Tropical forests store vast amounts of carbon, but much of this would be released into the atmosphere if these forests were cut down. Protecting forests for their carbon is therefore an important strategy to mitigate climate change. This is incentivised under the UN REDD+ policy framework, and gained formal recognition in the Paris Agreement adopted at COP21.
There may be a win-win situation for carbon and biodiversity conservation if the forests with the most carbon are also the most biodiverse. Work on biodiversity–ecosystem function relationships suggests that this may be the case for tree diversity, as (for example) niche differences allow diverse plant communities to exploit more of the available resources. However, much of the work on biodiversity-ecosystem function relationships comes from relatively low diversity systems (e.g. communities of 1-60 species in the Jena Experiment), so we were interested in seeing if biodiversity benefits were also evident in high diversity tropical forests.
To examine the relationship between tree diversity and carbon across the tropical forest biome we (a team of 115 scientists from 22 countries) assembled a dataset of 360 1-ha forest inventory plots located across South America, Africa and south-east Asia . We found that tree diversity and carbon were unrelated, both across the biome and within each continent. Diversity effects were unlikely to have been obscured by the effects of environmental variables, as we still found no relationship between diversity and carbon after statistically controlling for the effects of climate and soil. We did find a weak positive effect of tree diversity when we looked at relationships among 0.04-ha subplots within 1-ha plots, indicating the diversity effects may be scale-dependent.
Tree diversity and carbon are unrelated across the tropical forest biome. From Sullivan et al. (2017).
The absence of a relationship between tree diversity and carbon means that many forests have high tree diversity but low carbon stocks, so may be missed by carbon-focussed conservation strategies. A more optimistic take on this result is that the lack of a negative diversity-carbon relationship means there is plenty of scope to develop conservation strategies that optimise both tree diversity and carbon. Of course, the conservation value of tropical forests depends on more than their trees. However, our findings echo those of a recent study from the TEAM network, which found no relationship between forest carbon stocks and the diversity of ground-dwelling mammals and birds (Beaudrot et al. 2016).
To find out more, read our paper here