Abundance, growth and mortality of very large trees in neotropical lowland rain forest

Abstract

Very large trees, arbitrarily defined as those over 70 cm diameter above buttresses, account for a major portion of the above-ground biomass in neotropical rain forests. Owing to the scarcity of individuals of a given species and the difficulty of accurate measurement, there are few species-level data on the growth, mortality, and abundance of species that regularly reach emergent status. We report such data for very large individuals from old-growth tropical wet forest at the La Selva Biological Station in the Atlantic lowlands of the Republic of Costa Rica. The landscape-scale abundance of all species reaching over 70 cm diameter was assessed using 515 0.01-ha quadrats located at grid points in a 500 ha area of old-growth forest. In the total sample of 2301 stems 10 cm or more in diameter, very large individuals accounted for 2% of the stems, 23% of the basal area, and 27% of the estimated above-ground biomass. Growth and survival for five species that regularly attain emergent status were measured in a 150 ha area within the 500 ha plot. Survival of 282 very large individuals of the five species was measured over 6 years. The mean annual mortality rate of the total sample was only 0.6% year-1. Mean annual diameter growth increments varied from 1.9 to 5.2 mm year-1 among species, and were negatively correlated with diameter in four of the five species. For a sample of 193 individuals measured over 7 years, growth almost exactly equalled losses in basal area and biomass due to mortality. Because all of these species are regularly recruiting new trees into the over 70 cm diameter class, the amount of biomass in the large-individual size class is increasing over the 150 ha old-growth study area. Historic disturbance and/or current climatic change are hypothesized to account for the increase. We identify lack of standard diameter measurement criteria, and small and potentially unrepresentative plot locations as two problems in assessing the role of very large trees in other neotropical forests. Future studies should sample larger areas; this will increase the generality of the conclusions and will make possible a species-level comparison of the ecology of very large tropical trees.