Influence of tree height and age on leaf drip-tip morphology in lowland tropical rainforest trees

Across biomes, plant form and function are driven largely by variation in climate, particularly the availability of light and water. Strong gradients of these resources also exist within plant communities. For example, light availability above the canopy of an old-growth tropical forest decreases to less than 2% photosynthetically active radiation at the forest floor, meaning that as an individual ages and grows taller it will encounter variation in light or other microclimate conditions in higher canopy layers. Exploring associations between leaf morphology and environmental variables can therefore illuminate the processes that drive selection on leaf traits. Drip-tips (long, acuminate leaf apices) are a characteristic functional trait of rainforest trees. They are thought to aid the removal of water from the leaf surface, enabling a rapid return to photosynthesis and evapotranspiration. The utility of drip-tips for this purpose should vary with a tree's microclimatic conditions and be especially beneficial to plants in the humid, light-limited forest understory. In this study, we assessed variation in leaf drip-tips in an ever-wet, hyper-diverse tropical rainforest, using a mixture of field and herbarium leaf samples from a 50-hectare forest dynamics plot in Yasuní National Park, Ecuador. We examined how tree developmental stage and species canopy height maxima are associated with variation in the absolute length of drip-tips. Within individual trees, we found that drip-tips were longer by 0.27 ± 0.06 cm (mean ± SE) in leaves collected 25 years ago than from the same tree today, and across species by 0.95 ± 0.09 cm in shorter than taller trees. These differences tended to be larger for species with a growth form that reached greater maximum canopy heights than species that inhabit the understory. Synthesis. These findings support the hypothesis that longer drip-tips are more advantageous in the dark, humid understory compared to the well-lit, dry canopy. Further, drip-tip variation is likely linked to the gradual changes in microclimate associated with tree development. 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. Read the free Plain Language Summary for this article on the Journal blog.