Assessing trends on tree species diversity and biomass change across human-dominated tropical forests

The importance of carbon sequestration as a forest ecosystem service has rapidly increased in relevance as a viable strategy to curtail the rate of global warming. Among biomes, tropical forests have the highest capacity of sequester carbon, but the total amount sequestered over time is dependent on the tree species assemblage of the forest. At the same time, many tropical forests are an important source of livelihoods and cash benefits for local communities, and today most are affected by human activities to some extent. The impacts of human uses on these forests are likely to accumulate with extended use over time. Therefore, to forecast tropical forest carbon sequestration potential and biodiversity levels into the future we need to understand temporal patterns of tree species and biomass changes in human-impacted tropical forests.
We analyzed two decades (1993-2013) of tropical tree diversity and tree size data collected in eight countries and three continents (from the International Forestry Resources and Institutions, IFRI, dataset). Tree species data was recorded in 92 different forests including more than 6500 surveyed plots (78.5 m2 each) that were sampled over at least two time periods. Tree size data (diameter) was collected at 82 of those forests. We assessed changes in tree species diversity and biomass (estimated as basal area, BA) in each forest over time. Overall, 52% of the forests experienced a decline in tree species diversity, and an increase was found in 6.8%. In India 88% of sampled forests experienced a reduction in the number of species, the highest of any country sampled. Nepal experienced the greatest number of gains: 17.8% of the surveyed forests showed an increase in tree species. Basal area also decreased among most forests, 53.6% experienced a decline, but in this case 46% gained in BA. These trends indicate that even if tree diversity is decreasing, forests may be able to, at least temporally, maintain, or even increase, their levels of biomass, and probably carbon sequestration capacity. These estimates, together with, physical and sociological features of these forests, can now be used to estimate future trends on biodiversity and biomass of locally used tropical forests.