COS 232-4 - Influence of local bioclimatic conditions on leaf trait morphology in tropical rain forest trees

Abstract: Across biomes, plant form and function are driven largely by variations in climate, particularly light and

water availability. Strong gradients of these resources also exist within plant communities, altering

microclimates throughout the vertical forest strata. For example, light availability above the canopy of an

old-growth tropical forest decreases to less than 2% photosynthetically active radiation (PAR) at the forest

floor. At the leaf level, morphology is a tool used to adapt to local bioclimatic conditions. Exploring

associations between leaf morphology and environmental variables can illuminate the processes that drive

leaf trait selection, and thus the species that might be favored in hotter and drier forests. Drip-tips (long,

acuminate leaf apices) are functional traits thought to aid in the removal of water from the leaf surface,

enabling a rapid return to photosynthesis and transpiration. Therefore, they should be especially beneficial

to plants in the humid, light-limited forest understory. In this study, we assessed variation in leaf drip-tips

in an aseasonal, hyper-diverse tropical rain forest using a mixture of field and herbarium leaf samples

from a 50-hectare Forest Dynamics Plot. We examined how light and water availability interact with

developmental stage and canopy height maximums to drive variation in the absolute length of drip-tips.

Both within individual trees and across species, we found that drip-tips were on average 0.27cm and

0.95cm longer in younger and smaller trees, respectively. Life-history strategy was not a significant driver

of drip-tip length, with one exception between tall trees and mid-sized trees at the individual tree level.

These findings support the hypothesis that longer drip-tips are more advantageous in the

dark, humid understory compared to the well-lit, dry canopy. These results also suggest that drip-tip

variation is associated more closely with gradual changes in microclimate associated with tree

development rather than established canopy height maximums associated with a species’ life history

strategy. Studies have shown that leaves with pronounced drip-tips drain water at faster rates than similar

leaves without elongated apices. Therefore, even minute increases in drip-tip length while trees occupy

the forest understory could play a substantial role in drying the leaf surface and moderating leaf-climate

interactions.