As tropical droughts grow more frequent and severe,scientists are racing to understand how these water-stressed ecosystems will respond. However, the specific impacts of drought on these forests and the underlying response mechanisms remain poorly understood.

In a study published in Journal of Geophysical Research: Biogeosciences, researchers from Yunnan University and Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences investigated the ecological response mechanisms of tropical forests under drought conditions. The results reveal that while abundant sunlight boosts tree growth in normal times, it can become a critical liability during severe drought, especially for large, canopy-dominant trees.

The researchers analyzed continuous dendrometer (tree growth) data from more than 2,700 trees over 12 years, capturing the severe 2019–2020 drought event in Xishuangbanna. Their aim was to understand why some trees are more vulnerable to drought than others, a question of growing urgency as El Niño-related dry spells become more frequent and severe.

The severe 2019–2020 drought had a dramatic negative impact on the forest. The forestwide biomass growth dropped significantly during the dry spell,but the response was far from uniform.The functional traits, such as leaf thickness, wood density, or nutrient content, predict a species' resilience, explained only about 10% of the variation in drought response. The decisive factors were a tree's size, the season, and critically, its exposure to sunlight which is measured by the Crown Illumination Index (CII).

Using linear mixed-effects models, the researchers showed that tree size, seasonal timing, and sunlight exposure interact strongly to determine a tree's fate during drought, together explaining 67.4% of the variation in drought effects.

The study uncovers a "double-edged sword" effect of high crown illumination. In normal years, abundant light supercharges photosynthesis, spurring rapid growth. But during drought, that same intense sunlight drives up atmospheric water demand and evaporative stress. For large, fully exposed canopy trees, that combination can be lethal.

The data showed that drought-induced growth decline intensified with tree size, but only for those in well-lit positions. In contrast, smaller trees in the shaded understory showed far weaker responses.And in some cases, even a slight positive effect.

“Our results show that the widely observed vulnerability of large trees is not a simple function of size, it is an interactive response mediated by how much light their crowns receive and the time of year,” said LIN Luxiang of XTBG. “This shifts our understanding from a static view of tree size to a dynamic, microenvironment-sensitive perspective.”

The study suggested that conserving forests will require not just protecting trees, but understanding the complex interplay of size, light, and water that governs their survival.

First published: 12 May 2026