Woody plants are changing Africa’s savannas — and this may have an impact on all of us

Posted on May 09, 2018

An increase in indigenous plant life does not sound like a bad thing, but when woody plants threaten to change savanna ecosystems (that cover approximately 13.5 million square kilometres of Africa alone), there is cause for concern. By woody encroachment we mean the increase in density, cover and biomass of indigenous trees and shrubs in grasslands and savannas. It has large-scale implications for ecosystems because it has the potential to cause a complete switch from open habitats to dense thickets. This is greatly changing the biodiversity, ecological functioning and our current understanding of grassy ecosystems.

Woody encroachment is not unique to Africa’s savannas. Increases in trees and shrubs have been documented in grasslands from around the world since the early Twentieth Century, and predictions are that it will intensify in the coming decades. While the pool of research is still growing, and the reasons for woody encroachment are still being considered, climate change, changes in land use and the high levels of carbon dioxide in the atmosphere (which favour woody-plant growth over that of savanna grasslands) are likely to contribute to the increase in woody vegetation at the expense of grassland savannas.

The University of Pretoria brings new research to the international community, with the Department of Zoology and Entomology’s Monica Leitner, under the supervision of Prof Mark Robertson and Dr Andrew Davies, providing the first quantification of how woody encroachment changes the functioning of an African savanna. In their paper, entitled “Woody encroachment slows decomposition and termite activity in an African savanna”, published in the journal Global Change Biology, Leitner and colleagues discuss the ecological implications of an aspect of environmental change that affects a substantial part of the globe.

The study compared grass decomposition rates and termite activity on open areas with nearby encroached areas in a semi-arid African savanna (Madikwe Game Reserve, North West Province). Plant litter decomposition is a fundamental aspect of the nutrient cycling process and plays an important role in global carbon dynamics. Rates of plant decay are controlled by a variety of elements, including climate, ultraviolet (UV) radiation, and the growth of fungi and invertebrates. Termites are particularly important invertebrates in savanna systems, because they play an essential role as the dominant decomposers of plant material; and in doing so, they return nutrients to the soil. They also serve as an important food source for many mammal and bird species.

Leitner and colleagues found a significant reduction in decomposition rates in woodland-encroached areas (with grass taking twice as long to decay in woodland areas compared with open savanna areas), as well as a reduced number of termites. These two factors are cause for concern, not only because encroachment is causing a large change to a fundamental ecosystem process, but because there are also potential knock-on effects. As a crucial organism in savannas, termites not only promote healthy soils by increasing nutrients, water content and aeration, but their mound-building activities have positive effects on plants, which grow well on mounds , and plants growing on mounds are often more nutritious for mammals and insects. As a consequence, termite activity can affect mammal and insect herbivores (which selectively feed on the plants that grow on mounds) and fire patterns (because termite mounds often host trees which are less prone to fires than grass) — all of which are essential aspects of ecosystem functioning.  

The slower decomposition in woody-plant encroached areas also has implications for the “carbon budget”, because plant material will remain on the surface longer without being incorporated into the soils, and the dominant plant material will change from grass to wood. Grasses store a significant amount of carbon in their roots, and the increase in wood may not always be enough to offset the carbon lost when grasses are replaced by trees and shrubs. The loss of grasses in encroached areas also reduces food for insects and grazing animals (wildlife and livestock alike), which has implications for stocking rates. Stocking rates is defined as the number of animals on a given amount of land over a certain period of time. Additionally, the loss of termites as a food source in these areas may affect a range of other animals that feed on termites.

One possible reason for the decrease in termites in encroached areas is that woody encroachment changes the environmental conditions, and termites do not seem to be adapted to these changes. Dichrostachys cinerea, commonly known as sicklebush, is the encroaching plant most commonly found in the study, and is said to be an undesirable food source for termites. “There could be several other reasons which explain the low termite numbers, including cooler temperatures in encroached areas and perhaps changes in soil moisture,” explains Leitner. Sicklebush is widespread throughout South Africa and is also causing serious encroachment in the neighbouring countries of Swaziland (eSwatini), Zimbabwe, Botswana and Namibia.

Ultimately, a 50% slower decomposition rate means that nutrients are taking twice as long to return to the soil in woodland encroached areas as opposed to open savannas, and this is cause for concern Savannas make up a massive expanse in the African continent, and the potential for a substantial (and likely negative) change in the way these ecosystems function, emphasises that woody encroachment of formerly savanna areas demands our attention and understanding. This is not a theoretical problem that affects only scientists, but also a problem that has a direct impact on farmers and those passionate about Africa’s protected areas.

While many studies have looked at methods to control woody encroachments (ranging from fire-control regimes to mechanical removal of woody encroachment), this UP study is the first to seek to understand what woody encroachments really means to the functioning of a savanna ecosystem (for example, the process of plant decomposition) and what might be causing this change (such as the reduction in termite populations). This study reveals substantive changes in the study of an ecosystem process and creatures that might, for the most part, go unnoticed.  The studies of changes to nutrient cycling could have a significant impact on ecosystem health and the benefits we derive from ecosystem services, which ultimately affect us all.

- Author Louise de Bruin

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