Despite the increasing abundance of tropical secondary forests, we still have a relatively poor understanding of the rates of change in plant community composition and ecosystem functions during succession, and the factors that control these processes. In Northwestern Costa Rica, tropical dry forest is regenerating following centuries of grazing. Using the chronosequence approach, we studied above- and belowground carbon and biomass stocks in forests patches growing on different soil types to determine how much variation is accounted for by forest age and soil properties. Aboveground stem biomass and soil carbon stocks were determined in sixty 0.1 ha plots, and coarse woody debris and fine root stocks were measured in 18 of these plots. Plots were stratified by forest type, which varies as a function of soils. Data were analyzed with regression and analysis of variance.
Results/Conclusions
Averaged across forest ages, stem biomass had the largest carbon stocks (96 Mg C/ha), followed by soil carbon in the top 10 cm (31.5 Mg C/ha) and coarse woody debris (3.3 Mg C/ha), while fine roots had the lowest stock (0.9 Mg C/ha). Both stem biomass and CWD increased significantly with forest age (P<0.01), but did not vary with forest type. In contrast, soil carbon also increased with stand age (P=0.002), but forest type explained a larger proportion of the variation than forest age (P<0.0001). Soil carbon stocks on rocky, poor soils were ~35% less than on richer soils. In contrast, fine root mass was more than 40% higher on poor soils compared to fertile soils (P=0.01), but did not vary with forest age. Our results suggest that above- and belowground carbon stocks respond differently to variation in edaphic properties and stand disturbance history. A greater amount of the variation in aboveground carbon stocks was explained by forest age, while belowground carbon pools were more tightly linked to soil properties. In conclusion, regenerating tropical dry forests have the potential to sequester carbon, but the processes that determine accumulation rates and maximum stocks differ for above- versus belowground carbon pools.