified, the water potential of tree tissues declined. Water potential is a measure of the energy status of water inside a plant, and increasingly negative values indicate that the tree is under greater tension as it pulls water from drying soil toward its leaves. At the same time, the trees’ embolism resistance remained largely unchanged. This combination narrowed what scientists call the hydraulic safety margin—the difference between the water conditions a tree normally experiences and the threshold at which its xylem becomes vulnerable to catastrophic water-transport failure.
Hydraulic safety margins function much like a buffer. When the margin is wide, a tree can tolerate a substantial decline in tissue water potential before air enters its xylem and disrupts the continuous water columns needed to supply leaves. When the margin narrows, comparatively small additional changes in soil or atmospheric dryness can push the tree toward hydraulic dysfunction. Once embolism spreads through enough of the xylem, leaves may be starved of water, photosynthesis can collapse, and entire branches or trees may die. The global experiments indicate that trees under prolonged drought were increasingly close to this threshold without making a corresponding physiological adjustment to move the threshold farther away.
The study also identified a striking separation between carbon uptake and carbon storage. Net photosynthesis declined under drought, largely because stomata—the microscopic pores on leaf surfaces that regulate gas exchange—closed to limit water loss. Stomatal closure helps prevent dehydration, but it also restricts the entry of carbon dioxide, reducing the raw material available for photosynthesis. Despite this decline in carbon assimilation, the researchers found that nonstructural carbohydrates, including soluble sugars and starch, remained relatively stable. These compounds serve as internal reserves that can support maintenance, growth, defense, and recovery when conditions improve.
The stability of stored carbohydrates suggests that reduced photosynthesis did not necessarily translate into immediate carbon starvation during the drought periods represented in the experiments. Trees may have drawn on existing reserves, reduced growth, or adjusted the allocation of carbon among different tissues while preserving their internal stores. This finding complicates a common explanation for drought-related tree decline, in which mortality is expected to result primarily from the gradual exhaustion of carbohydrates. The results instead point to hydraulic failure as a potentially more immediate danger, especially when drought causes water transport to approach its physical limits before stored carbon is substantially depleted.
The researchers propose that maintaining existing hydraulic and photosynthetic capacities could still provide an advantage during brief periods of improved water availability. After rainfall, a tree that has not significantly reduced its photosynthetic machinery may be able to resume carbon assimilation rapidly and take advantage of favorable conditions. However, that strategy may carry increasing risks as drought becomes more intense or persistent. A tree can remain capable of rapid carbon gain after rainfall while simultaneously operating with a dangerously narrow hydraulic safety margin. In other words, physiological capacity does not automatically equal physiological safety. The ability to continue functioning under favorable conditions may coexist with a limited ability to withstand further drying.
The findings have broad implications for ecological models and projections of future forests. Many models assume that plants will acclimate to climate change by altering traits such as hydraulic resistance, photosynthetic capacity, leaf chemistry, or drought tolerance. If trees in natural forests generally make only limited adjustments to these traits during prolonged drought, models that assume substantial acclimation could overestimate forest resilience. As droughts become more frequent and severe, the decisive factor may not be whether trees can preserve their carbon reserves, but whether their water-transport systems can continue operating as soil moisture falls and atmospheric demand for water rises. The study’s central warning is therefore clear: forests may retain the machinery needed to recover after rainfall while moving ever closer to irreversible hydraulic damage. Recognizing that narrow margin will be essential for predicting forest mortality, improving climate forecasts, and protecting ecosystems that support biodiversity and human societies.
Subject of Research: The physiological acclimation of forest trees to prolonged drought, with emphasis on hydraulic safety, water transport, photosynthesis, and carbon storage.
Article Title: Declining hydraulic safety in a drier world
News Publication Date: 24-Aug-2026
Web References: https://doi.org/10.1073/pnas.2622754123
References: Proceedings of the National Academy of Sciences; DOI: 10.1073/pnas.2622754123
Image Credits: Image by CHEN Zhicheng
Keywords: forests, drought, climate change, tree physiology, hydraulic safety margin, embolism resistance, xylem, photosynthesis, nonstructural carbohydrates, forest ecosystems, drought acclimation, water stress, plant hydraulics
