As tropical droughts become 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 their underlying response mechanisms remain poorly understood.

In a new study published in Journal of Geophysical Research: Biogeosciences, researchers from Yunnan University and the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences examined how tropical forests respond ecologically to drought. Their results reveal that, while abundant sunlight typically 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, an increasingly urgent question as El Niño–related dry spells become more frequent and severe.

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

Using linear mixed-effects models, the researchers demonstrated that the interaction between tree size, seasonal timing, and sunlight exposure strongly influences a tree's fate during drought. Together, these factors explain 67.4% of the variation in drought effects.

The study revealed a "double-edged sword" effect of high crown illumination. In normal years, abundant light supercharges photosynthesis, spurring rapid growth. However, during droughts, that same intense sunlight increases atmospheric water demand and evaporative stress. For large, fully exposed canopy trees, this 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 simply a function of size; rather, 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 suggests that conserving forests requires more than just protecting trees; it requires understanding the complex interplay of size, light, and water that governs their survival.