Biomass and forest structure across topographic position in a Costa Rican secondary tropical dry forest

Background/Question/Methods

Tropical forests are key to understanding the global carbon cycle as they hold approximately a quarter of the world’s biomass carbon. Quantifying and modeling the distribution of and changes in tropical forest biomass are key steps in clarifying the global role of tropical forests. Topography has important affects on forest biomass because the availability of light, nutrients, and water can all change with topographic position. Forest biomass has been shown to be associated with topographic position in other ecosystems but has not been tested in tropical dry forest. We quantified secondary forest sites estimated to have been recovering from clearing for between 25 and 80 years. We expected to find greater biomass at toeslope sites and that biomass would decrease as topographic position moved up to the midslope and summit. Biomass and forest structure were measured in 24 20 × 50m forest inventory plots. Each set of three plots was positioned along a topographic gradient at the summit, midslope, toeslope. Aboveground biomass was estimated with an allometric equation that incorporates diameter at breast height (DBH) and species-specific wood density. Forest structural indices were calculated and we took soil samples to measure soil physical and chemical characteristics. 

Results/Conclusions

Our analyses show that biomass of trees above 10cm DBH did not follow our expectations of increasing up the slope gradient. Mean aboveground biomass was 170, 225, and 208 Mg/ha for toeslope, midslope, and summit sites respectively and did not differ significantly among topographic positions. Differences in mean stem density, basal area, and species richness were also not statistically significant among topographic positions. However, the variation in stand age across the data set may account for this lack of differences.

Regression analysis of aboveground biomass as a function of log-transformed stand age revealed a strong relationship between these variables in toeslope and summit sites (R² = 0.83 and 0.52 respectively). Midslope sites did not show a significant relationship between age and aboveground biomass, indicating that other factors control biomass accumulation of sloped sites. These may include alternative disturbance history or differences in nutrient, water, and light availability. Further analysis of the relationships between aboveground biomass and soil physical and chemical characteristics may provide insights into these differences.