Carbon cycling in secondary tropical dry forest from species to global scales

Tropical forests are important in the global carbon cycle but questions remain about the distribution of carbon stocks and the controls over carbon cycle processes in these forests. This is especially true with tropical dry forests which comprise as much as 43% of global tropical forests, but have been studied less and impacted more by human activities. In many areas, tropical dry forests are recovering from clearing and few studies have investigated the carbon cycle implications of this increasing area of secondary tropical dry forests. This dissertation describes three projects that examined the global, regional, and stand-scale patterns of carbon uptake and storage in secondary tropical dry forests. In Chapter 2, I reviewed literature on carbon stocks in mature and secondary tropical dry forests around the world and analyzed the global contribution of these forests. I found that carbon stocks in mature tropical dry forests increased linearly with mean annual precipitation and I conclude that restoring tropical dry forest could make a significant global contribution to climate change mitigation. In Chapter 3, I studied landscape level controls of aboveground biomass and plant functional trait distributions in secondary tropical dry forests of northwestern Costa Rica. I found that edaphic factors, and plant functional traits explain little additional variation in biomass after accounting for stand age but that the dominant plant traits in a community are well correlated with succession and soil properties. I conclude that plant trait distributions indicate soil and succession driven changes in stand level function, but that these changes are not reflected in the landscape scale distribution of biomass. In Chapter 4, I measured aboveground net primary productivity in 18 secondary forest plots in northwestern Costa Rica over three years. I found that large inter-annual differences in precipitation do not significantly affect ANPP and that variation in litterfall and biomass among our plots is explained by different factors. I conclude tropical dry forest ANPP is well buffered against future climate change driven variation in precipitation and that community weighted means of plant functional traits explain significant portions of the variation in ANPP and its components among our plots.