Relationships between biodiversity and carbon dynamics in tropical forests

Biological diversity and ecosystem functions, such as the production of wood and the storage of carbon, are important ecological attributes, but how are they related? Tropical forests are the most diverse terrestrial ecosystems and key players in global carbon cycling, so preserving both functions are important conservation goals. Yet little is known about diversity–function relations here. I investigate tree diversity–function relations in African and South American old-growth tropical forests, using 323 forest plots (mostly 1-ha) from a pan-tropical network sampled using standardised techniques. I focus on aboveground biomass (AGB) and coarse woody production (AGWP). I develop methods to deal with issues that arise in calculating AGWP over long time-spans and in estimating diversity when only some individuals are identified. Diversity–function relations are assessed using two main approaches. Firstly, I use linear models to assess whether AGB and AGWP covary with diversity across large spatial extents. These models include climate and soil variables that may drive AGB and AGWP to statistically account for these, and filters to account for spatial autocorrelation. There is no evidence of a relationship between diversity and AGB. For AGWP, there is a positive relationship with genus and family-level diversity in South America, but not in Africa. Secondly, I investigate whether diversity is related to AGB or AGWP within the plots, thereby removing environmental differences among plots. Using mixed effects models on 0.04-ha subplots again shows AGB is unrelated to diversity. However, AGWP is positively associated with diversity in both continents, with a doubling of species richness increasing wood production by 11%. Taken together the results represent the first evidence of widespread positive diversity–productivity relations across the two largest tropical forest continents. The lack of positive association between diversity and biomass implies a trade-off between the conservation of tropical biodiversity and carbon storage.