Forests are often portrayed as collections of distinct species, each with a relatively fixed set of biological characteristics. But a new study suggests that this view may miss one of the most important factors determining whether tropical forests survive intensifying drought: the ability of individual species to vary within their own populations. Research conducted across Puerto Rico’s dramatic rainfall gradient has found that many tropical tree species can adjust key hydraulic traits as conditions become drier, potentially giving forests a hidden reservoir of drought resistance.
The study, published in Nature, examined 290 trees from 18 species growing in forests that receive between 1,000 and 4,000 millimetres of rain each year. This natural gradient allowed the researchers to compare trees exposed to substantially different levels of water availability without relying solely on short-term laboratory experiments. Across the sampled trees, the team measured 11 hydraulic traits associated with the movement, regulation and loss of water through plants. Together, these traits provide a detailed picture of how trees manage the tension between capturing carbon dioxide for photosynthesis and avoiding catastrophic dehydration.
Drought is especially dangerous for trees because water transport through the plant can fail in several ways. Trees move water from the soil to their leaves through a continuous column inside microscopic conduits in the xylem. When drought creates sufficiently negative pressure in this water column, air bubbles can form and spread through the conduits, a process known as embolism. Once enough conduits become blocked, water can no longer reach the leaves and tissues above the damaged region. Severe hydraulic failure can eventually stop photosynthesis, cause tissue death and, in extreme cases, kill the entire tree.
One of the most important measurements in the study was embolism resistance, which describes how much negative pressure a tree’s water-transport system can withstand before it begins to fail. The researchers also examined stomatal safety margins, the distance between the water potential at which a tree closes its stomata and the level at which serious hydraulic damage occurs. Stomata are microscopic pores on leaves that regulate gas exchange. By closing them, trees reduce water loss, but they also limit carbon uptake and photosynthesis. A wider safety margin means that a tree can continue exchanging gases while remaining farther from the point of hydraulic failure.
The results challenge a long-standing assumption based largely on research in temperate forests. Earlier studies had often found limited variation within a species in traits such as resistance to xylem embolism. That apparent uniformity raised the possibility that hydraulic traits were strongly constrained by evolution, leaving species with only limited capacity to adapt to changing climates. The Puerto Rican data tell a different story. Substantial differences occurred among individuals of the same species, and those differences were organized along the rainfall gradient rather than appearing as random noise.
Most of the species examined showed greater embolism resistance in the drier forests. Trees living where rainfall was lower also tended to maintain wider stomatal safety margins, indicating that their leaves and water-transport systems were better positioned to avoid damage during periods of water stress. These shifts were not restricted to differences between species, such as a wet-forest species being replaced by a naturally drought-tolerant species. Instead, many of the same species appeared to express different hydraulic characteristics depending on the environments in which they grew.
That distinction is crucial for predicting the future of tropical forests. If scientists consider only species turnover, they may assume that a forest becomes more drought resistant mainly because vulnerable species disappear and drought-tolerant species become more common. The new findings suggest that forests can also change through adjustment within species. Individuals of a single species may differ in the construction and operation of their hydraulic systems, allowing populations to persist across a wider environmental range than their average traits would suggest.
The study does not imply that all trees can adapt indefinitely or that tropical forests are protected from climate change. Intraspecific variation may arise from genetic differences, developmental conditions, environmental plasticity or a combination of these factors, and the research does not establish that every observed difference will be inherited by future generations. There may also be limits to how far a tree can shift its hydraulic traits before growth, reproduction or carbon gain are compromised. A tree that closes its stomata early may avoid embolism, for example, but it may also photosynthesize less and grow more slowly.
The researchers therefore emphasize that species lacking meaningful hydraulic variation could be especially vulnerable as droughts intensify. Climate models increasingly project changes in rainfall patterns, longer dry seasons and more frequent extreme droughts in many tropical regions. Forests that appear stable under present conditions may cross critical thresholds when atmospheric demand for water rises or when soils remain dry for longer periods. Forecasting those changes will require models that represent not only which species are present, but also how individuals within each species differ in their capacity to transport and conserve water.
The findings offer a more dynamic view of forest resilience, in which tropical trees are neither hydraulically identical nor locked into a single drought strategy. Instead, resilience emerges from the combined effects of species composition and variation among individuals. By incorporating both sources of diversity, scientists may be able to identify forests with hidden adaptive capacity, detect populations at greatest risk and improve predictions of which ecosystems will withstand a hotter and drier future. For tropical forests facing accelerating climate pressure, the most important survival trait may not belong to a species as a whole, but to the variation hidden within it.
Subject of Research: Intraspecific variation in hydraulic traits and drought resistance among tropical forest trees.
Article Title: Species intraspecific variation drives tropical forest drought resistance.
Article References: Smith-Martin, C.M., Muscarella, R., Brodribb, T.J. et al. Species intraspecific variation drives tropical forest drought resistance. Nature (2026). https://doi.org/10.1038/s41586-026-10870-4
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41586-026-10870-4
Keywords: Tropical forests, drought resistance, climate change, tree mortality, hydraulic traits, embolism resistance, stomatal safety margins, intraspecific variation, forest resilience, Puerto Rico.
