New research shows that high forest productivity is not only the result of how many different species occur in a forest, but also how abundant the species are.
Typically, a greater number of species increases an ecosystem’s productivity. However, a new study shows that the effects of diversity are not only a matter of species richness. If many species coexist in a forest, even distribution of species abundances is key for high ecosystem diversity and productivity.
Biodiversity is an essential factor for ecosystem health and productivity. A higher biodiversity generally leads to a higher productivity. Indeed, the number of species in an ecosystem has been found to positively correlate with forest productivity. Previous research, however, has also pointed to a limit in increasing forest productivity at a high number of coexisting species. What is the reason?
A new publication in Journal of Ecology provides evidence at global scale that the number of species, also known as species richness, is not sufficient to measure the effect of biodiversity on forest productivity. Biodiversity is a product of species richness, but also the relative abundance of species in the community (evenness). Evenness refers to the distribution of species abundance: for example, a plot with 50 trees and 5 species at each 10 trees can be considered even. However, a forest plot with 46 trees of one species but one each of the others would be considered uneven.
Forest diversity and productivity is not just a matter of species richness
Based on data from over one million forest plots worldwide, the study shows that there is a negative relationship between the richness and the evenness of species within forests around the world. Forests with a high species count therefore tend to be highly uneven, with a few species dominating the community, while the majority of species are rare. In these cases, evenness will be more important for improving forest productivity than increasing the number of species.
Dr. Iris Hordijk, lead author of the study summarises: “This study suggests that trees in a species rich forest need to be present in high enough numbers to contribute to the productivity of the forest.”
Important insights for guiding ecosystem conservation and management
Overall, this study demonstrates the importance of species evenness in influencing forest diversity and productivity. At a time when diverse and productive forests are needed to fight climate change and biodiversity loss, understanding the role of species abundances is critical for guiding ecosystem conservation and management efforts.
Prof. Thomas Crowther of Crowther Lab, ETH Zurich and senior author of the paper explains: “Increasing a forests’ diversity doesn’t only mean increasing the number of species. It can also mean working to promote the natural evenness of those species.
The Journal of Ecology publication “Evenness mediates the global relationship between forest productivity and richness” can be accessed here: https://doi.org/10.1111/1365-2745.14098
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