This study is part of the OPTIMASS, an interdisciplinary project funded within the SPACES-programme of the BMBF with the aim of implementing sustainable management of savanna ecosystems.
There is yet no consensus about the main mechanisms that drive shrub encroachment in southern African savannas, and thus recommendations regarding management are difficult to make. Much work has been done on effects of fire, grazing, water and nutrient availability on rangeland productivity but the role of climate change, namely changes in CO2 concentrations, temperature, and rainfall and their potential interaction with manageable factors (grazing, fire) is not understood. A unique feature of tree-grass interactions in savannas is the fact that the two life forms are characterized by two different photosynthetic pathways (C4 grasses vs. C3 trees and shrubs). Therefore, CO2 should play a key role in shrub encroachment, but experimental evidence is missing in the field and rare in common gardens, and interactions between CO2 and factors amenable to manipulation (i.e. adaptive management of fire, grazing, and nutrients) are unknown.
Within the OPTIMASS project, we investigate the combined role of elevated CO2, temperatures, drought and land use on the balance between shrubs and grasses in African Savannas. We use a climate gradient in Namibia which spans from approx. 600mm average annual rainfall in the North to 250mm in the South to set up experiments at a nested scale of realism and detail. On the large scale (climate gradient), we monitor shrub-grass interactions, and we perform reciprocal transplant of shrubs and grasses. Within one focal site, we manipulate climate in a realistic manner and measure how increased drought affects interactions among shrubs and grasses. On a more controlled level, we study, by means of greenhouse experiments, how shrub-grass interactions at a seedling stage are mediated by drought and land use. Finally, climate chamber experiments will look at CO2 as an additional independent variable. Our overall goal is to determine the relative importance of drought (which will favour C4 grasses over C3 shrubs) and CO2 (which will favour the shrubs) on shrub encroachment in African savannas.