Savanna vegetation structure. Top down or bottom up?

Background/Question/Methods

Bill Robertson of the Mellon Foundation may be well known for sponsoring high risk high reward research but he was less well known for his commitment to both racial and gender equity. His belief was that he had to find bright kids from disadvantaged backgrounds who could make it, but not without a little help from the Mellon Foundation, and get them through and into academic careers. Along with addressing some of the social inequalities in South Africa he also prioritised supporting research in National Parks that would broaden our understanding of how savanna systems work. This focus resulted in a number of collaborative programs that have given us a mechanistic understanding of savanna systems. Until very recently it was understood that water, nutrients, fire and herbivory interacted to determine savanna vegetation structure, we were, however, not sure how, if at all, this worked. Our Mellon funded research has resulted in a much broader mechanistic understanding of savanna systems.

Results/Conclusions

We now understand that the availability of water and nutrients not only determine grass biomass but also grass species composition and that grasses rather than trees are superior competitors for resources. Trees avoid competition with grasses by leafing out early and taking up nutrients before grasses are able. These results suggest that trees are only able to establish when grass biomass is low through drought or intense grazing. Funding for the manipulative experiments necessary to develop this understanding was not possible without the Mellon Foundation and here I reflect on how this understanding was achieved in tandem with Bill Robertson’s commitment to racial and gender equity.