Stocks and flows of coarse woody debris across a tropical rain forest nutrient and topography gradient

Abstract

Large pieces of standing or fallen dead wood, known as coarse woody debris (CWD), play important roles in temperate forest carbon and nutrient cycles, and affect the abundance and distribution of many classes of organisms. CWD biomass and inputs are poorly documented in tropical rain forests (TRF), and the causes for their variation at landscape-scales in this biome have not been studied. We quantified standing and fallen CWD stocks and inputs in upland (non-swamp) old-growth TRF at the La Selva Biological Station, Costa Rica. We used a network of 18 0.5 ha plots sited in three edaphic conditions to analyze soil nutrient effects on CWD stocks and inputs controlling for topography, and to examine topographic effects controlling for soil nutrients. The edaphic conditions were flat inceptisols, flat ultisols, and steep ultisols. Chemical analyses confirmed the existence of an almost three-fold gradient in total P and K in the upper 1 m of soil. We also annually censused all live woody stems ≥10 cm diameter above buttresses in each plot in September/October from 1997 to 2000 to obtain data on stand structure and dynamics. Fallen CWD stocks averaged 46.3 Mg ha −1 (22.3 Mg C ha −1), while standing CWD averaged 6.5 Mg ha −1 (3.1 Mg C ha −1). There were no significant differences in volume or mass of standing or fallen CWD among edaphic conditions. Annual inputs of CWD averaged 4.9 Mg ha −1 (2.4 Mg C ha −1). Turnover time of fallen CWD was ca. 9 year. Neither stocks nor inputs were correlated with stand structure (number of trees per plot, plot basal area, or plot estimated above-ground biomass). Potential differences in CWD stocks and inputs among sites with different edaphic conditions may have been obscured by a 10-fold variation in tree mortality among plots and a two-fold variation in mean CWD input among years. Analysis of sample variance showed that stocks of CWD were adequately sampled with the 18 0.5 ha plot design, but that inputs were measured with low precision. At La Selva fallen and standing CWD stocks together equaled ca. 33% of estimated above-ground live woody biomass. Tropical rain forest CWD and its associated carbon are intermediate in pool size and turnover rate between fine litter and live trees. Our results show that scaling up TRF CWD estimates to larger spatial scales may be more constrained by the quality of data obtained over single landscapes than by variation due to zonal soil nutrient and topographic conditions. Both the magnitude and vagility of TRF CWD pools are likely to change with global climate change, but the overall direction of change is uncertain.