Livestock grazing in an agro-pastoral land outside the Serengeti National Park, Tanzania. Photo: Per Harald Olsen/AfricanBioServices
Shel Silverstein’s classic children’s novel, “The Giving Tree”, details the relationship between a boy and a tree. In the course of the boy’s life the tree provides the boy with everything: a place to rest and play, apples to eat and sell, and, eventually, branches and a trunk to construct a house and boat. As Silverstein’s book progresses the boy’s demands from the tree increase; in the end, all that is left of the tree is a stump on the ground – and a sad old man that the boy has grown into.
I have read this story many times to my daughter, who, despite the story’s sadness, is fascinated by it. The story has also made me reflect on my own work in East African savannahs, where trees provide multiple services to plant, animal and human communities: they provide habitats for bugs and birds, shade and food for wildlife and livestock, and water and nutrients for grasses growing beneath them. As in Silverstein’s story, the demands on savannah trees are increasing. But is there a point when savannah trees stop giving?
Continuous livestock grazing underneath trees in Mwantimba village, Bariadi district, Tanzania bordering the Serengeti National Park. Photo: Vegard Martinsen
In East Africa, increasing numbers of livestock are replacing wild herbivores. More livestock eat more grass, potentially undoing the positive effects that trees exert on understorey grasses. Under livestock grazing grass species only survive if they tolerate heavy grazing and trampling and these may not always be the most productive grass species. As part of the AfricanBioServices project, we wished to investigate how increasing livestock pressure at the borders of the Serengeti National Park, Tanzania, is influencing the grass species growing beneath trees.
Over the course of two years, we measured the grass underneath and away from savannah trees on a monthly basis. We also measured grass biomass inside and outside small cages to test the effect of excluding herbivores. Unsurprisingly, we found that more livestock reduced the amount of grass. Yet, trees limited the negative influence of herbivory on grass biomass. The capacity of a tree to limit the impact of herbivores related to specialist grass species found beneath the tree. We found a number of productive understorey specialist grasses, such as Panicum maximum. These grass species tolerated low to moderate grazing, but only persisted inside exclosures at the highest livestock intensities.
A herbivore exclosure tree understorey, dominated by a thick sward of understorey specialist grass Panicum maximum. Photo: Stuart Smith
The focus of Silverstein’s “The Giving Tree” is very much on the tree and the boy. As is so often the case, ecological relationships are shaped by indirect interaction, such as the involvement of understorey grasses. Heated scientific and policy debate rages as to the appropriate number of trees in African savannahs. Suggestions to afforest savannahs are viewed as ecologically inappropriate for an ecosystem that should have naturally low tree densities. Cutting down and clearing trees from savannahs, similar to the fate of the tree in Silverstein’s book, is equally not a desired outcome. Perhaps in all this debate, an ecological warning that our tree-human relationship is on the verge of a breakdown is the loss of specialist grass species found beneath savannah trees.
Our research team discussing the importance of grazing management, large trees and understorey grasses with farmers from Mwantimba village. Photo: Per Harald Olsen/AfricanBioServices
The paper ‘Savannah trees buffer herbaceous plant biomass against wild and domestic herbivores’ published in Applied Vegetation Science was a collaborative effort between researchers from the Norwegian University of Science and Technology, Tanzanian Wildlife Research Institute, Sokoine University of Agriculture and the Nelson Mandela African Institute of Science and Technology was part of the AfricanBioServices project funded by EU Horizon 2020 (grant 641918).
Stuart W. Smith on behalf of all co-authors
Publication
Savannah trees buffer herbaceous plant biomass against wild and domestic herbivores (PDF)