Leaf demography and phenology in Amazonian rain forest : a census of 40,000 leaves of 23 tree species

The periodicity, synchrony, and causes of variability in the demography of tree leaves in ecosystems with relatively aseasonal climates, such as tropical rain forests, is still poorly understood. To address this issue, we surveyed the timing of birth and death of >40 000 leaves of 1445 individuals of 23 evergreen rain forest species in several late primary and early secondary successional plant communities at San Carlos de Rio Negro, Venezuela, in the northern Amazon basin. In all species, the mortality rate generally increased with leaf age. However, in many species, deceleration of death rates with extreme leaf age was noted. In general, for each species, the age structure of leaf populations and the frequency distribution of leaf life span were broad. Species differed substantially in their leaf demography. Measured in their native habitats, seven species common to disturbed open sites had shorter median life spans (0.7 yr) than five species common to open but infertile Bana primary communities (1.9 yr average) or six species common to two tall primary forest communities (Tierra Firme and Caatinga), when measured in high-light conditions in the canopy (2.0 yr average). Variation in light availability had consistent effects on leaf life span in all species. Species native to Tierra Firme forest had average leaf life spans of 3.2, 1.9, and 1.6 yr, respectively, in deeply shaded understory microsites, in small gaps, and in sunlit mature tree canopies. Species native to Caatinga forest had average leaf life spans of 4.2, 3.4, and 2.5 yr, respectively, in these same microsite types. Two species common in gaps and in disturbed sites had much longer leaf life span in shaded understory than in open, disturbed microsites. For all species, responses were similar when trees were planted in sites differing in light availability, as when trees naturally established across light gradients. The rate of leaf production, the risk of leaf mortality, and the leaf life span were not periodic or related consistently to seasonality of climate. Negligible relationships existed between the mild annual dry season and either leaf production or leaf mortality in all species. Thus, leaf phenology and demography were essentially aseasonal in this tropical forest environment.