Tree-grass interactions on an East African savanna : the effects of facilitation, competition, and hydraulic lift

Keywords: Rangelands, Semi-arid areas, stable isotopes, Acacia, C _4- grasses, plant nutrients, soil nutrients, soil water, plant water relations

Savanna trees can either increase or decrease the productivity of understorey grasses. Trees reduce grass growth through competition for nutrients, water and light and can facilitate grass production through increased soil nutrient availability, shade and hydraulic lift. In an East African savanna in Tarangire, I studied what determines whether Acacia tortilis trees interfere or facilitate understorey grass growth and especially whether trees stimulate grass growth through hydraulic lift.

The availability and concentration of all major nutrients is much higher under trees compared to open grassland. This increased nutrient availability under trees changes the nutrient limitation of the herbaceous layer from nitrogen limited in open grassland to phosphorus limitation under the tree canopy. The water availability however was lower under compared to outside tree canopies although we found clear evidence of hydraulic lift. However, exuding large amounts of water into the topsoil (up to 235 l. per night per tree) by large trees through hydraulic lift, could not compensate for water competition between trees and grasses. However grasses probably have access to hydraulically lifted water which indicates that hydraulic lift reduces the severity of water competition between trees and grasses.

So the main processes regulating tree-grass interactions in this East-African savanna are water competition and increased soil nutrient availability. This balance between positive and negative effects of trees on grass growth resulted in equal grass productivity under and outside tree canopies.

Although Acacia tortilis trees did not increase grass productivity, they did have a positive effect on the grass quality for herbivores. Grasses growing under trees have higher nutrient and protein concentrations. Grasses from open grassland, however, have such a low quality that wildebeest cannot maintain a stable body weight by only selecting food from open grassland but need forage from under trees. Large trees are thus essential for the survival of wildebeest in Tarangire National Park.