May 28, 2018 | Resource degradation

Woody savannas, an ecosystem characterized by grasses and other herbaceous plants, have quickly become a global prevalence within the last two centuries. Scientists still don’t know which drivers specifically are responsible for the increased vegetation. The African savanna grasslands sustain the livelihoods of millions of people and are also a vital source of food for livestock, therefore the woody encroachment on these savannas is a threat of note, suppressing local herbs and grasses that are essential for these ecosystems continued development.

These savannas are home to millions of species that are threatened directly by this widespread phenomenon. The diverseness of these landscapes is directly vulnerable to these woody vegetations which have implications for invertebrates, birds and large mammals, who depend on this directly for sustenance. That’s not all, when there is a substantial change in a landscapes vegetation there is a direct feedback implication as far as earth–atmosphere feedback processes are concerned and can also alter energy, carbon and water budgets.

The proposed drivers for woody encroachments in savannas in Africa have been divided into two groups; global and local.

The Global drivers proposed are climatic interactions, soil type and nutrient availability and carbon dioxide enrichment while the local drivers are fire regimes and herbivory. The patterns observed indicate that these drivers are an integral part of the process but, not the only cause.

Several recent studies have allowed scientists to delve deeper to synthesize the latest information and try to create a more understandable view to allow for targeted suggestions on the way forward. However, it is still difficult for scientists to understand how these drivers actually interact with each other. While herbivory, fire, and soil properties are likely to alter woody cover and rates of encroachment in both wet and dry savannas at a local level, it’s difficult to say this for both overall on a global scale. Particularly because there are substantial differences between dry (stable) and wet (unstable) savannas, particularly with regards to the role that water plays in altering the tree-grass competition.

Still, the causes of this situation are still unknown, in a journal, (Aisling P. Devine)  conclude, “resolving the puzzle of woody encroachment (almost by definition), requires that savanna ecosystem responses to changing conditions can be understood and predicted. Additionally, examining dry savanna regions that are not experiencing woody encroachment could shed light on the most important drivers causing woody encroachment in dry savanna systems. While it is always possible to propose sophisticated hypotheses and predictive models, it is only by testing these against data, that these can be convincingly validated. Until long-term experiments that investigate the causes have been carried out, it is unlikely that the phenomenon of woody encroachment will be fully understood.

Overall, however, we propose that increased atmospheric CO2 is the over-riding factor driving woody encroachment, though the ecological mechanisms involved differ depending on whether the savanna in a wet or a dry system. At a local level, changes in precipitation, burning regimes or herbivory could be driving woody encroachment, but are unlikely to be the explanation of this global phenomenon.



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