Review of biodiversity–productivity relationships in forests published in Oecologia

Forests are a massive store of carbon and also contain a large fraction of the earth’s species. Conserving forests is thus essential for mitigating both climate change and biodiversity loss. But how synergistic are these goals? Do forests that sequester more carbon also have high biodiversity? If so, that suggests we can get more bang for our conservation buck by protecting such forests. If not, we may need largely complementary approaches to carbon conservation and biodiversity conservation. The answer depends on the form of the biodiversity–productivity relationship.

In a just-published review paper by Ryan and Tanvi Dutta Gupta, an undergraduate student at Stanford University who has been an occasional visitor to our lab, we assess current knowledge about the forest biodiversity–productivity relationship and discuss its implications for conservation. We find that the relationship is generally positive: forests with more tree species tend to be more productive, and thus sequester carbon more rapidly. However, there are several important caveats to this that may limit the relevance to conservation. Firstly, even though on average more species means more productivity, in many cases the most-productive forest stands are monocultures of particular fast-growing species, such as eucalpyts. Secondly, the productivity gains of having more than ten species are hard to perceive. Thirdly, the relationship between biodiversity and productivity may not be causal, but driven by some third variable, such as tree density. Fourthly, the relationship varies markedly across spatial scales and may not be strongly positive at scales relevant to conservation. Lastly, current methods for estimating productivity in forests have large errors associated with them. Clearly, more work is needed before we can confidently quantify the carbon benefits of protecting or planting diverse forests.

Chisholm, R. A., T. Dutta Gupta (2023) A critical assessment of the biodiversity–productivity relationship in forests and implications for conservation. Oecologia (in press)

Pasoh Forest Reserve, Malaysia, has very high tree diversity. Such forests also tend to have higher productivity, but the strength of this relationship depends strongly on factors such as the scale of observation and whether confounding variables are controlled for.