Document: KER-3-37-20

Bush encroachment as an integral part of savanna dynamics.

WIEGAND, K.* ^1,2, D.WARD ² and D.SALTZ ^2,3

Princeton University, Princeton, NJ 08544 USA ¹
Ben Gurion University of the Negev, Sede Boqer 84990 Israel ²
Israel Nature Reserves and National Parks Authority, Jerusalem 95463 Israel ³

Abstract:
For many years, the coexistence of woody and grassy plants in savannas has been attributed to a rooting niche separation. It was assumed that water is the limiting resource for both growth forms and that grasses take their water from the upper soil layer and trees and bushes from the lower. However, field studies showed that such a niche separation is the exception rather than the rule among different savannas. Furthermore, with a spatially explicit simulation model it has been shown that even if the two-layer hypothesis were true, it would not be sufficient to allow coexistence under a range of climatic situations. Bush encroachment (i.e. the suppression of palatable grasses and herbs by encroaching woody species often unpalatable to domestic livestock) is a serious problem in many savannas because it threatens the living of many pastoralists. In the literature, the occurrence of bush encroachment is almost exclusively attributed to overgrazing. We tested the hypothesis that under certain (climatic) conditions bush encroachment is part of a cyclic succession between open savanna and woody dominance driven by only two factors: Rainfall highly variable in space and time, and between-tree competition. In this case, savannas are patch dynamic systems, i.e. savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance. Taking a savanna in Namibia as an example, we analyzed local tree size distributions, grass production, and soil nutrient concentrations across the savanna. Furthermore, we analyzed the results of removal experiments testing for the effect of competition on tree growth and combine modelling and field observation to elucidate the effect of inter-tree competition on tree mortality. All analyses clearly showed that the savanna under study is a patch dynamic system as described above.

Keywords: savanna, tree-grass coexistence, patch dynamic system, Namibia

This abstract is being presented at: 1:30 PM in session:
Oral Session #16: Plant Demography: Trees and Shrubs.