Ontogenetic changes in leaf size in Malaysian rain forest trees

It is common lore that saplings of tropical rain forest trees have larger leaves than do adult trees; however, there is very little data available to quantitatively evaluate such ontogenetic changes in leaf size. To address this issue, we measured randomly sampled fully expanded leaves from saplings and adult trees in 51 species found in primary lowland tropical rain forest at Pasoh Forest Reserve, West Malaysia. Length and width of the leaf laminae were measured on 8 leaves/tree of 3 saplings and 3 adults of each species, using fallen leaves to sample the adult trees of larger species. Of the species examined, about half (26/51) exhibited smaller leaves on adult trees than saplings, while 13 species had larger leaves on adults, and 12 species showed differences of <10 percent on an area basis. The direction and degree of ontogenetic changes in leaf size was related to tree stature: understory treelets tended to have larger leaves as adults, while canopy trees tended to have smaller leaves as adults than as saplings. However, a few canopy species deviated from this pattern, having significantly larger leaves on adult trees. For one such canopy species (Garcinia nervosa) and a smaller-statured congener (G. malaccensis) we sampled leaves from trees spanning a wide range of size classes. Both species exhibited a non-monotonic relationship between leaf size and tree size, showing a peak in leaf size at stem diameters dose to the species-specific size at reproductive onset. We suggest that decreases in leaf size following reproductive onset may in many cases result from the physiological effects of carbon allocation to reproductive structures, rather than adaptive acclimation to altered light regimes.