IMPACT OF CLIMATE VARIABILITY ON PRESENT AND HOLOCENE VEGETATION: A MODEL-BASED STUDY WITH EXAMPLES FROM CHINA AND NORTHERN AFRICA

Changes in the mean state of climate may be accompanied by changes in interannual and interdecadal variability. The impacts of climate change on ecosystems may derive as much from changes in variability as from the mean state of climate. However, relationships between mean climate state and climate variability and impacts of interannual and interdecadal climate variability on regional vegetation have not been extensively investigated. A series of nine simulations have been made with the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM) in order to explore the impacts of climate variability and Holocene changes in variability (as simulated by the Fast Ocean-Atmosphere Model, FOAM) on vegetation in three regions of China (northeastern China, central China and southern China) and in the Sahelian region of northern Africa. The simulations show that changes in both the magnitude of climate variability and the persistence of above/below average conditions have the potential to substantially modify the vegetation response to changes in mean climate. Changes in moisture availability can affect vegetation through drought stress and, in regions where fires are limited by lack of fuel, through changing the availability of fuel load. Increasing moisture availability causes trees to replace grasses in China by reducing drought stress; increasing moisture availability in the Sahel increases the available fuel and hence reduces fire return times leading to a reduction in woody plants and favouring grasses. The modelling results imply that climate variability is important to vegetation dynamics; that not only the magnitude, but also the temporal structure of variability is important; and that simulating vegetation changes in response to climate variability requires a realistic simulation of plant community composition. Furthermore, they indicate that the impacts of climate change on ecosystems may derive as much from changes in variability as from changes in mean climate.