High CO2 helps savanna trees escape fatal herbivory

Brad S. Ripley, Sarah L. Raubenheimer, Lavinia Perumal, Maurice Anderson, Emma Mostert, Barney S. Kgope_, Guy F. Midgley, Kimberley J. Simpson

This is a plain language summary of a Functional Ecology research article which can be found here.

When you picture a savanna in your mind, you likely think of a wide and open tropical scene dominated by grasses with a scattering of trees. However these characteristic landscapes are under threat. Savannas across the world are seeing dramatic increases in the number of trees in them (a phenomenon called ‘encroachment’), which threaten to turn these open ecosystems into dense forest. Such a transformation threatens many important services that savannas provide to local people, such as grazing land for livestock and eco-tourism, as well as threatening flora and fauna that are adapted to open landscapes.

In attempting to understand what is causing encroachment, ecologists have uncovered a number of factors that are increasing the survival of savanna trees and causing their numbers to rocket. One key factor is increasing levels of CO₂ in the air which fertilises the growth of savanna trees, and helps them to survive wildfire. However, another important disturbance in savannas that controls tree numbers is herbivory. Here, using Sweet Thorn (Vachellia karroo; see photo), a tree that is encroaching in savannas worldwide and causing serious land-management issues, we aimed to find out how rising CO₂ impacts on the likelihood of trees to experience herbivory and their ability to survive it.

“Sweet Thorn (Vachellia karroo)” by berniedup is licensed under CC BY-SA 2.0.

We found that increasing CO₂ allowed trees to grow quickly early in their life, which meant they were vulnerable to the fatal effects of herbivory for shorter periods of time. However, increased CO₂ did not mean these trees were any better defended against herbivores – their leaves were still highly palatable and their branches were not spinier when grown under elevated CO₂.

Our results suggest that historic increases in CO₂ levels have contributed to savanna encroachment by allowing trees to grow faster and escape being eaten when they are most vulnerable. However, they also suggest the worse may still be to come, with predicted future increases in CO₂ associated with even greater tree survival, and therefore greater encroachment.