Seedlings of shade-tolerant tree species are more vulnerable to chilling rain under a forest gap

Abstract

The manner in which tree species differ in their responses to chilling rain in warm and species-rich (sub-)tropical forests is not well understood. Understanding this variation between species is essential for linking the responses of individual plants to chilling rain with ecological consequences at the forest community and ecosystem levels. We hypothesized that chilling rain can induce detrimental effects on leaf photochemical processes, and the negative impacts are more evident for shade-tolerant species than light-demanding species. This trade-off between species' tolerance to shade and chilling rain may depend on the light environment to which plants are exposed. To test these hypotheses, we conducted two sequential experiments with five subtropical tree species, measuring their leaf photochemical processes during the phases of a chilling rain event and assessing their resistance and recovery. We determined species shade tolerance by integrating their functional traits associated with resource acquisition in a shade environment. Our results showed that Fv/Fm, a measure of maximum quantum yield of photosystem II, was co-determined by the plant's exposure to light and chilling rain. Seedlings exposed to a gap before the chilling rain generally had lower Fv/Fm than those exposed to shade before the rain treatment. Chilling rain, relative to ambient rain, significantly reduced Fv/Fm during the cold and sunny phase. However, the effects of chilling rain were only evident for shade-tolerant species with gap-exposure history. The reduction in Fv/Fm induced by chilling rain led to higher resistance of Fv/Fm to chilling rain for light-demanding species than shade-tolerant species, leading to a trade-off between species' tolerances to shade and chilling rain for plants with gap-exposure history, thus supporting the hypotheses. This trade-off was primarily due to variation in functional traits related to light-use strategy, such as specific leaf area, leaf area ratio, leaf thickness, relative volume growth rate, and root-shoot ratio. As light availability and species' tolerance to shade are critical abiotic and biotic factors determining forest community succession, respectively, these findings may improve the scalability of the physiological responses of individual plants to chilling rain to the dynamics of forest communities. Our results suggest that frequent chilling rain events and canopy disturbances may favor light-demanding species and alter the species composition of subtropical forests.