Structure and Biomass of a Subtropical Dry Forest in Puerto Rico

Abstract

Guanica Forest, with seasonal rainfall averaging 860 mm annually, is among the driest of tropical or subtropical forests studied to date. It is composed of over 12,000 live tree stems per hectare, only 2.3 and 12 percent of which exceed 10 cm DBH or 5 m in height, respectively. Of all stems greater than 2.5 cm DBH, 57 percent are stump or root sprouts, attributable to forest cutting 50 years earlier. The dry winter months induce maximum deciduousness and are reflected in a 50 percent reduction in leaf area index, from approximately 4.3 to 2.1. Although less in magnitude, leaf fall was also observed in the moderately dry midsummer months. Relative to wetter forests, tree species richness and total community biomass is low. Approximately 50 percent of the total live-plant biomass of 89.9 t/ha occurs below ground, a higher proportion than for any other comparable forest measured thus far. IN RECENT YEARS, tropical forests have received unprecedented ecological attention. Interest in these ecosystems has been stimulated, in part, by the alarming rate at which they are being modified or completely destroyed. But interest in them is also attributable to their vast stores of carbon and the potential effects of their disruption on the world's carbon balance. Most studies in the tropics have focused on forests growing in humid climates even though they account for a relatively small portion of the forested tropical landscape. Of the total global extent of tropical forest, Brown and Lugo (1982) estimated that about 25 percent is tropical and subtropical wet and rain forest and 33 percent tropical or subtropical moist forest. The remaining 42 percent is tropical or subtropical dry forest (sensu Holdridge 1967). Unlike humid forest, tropical and subtropical dry forest has been very little studied, particularly with respect to taxonomic composition, stand structure, biomass, primary productivity, and rates of carbon turnover. It is, therefore, difficult at the present time to evaluate the significance of tropical and subtropical dry or seasonal forest relative to the global carbon cycle. Additionally, data regarding biomass and related characteristics (e.g., growth and primary productivity) would be useful in assessing the resource potential of dry forest and in furthering our understanding of forest function, especially with respect to climate and seasonality. In 1981 we initiated a comprehensive, long-term study of structure, primary productivity, and plant succession in a subtropical dry forest in southwestern Puerto Rico. This paper reports on the taxonomic composition (woody plants), structure, and biomass of the forest.