Forests support much of the world's biodiversity and play a vital role in regulating Earth's climate, but they are increasingly exposed to drought. To understand the potential consequences of drought, a key question is whether trees can acclimate to prolonged water stress by altering their physiology.
Many previous attempts to answer this question have relied on evidence from potted plants or natural rainfall gradients, leaving uncertainty over whether trees growing in natural forest environments respond in the same way. Now, a new global forest study that better reflects natural conditions has found that trees do not substantially alter their physiological traits to cope with prolonged drought.
The study, led by researchers from the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS), is based on 40 throughfall reduction experiments conducted across forests worldwide and represents one of the most comprehensive field-based assessments of drought acclimation to date.
It was published in Proceedings of the National Academy of Sciences (PNAS) on Aug. 24.
In the experiments, gutters installed beneath the forest canopy diverted part of the incoming rainfall before it reached the soil, reducing water availability while trees remained rooted in natural soils and surrounded by their forest communities. This approach allowed researchers to examine long-term drought responses under realistic forest conditions, rather than in artificial pot experiments or across natural rainfall gradients.
The researchers examined 24 physiological traits related to water transport, drought resistance, and carbon gain. Across forests spanning contrasting climates and drought conditions, traits including embolism resistance, hydraulic efficiency, leaf nutrient concentrations, and photosynthetic capacity showed little change.
"Trees showed remarkably little adjustment in their hydraulic and photosynthetic capacities despite prolonged reductions in water availability," said LIANG Xingyun from SCBG, first author of the study. "By maintaining these capacities, trees may be able to maximize carbon assimilation during favorable periods following rainfall, when water becomes available and conditions for photosynthesis improve."
However, this stability was associated with an important consequence.
Drought caused tree tissue water potentials to decline, while embolism resistance remained largely unchanged. As a result, hydraulic safety margins—the buffer between the water status a tree experiences and the point at which its water-transport system becomes vulnerable to damage—became significantly narrower. Trees were therefore operating closer to their hydraulic limits without substantially increasing their resistance to hydraulic damage.
The researchers also revealed a contrasting pattern in carbon uptake and storage. Net photosynthesis declined under drought, largely because stomata closed to conserve water. Yet nonstructural carbohydrates, including starch and sugars, remained stable. Thus, reduced carbon uptake did not translate into clear depletion of stored carbon during the experimental droughts.
These results suggest that carbon depletion may not be the main limitation for trees exposed to prolonged drought. Instead, narrowing hydraulic safety margins may represent a more immediate threat.
"The key finding is that maintaining physiological capacity does not necessarily make trees safer during drought," said YE Qing from SCBG, corresponding author of the study. "Trees may remain capable of taking advantage of favorable periods following rainfall, while operating closer to their hydraulic limits as drought intensifies."
According to the researchers, the findings challenge the assumption that forest trees will substantially acclimate to a drier climate. The study also provides an important benchmark for improving projections of future forest responses to drought by refining assumptions about drought-induced changes in key plant traits.
More broadly, the findings highlight narrowing hydraulic safety margins—not carbon depletion—as a major concern for forests facing prolonged drought. As droughts become more frequent and severe, recognizing trees' limited capacity for acclimation will be critical for better understanding and protecting the future of forests.
