Soil and Stem Water Storage Determine Phenology and Distribution of Tropical Dry Forest Trees

Many trees of tropical dry forests flower or form new shoots soon after leaf shedding during the dry season, i.e., during a period when trees are likely to be severely water stressed. To resolve this apparent paradox, phenology and seasonal changes in tree water status were monitored during two consecutive dry seasons in > 150 trees of 37 species growing at different sites in the tropical dry lowland forest of Guanacaste, Costa Rica. Tree development during the dry season varied considerably between species and be- tween sites of different moisture availability. Leaf shedding, flowering, and shoot growth (flushing) were strongly correlated with seasonal changes in tree water status, measured by conventional and newly developed techniques. Tree water status varied with the availability of subsoil water and a variety of biotic factors such as structure and life-span of leaves, time of leaf shedding, wood density and capacity for stem water storage, and depth and density of root systems. Observed tree species differed widely in wood density (from 0.19 to 1.1 g/cm3) and stem water storage capacity (400-20% of dry mass), which was highly correlated with the degree of desiccation during drought. Only hardwood trees at dry upland sites, lacking stem water storage and access to subsoil water, desiccated strongly (stem water potential <-4 MPa) and remained inactive throughout the dry season. In all other trees elimination of transpirational water loss during leaf shedding in conjunction with utilization of residual water enabled rehydration of stem tissues and subsequent flowering or flushing during the dry season. Rate and degree of rehydration varied strongly with the availability of water stored in tree trunks or in the subsoil. Stored water buffers the impact of seasonal drought and enables flowering and flushing during the dry season. Climatic data are thus not sufficient to explain tree phenology in seasonally dry tropical forests. Phenology, seasonal changes in water status, and water storage capacity of tree species are highly correlated. Tree species cluster into a number of distinct functional types ranging from deciduous hardwood trees and water-storing lightwood trees in dry upland forests to evergreen light- and softwood trees confined to moist lowland sites. Seasonal variation in tree water status thus appears to be the principal determinant of both phenology and distribution of tree species in tropical dry forests.