Seasonal patterns of carbohydrate storage in four tropical tree species

We examined the seasonal variation in total non-structural carbohydrate (TNC) concentrations in branch, trunk, and root tissues of Anacardium excelsum, Luehea seemannii, Cecropia longipes, and Urera caracasana growing in a seasonally dry forest in Panama. Our main goals were: (1) to determine the main sites of carbohydrate storage, and (2) to determine if seasonal patterns of carbohydrate storage are related to seasonal asynchronies in carbon supply and demand. We expected asynchronies to be related to seasonal variation in water and light availability and to foliar and reproductive phenology. Cecropia and Urera are fully drought-deciduous and so we expected them to exhibit the most dramatic seasonal variation in TNC concentrations. We predicted that maximum carbon supply would occur when canopies were at their fullest and that maximum carbon demand would occur when leaves, flowers, and fruits were produced. The concentration of total non-structural carbohydrates was assessed monthly in wood tissue of roots and in wood and bark tissue of terminal branches. Trunk tissue was sampled bimonthly. All tissues sampled served as storage sites for carbohydrates. As predicted, TNC concentrations varied most dramatically in branches of Cecropia and Urera: a 4-fold difference was observed between dry season maxima and wet season minima in branch wood tissue. Peak concentrations exceeded 25% in Urera and 30% in Cecropia. Less dramatic but significant seasonal variation was observed in Anacardium and Luehea. In all species, minimum branch TNC concentrations were measured during canopy rebuilding. In Anacardium, maximum branch TNC concentrations occurred when canopies were at their fullest. In Cecropia, Urera, and Luehea, TNC concentrations continued to increase even as canopies thinned in the early dry season. The greater photosynthetic capacity of leaves produced at the beginning of the dry season and the potential for the export of carbohydrates from senescing leaves may explain this pattern. In all species, the phenology of carbon gain was more important than the phenology of reproduction in influencing seasonal carbohydrate patterns. The combination of high TNC concentrations and the large biomass of branches, trunks, and roots indicates these species are storing and moving large quantities of carbohydrates.