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

Forests are intricate biological systems where multiple tree species coexist and interact, often enhancing ecosystem function through complementary resource use and mutualistic relationships. Previous studies have suggested that increased tree species diversity generally improves forest stability and productivity, particularly under stressful conditions like drought. However, the new findings indicate that the benefits of tree diversity are contingent on drought duration and intensity, with positive effects evident during short drought events but potentially reversed when droughts extend beyond a single season or span multiple years. This bidirectional response underscores the intricate balance between facilitative and competitive interactions shaped by environmental pressures.

The research team employed an extensive dataset from TreeDivNet, the world’s largest network of tree diversity experiments. This network encompasses nine large-scale, controlled tree plantations distributed across diverse climatic zones in Europe, ranging from Mediterranean to boreal environments. By analyzing tree ring data from 948 samples representing 21 species grown in monocultures and mixed species plots, the scientists isolated the functional outcomes of diversity under drought conditions. Such dendrochronological analyses, coupled with advanced X-ray tomography, enabled precise quantification of annual growth increments as well as internal wood structure changes related to water transport efficiency.

Their analysis revealed an intriguing temporal dimension to the functional diversity effects. During single-season droughts, mixed-species stands often exhibited enhanced growth responses compared to monocultures, likely due to complementary water uptake and improved microclimatic buffering. Different species’ root architectures and hydraulic strategies may allow more efficient soil moisture utilization, thus ameliorating drought impacts for the community. Conversely, under multi-year drought scenarios, these positive effects diminished or even reversed. The prolonged water scarcity intensified interspecific competition, leading to reduced growth across mixtures. This finding suggests that drought duration critically modulates the net outcomes of biodiversity on forest productivity.

The spatial heterogeneity intrinsic to these results further complicates the interpretation. Climatic, edaphic, and stand-scale factors influenced whether tree diversity conferred drought resilience or amplified stress effects. In some geographic locations, species mixtures mitigated drought-driven decline; in others, they exacerbated water competition and hydraulic failure risk. This spatial variability points to an urgent need for forest managers to tailor species selection and silvicultural approaches to local environmental contexts rather than relying on generalized prescriptions predicated solely on species richness.

Technically, the combination of dendrochronology and X-ray tomography represents a significant advance in ecological research methods. Dendrochronology provides time-resolved insights into growth variability and stress episodes over decades, while X-ray tomography offers a non-destructive window into wood anatomical traits that govern hydraulic function. Together, these methodologies elucidate the mechanistic underpinnings linking species diversity to physiological performance under drought. By mapping annual growth rings alongside microstructural changes in xylem vessels, researchers can better understand how trees adjust their water transport capacity in response to environmental fluctuations.

Beyond academic insights, this study delivers a crucial message for forest conservation and climate adaptation policies. It challenges the simplistic narrative advocating increased species numbers as a universal drought mitigation strategy. Instead, achieving resilient forest ecosystems necessitates selecting species assemblages whose physiological traits and water use strategies complement each other, minimizing antagonistic interactions during extended dry spells. Adaptive management regimes incorporating ecological knowledge and local forestry experience will be paramount in navigating this complexity.

Lead author Hernán Serrano-León emphasizes that the study’s results do not dismiss the value of biodiversity but advocate for a shift towards precision forestry. “Our findings show that diversity’s role in drought resilience is fundamentally context-dependent,” he explains. “To construct forests capable of thriving amid escalating climatic extremes, we must harmonize species choice with site-specific conditions and dynamic environmental feedbacks.” This integrative approach may involve mixing drought-tolerant species with others that maintain hydraulic function under stress, optimizing the community assembly over time.

The significance of this research extends beyond European temperate forests to global biomes vulnerable to increasing drought frequencies and magnitudes. Mixed-species plantations, which are gaining prominence as sustainable alternatives to monoculture forestry, stand at a crossroads where their design critically influences ecosystem stability. Detailed mechanistic understanding, such as that provided by the MixForChange and CAMBIO projects supporting this work, is essential to guide afforestation efforts worldwide to maximize carbon sequestration and biodiversity conservation while enhancing resilience to climate perturbations.

Moreover, the long-term ecological data harnessed in this study illuminate the potential pitfalls of adopting uniform forestry prescriptions in an era of rapid environmental change. The nuanced responses of tree growth to multi-year drought dynamics underscore the complex feedback loops governing ecosystem productivity. As climate models predict amplified drought regimes across many regions, forestry science must integrate these temporal dimensions into species selection and management planning to safeguard ecosystem services.

This pioneering investigation also presses the scientific community to pursue further interdisciplinary studies combining field experiments, remote sensing, and physiological modeling. Only by bridging scales from cellular hydraulics to landscape processes can researchers unravel the conditional benefits and trade-offs associated with tree diversity under fluctuating moisture availability. Such comprehensive knowledge will be indispensable in crafting forest systems that are not only diverse but dynamically resilient under global change.

In summary, the University of Freiburg-led study represents a critical advance in forest ecology, revealing that the interplay between drought duration and tree diversity decisively shapes forest growth outcomes. Moving beyond simplistic biodiversity slogans, it calls for sophisticated, context-aware strategies that leverage species-specific traits and local conditions. As droughts lengthen and intensify worldwide, these insights form a foundational blueprint for fostering forests capable of enduring and thriving in a warming and drying world.

Subject of Research: Effects of tree species diversity on forest resilience to prolonged drought conditions.

Article Title: Multi-year drought strengthens positive and negative functional diversity effects on tree growth response.

Web References:

• MixForChange project – https://mixforchange.cirad.fr/
• CAMBIO project – https://www.cambio-treediversity.com/
• DOI link to article – https://doi.org/10.1111/gcb.70394

References:
Serrano-León H, Blondeel H, Glenz P, Steurer J, Schnabel F, Baeten L, Guillemot J, Martin-StPaul N, Skiadaresis G, Scherer-Lorenzen M, Bonal D, Boone M, Decarsin R, Druel A, Godbold DL, Gong J, Hajek P, Jactel H, Koricheva J, Mereu S, Ponette Q, Rewald B, Sandén H, van den Bulcke J, Verheyen K, Werner R, Bauhus J (2025) Multi-year drought strengthens positive and negative functional diversity effects on tree growth response. Global Change Biology. 10.1111/gcb.70394

Keywords: Forests, Ecology, Tree Diversity, Drought Resilience, Climate Change, Tree Rings, Hydraulic Function, Mixed-species Plantations