June 29, 2012
Grassland in Kenya
The explanation for this vegetation shift lies in a key difference between tropical trees and grasses: trees use the C3 photosynthetic pathway, which is favored under high CO2 concentrations, whereas most tropical grasses use the C4 photosynthetic pathways.
Temperature also influences tree versus grass ecosystems, with higher temperatures primarily selecting for the C4 grasses. Still, the study suggests that “at the leaf level, the impacts of predicted CO2 changes overwhelm the impacts of predicted temperature changes.”
Trees also have a higher carbon demand than grasses, and the higher CO2 concentrations in the atmosphere set up a cascade of conditions favoring trees. More trees will also suppress grass production through both light competition and decreased fire severity. According to the study, this will result in a positive feedback loop which “further increases the likelihood of the system tipping from a C4-dominated state to a C3-dominated state,” or tree ecosystems superseding grass ecosystems.
The study looked at all of Africa from 1850 – 2100 and found a pattern suggestive of drastic, rapid changes at local levels. However, viewing these transformations over a larger area and period of time also changes the observed pattern. Because the environmental shifts occur in different locations at different times, the averaged change looks like a smooth transition from grassland to trees.
This prediction of increased tree dominance contradicts projected forest declines in the Amazon, but the discrepancy may be due to the high uncertainty of precipitation change in Africa. Because of this, the authors explain, “our analysis assumes that rainfall remains constant, whereas projections of Amazon forest dieback are based on climate simulations that project decreased rainfall.” The scientists add that, “future studies should consider changes in rainfall as well as feedbacks between vegetation and rainfall.”
Citation: Higgins, S. I. & Scheiter, S. Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally. Nature (2012).doi:10.1038/nature11238
Tyler Lark is an Environment & Resources graduate student researching agricultural land use change at the Center for Sustainability and the Global Environment (SAGE) at the University of Wisconsin-Madison.