Nutrient and Light Limitations on Vegetation Dynamics in the Savannas of Southern Africa

The role of precipitation as a limiting factor on savanna vegetation structure and function has been comprehensively documented. However, the distribution and availability of soil nutrients (e.g. nitrogen and phosphorus) may also play pivotal roles on vegetation dynamics in savannas, particularly those found in the Kalahari region of southern Africa. The Kalahari is a sand-filled basin that occupies approximately one third of southern Africa. The International Geosphere-Biosphere Programme designated the Kalahari Transect (KT) as a unique series of research sites along a latitudinal precipitation and vegetation gradient. The distribution of and limitation on aboveground grass production from soil nutrients, in particular phosphorus (P), is investigated for four sites along the KT to characterize potential nutrient limitations. Research sites lie along this dramatic precipitation gradient ranging from ~920 rain/year in the north to ~260 mm rain/year in the south on structurally homogeneous sandy soils. Although the savannas at each of these sites host distinct species, they are characterized by the unique co-existence of two distinct life forms; trees and grasses. Total [P] does not vary along the transect but plant available [P] is negatively correlated with precipitation. In contrast to island of fertility hypotheses, P soil concentrations did not show horizontal heterogeneity under and between canopies for any sites. Generally, under increased conditions of plant available [P], grass species exhibit higher levels of foliar nutrients although relative differences between sites was not as apparent. There were no changes in aboveground biomass due to N and P additions but inter-annual differences in biomass between sites were observed. In Pandamatenga, an Lydia Polek Ries (2007) 4 open woodland savanna, there were differences in biomass production but no differences in physiological response variables net carbon assimilation or stomatal conductance. Limitations may exist but are masked by factors operating at commensurate time and spatial scales such as precipitation or inter-species competition for light. Inter-annual and site specific differences emphasize variability in resource availability and use in these systems. This may be the mechanism that allows for the coexistence of such disparate life forms uniquely characteristic of savanna ecosystems in Africa and throughout the earth. Furthermore, we now see that the role of nutrients in landscape scale productivity models should not be overlooked as we begin to employ and depend on predictive models to assess ecosystem change under climate change scenarios.

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