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The African savannas appear peaceful but beneath the wings of birds and the hooves of mammals, a millennia-long battle is being fought. This struggle determines whether vast regions of the tropics and subtropics are covered in grasslands, savannas or forests. But a new study shows that rising concentrations of CO2 are shifting the odds to favour trees over grasses, suggesting that large regions of Africa’s savannas may be forests by the end of this century.
This study, conducted by Steven Higgins, a professor of Applied Physical Geography at Goethe University and a researcher at the Biodiversity and Climate Research Centre (BiK-F) in Frankfurt am Main, Germany, and Simon Scheiter, a postdoctoral researcher at BiK-F, investigated how increasing CO2 levels could influence tropical and subtropical grasslands, savannas and forests — dynamic ecosystems that the authors refer to as the “savanna complex”.
Historically, the savanna complex has always fluctuated between grasslands, savannas and forests due to local variations in temperature, fire and rainfall. But CO2 concentrations also affect this age-old struggle between grasses and trees.
“[A] study by a team based in Cape Town, South Africa showed that savanna trees were essentially CO2 starved under pre-industrial CO2 concentrations, and their growth really starts taking off at the CO2 concentrations we are currently experiencing,” [Kgope, Bond & Midgley (2010)] explained Professor Higgins in email. “The upshot is that savanna trees stand to profit enormously from the CO2-rich atmosphere [that] industrial activity is producing.”
This is due to basic plant physiology: trees use the C3 photosynthetic pathway, whereas most tropical grasses use the C4 photosynthetic pathway. The C4 pathway, which evolved more recently than the C3 pathway, is more elaborate and thus, more energetically expensive, but it functions equally well at both higher and lower CO2 levels. In contrast, the C3 pathway is inexpensive but requires high CO2 concentrations to be competitive.
“C4 plants have a cunning mechanism to concentrate CO2 and thereby run photosynthesis efficiently at low atmospheric CO2 concentrations. Of course, these cunning mechanisms have costs,” explains Professor Higgins. “At high atmospheric concentrations the costs outweigh the benefits.”
Life history traits are also important: trees have higher carbon demands than grasses because, being trees, they have many more leaves to support and that takes a lot of carbon. Additionally, trees grow rapidly so they can become too big to be seriously injured by the inevitable grass fires. It is the interaction of these physiological, structural and life history differences that influence the ability of grasses and trees to capture and use resources, affects their responses to fire — and drive this ongoing battle for dominance over the African landscape.
Based on this physiological difference, the authors predicted that high temperatures and low CO2 concentrations favour plants using the C4 photosynthetic pathway (grasses), whereas low temperatures and high CO2 concentrations select for plants using the C3 photosynthetic pathway (trees). Accordingly, the authors ran the dynamic global vegetation model so it considered how increasing CO2 levels would affect the dominance of four basic types of plants: a savanna tree, a forest tree, a C4 grass and a C3 grass (Figure 1):
As you can see in the figure above, this model predicts strong directional shifts: deserts are replaced by grasslands, grasslands by savannas and woodlands, and savannas by forests (see Figure 1a; the width of the arrows indicates the relative frequency of vegetation transitions projected to occur between 1850 and 2100).
Overall, the area occupied by woodland, deciduous forest and evergreen forest is predicted to increase from 31% to 47%, whilst savannas decrease from 23% to 14%, grasslands from 18% to 16% and deserts from 28% to 23% (Figure 1b).
The model also indicates a geographic pattern for where these transitions will probably occur; savanna and C4 grasslands located next to regions with hot semi-arid climates will likely transition to woodland, deciduous forest and evergreen forest. Further, areas that could be forested in the absence of fire, but are savannas if fires occur (bistable areas) are projected to shift location (Figure 1c). Additionally, the model predicts only a small overlap in the locations of past and future bistable areas. Moreover, most sites are predicted to remain savanna and C4 grasslands (Figure 1b, red pixels) are projected to switch to bistable states by 2100 (Figure 1c, yellow pixels).
These shifts to woodland and forest have, according to the model, been occurring at low rates since 1850, but the rate of transition accelerates from 1.9% of the African land surface between 1990 and 2010 to 4.5% between 2010 and 2030, and to 11.2% between 2030 and 2050 (Figure 1d).
But, as anyone who has forgotten to water their houseplants will tell you, variations in rainfall are also important. So the authors examined the probability for how the savanna complex changes as both CO2 concentration and rainfall change (Figure 2a):
The authors found high CO2 concentrations favour transition of deserts (rainfall of less than 250mm) to grasslands whilst locations with high rainfall (more than 1,500mm) will shift to forests and woodlands. Further, these data show that savannas and grasslands extended to regions with high rainfall at pre-industrial CO2 concentrations, whereas savannas and grasslands will be restricted to regions with lower rainfall (less than 750mm) at the CO2 concentrations expected by 2100.
But local conditions and plant species differ, so overall photosynthetic rates should respond differently to CO2 levels. As expected, the authors found that sensitivity to these extra carbon concentrations varies regionally (Figure 2b):
“This figure shows that this sensitivity varies, so in some regions we get large changes in tree biomass associated with only small changes in photosynthesis [dark green pixels], but in other regions this sensitivity is smaller” [grey or red pixels].
Thus, “changes in woody biomass [are] not purely driven by changes in photosynthetic rates but by what the trees do with this extra carbon and the consequences of this allocation for their growth and competitive interactions with grasses,” explained Professor Higgins in email.
The practical implications of these findings are far reaching. For example, the study identified a belt than spans northern central Africa where fire suppression would encourage savannas to transition to forests.
“So if you wanted to sequester carbon as part of a carbon mitigation action, this is where you should do it,” explained Professor Higgins. “[W]ith the caveat that where this will work is shifting as atmospheric conditions change.”
Carbon mitigation actions notwithstanding, I think this study’s findings are scary. Whilst rewriting and editing the manuscript for publication, I mentioned my concerns to Professor Higgins about what appear to be catastrophic ecosystem shifts and the potential effects these could have on Africa’s unique but already threatened floras and faunas.
“Even though a single location may experience a catastrophic regime shift, the signal of vegetation change will be almost linear when averaged over a region,” Professor Higgins replied. Although he did agree that my concerns are valid.
“This may seem reassuring, as this smoother transition will reduce the potential for shocks to the earth system,” continued Professor Higgins. “But we have to bear in mind that these changes are still rapid when viewed on geological time scales.”
In fact, the impact of land-use and climate change may spell the end of the open ecosystems of the savanna complex.