Bisley 40 X 40 grid vegetation and site characteristics

Relationships between landforms, soil nutrients, forest structure, and the relative importance of different disturbances were quantified in two subtropical wet steepland watersheds in Pueno Rico. Ridges had fewer landslides and treefall gaps, more above-ground biomass, older aged stands, and greater species richness than other landscape positions. Ridge soils had relatively low quantities of exchangeable bases but high soil organic matter, acidity and exchangeable iron. Valley sites had higher frequencies of disturbance, less biomass, younger aged stands, lower species richness and soils with more exchangeable bases. Soil N, P, and K were distributed relatively independently of geomorphic setting, but were significantly related to the composition and age of vegetation. On a watershed basis, hurricanes were the dominant natural disturbance in the turnover of individuals, biomass, and forest canopy. However. turnover by the mortality of individuals that die without creating canopy openings was faster than the turnover by any natural disturbance. Only in riparian areas was forest turnover by treefall gaps faster than turnover by hurricanes. The same downslope mass transfer that links soil forming processes across the landscape also influences the distribution of landslides, treefall gaps, and the structure and composition of the forest. One consequence of these interactions is that the greatest aboveground biomass occurs on ridges where the soil nutrient pools are the smallest. Geomorphic stability, edaphic conditions, and biotic adaptations apparently override the importance of spatial variations in soil nutrients in the accumulation of above-ground biomass at this site. Support for this work was provided by grants BSR-8811902, DEB-9411973, DEB-9705814 , DEB-0080538, DEB-0218039 , DEB-0620910 , DEB-1239764, DEB-1546686, and DEB-1831952 from the National Science Foundation to the University of Puerto Rico as part of the Luquillo Long-Term Ecological Research Program. Additional support provided by the University of Puerto Rico and the International Institute of Tropical Forestry, USDA Forest Service.