About 60% of all tropical forests are degraded or in secondary succession, and the potential for tropical forest regeneration to serve as an important carbon sink is drawing increasing attention from scientists and policy makers. Characterizing successional dynamics and quantifying rates of biomass accumulation in secondary forests is critical to evaluating their potential as carbon offsets, but there are relatively few permanent plot studies in tropical secondary forest. We established four 30 m by 30 m permanent plots in lowland secondary rain forest near Dominical, Costa Rica on land abandoned from logging and grazing 15 years ago. From 2006-8, all trees with diameter at breast height (dbh) greater than 2 cm were measured and tagged, identified to species and mapped. In 2008 and 2009, previously tagged trees were remeasured and new recruits identified, measured and tagged. We estimated tree mortality, growth and recruitment rates as well as changes in stand densities and biomass over the three years of the study and compared them to values reported for other similar-aged forests in the literature.  

Results/Conclusions

Plots were dominated by individuals in the smallest size classes (2-4 cm) from a few pioneer species, including Piper colonense, Psidium guajava, Vernonia canescens, and Miconia spp. We found smaller average sizes, lower basal areas and higher mortality rates than published values for forests of similar ages. Densities of trees with dbh ≥ 5cm ranged from 1033-1433 stems per hectare, compared with 1067-1131 stems/ha for 12-15 year old secondary forests in northern Costa Rica (Chazdon et al. 2005). However, our plots consisted mostly of smaller individuals and had substantially lower basal areas (9.86-11.04 m²/ha compared to 17.13-18.11m²/ha). Tree mortality rates were also much higher; for example, we observed up to 13% annual mortality for 5-9.9 cm dbh individuals in some plots, compared to about 6% for the same size class in one similar study. In sum, our plots appear to be regenerating more slowly than previously documented for forests of similar ages, for reasons that are not clear. Accurately forecasting carbon fixation rates in regenerating tropical forests requires more studies that capture what is likely to be a substantial amount of variability in recovery processes. Long-term research on secondary forest demography will contribute to greater understanding of these processes and how they drive biomass accumulation in successional forests.