Study finds African grass could become trees by 2100 AD

Grass growing among young tree plantation. Image copyright: ICRAF

Climate change is expected to have different effects on different regions and ecosystems. A recent study in the Advance Online Publication (AOP) of Nature shows that in Africa, increasing atmospheric carbon dioxide will favour tree growth over tropical grasses. Even so, lead author Steven Higgins cautions that such a change will take many years and the rate at which it will happen will vary across different parts of the African savanna grassland therefore reducing some of the anticipated negative effects of the shift.

The main motivation for this study was the growing evidence that ecosystems may be changed locally by factors such as increased carbon dioxide concentration.  Steven and fellow scientist Simon Scheiter wanted to see whether such local changes could lead to changes at a regional scale.

Focusing on the African savanna complex which is made up of grassland, savanna and forest ecosystems of tropical and subtropical regions, they analysed the effects increased carbon dioxide would have on trees and grasslands as a function of temperature. They found that increasing carbon dioxide from 350 to 700 parts per million favoured trees over tropical grasses when temperature is increased from 2°C to 5°C. This is so because “high temperatures and low CO2 concentrations select for the C4 photosynthetic pathway (favoured by tropical grasses), whereas low temperatures and high CO2 concentrations select for the C3 photosynthetic pathway (favoured by trees).”

The two found that not only did trees increase in number but also grasslands dominated over deserts and savanna complexes became forests.

Using the years 1850 to 2100, they found that trees comprising deciduous forests and evergreen forests increased from 31% to 47% while grasslands decreased from 18% to 16% with the other percentage decreases attributed to savannas and deserts.

Their results show that the shift was accelerating very slowly from 1850 peaking at 1.9% of the land surface of Africa by 1990 but then the shift rapidly accelerates to roughly 5 times faster between 1990 and 2050 to 11% of the land surface.

The scientists warn that the shift towards trees is not simply due to increase in carbon dioxide concentrations but rather due to the advantage trees have over grass. Increasing number of trees block sunlight for the grass which leads to less grass growth and this in turn means less fuel for bush fires and hence higher number of trees retained even after fires.

There are other studies conducted in the Amazon which show that with the same effects, trees actually die out in the Amazon but the scientist say this may be due to how rainfall was treated in the separate studies. The study by Steven Higgins and Simon Scheiter assumed that rainfall was constant whereas models of the Amazon assume decreasing rainfall. The scientists decided that the available evidence was not strong enough to suggest that rainfall either increased or decreased, but they encourage future studies to consider the effects varied rainfall patterns have over the number of trees and grasslands.

The full study can be accessed via the nature website

Citation: Higgins, S. I. & Scheiter, S. Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally. Nature (2012). doi:10.1038/nature11238'

Christopher Mesiku

Chris Mesiku is a science communicator volunteering at the World Agroforestry Centre in Nairobi, Kenya. In the last 5 years, he has worked as a communicator for various scientific institutions. He holds a Bachelor of Science, Graduate Diploma in Science Communication (ANU) and a Masters in Philosophy of Science (UQ).

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