MECHANISMS OF SHRUBLAND EXPANSION - LAND-USE, CLIMATE OR CO-2

Encroachment of trees and shrubs into grasslands and the 'thicketizati on' of savannas has occurred worldwide over the past century. These ch anges in vegetation structure are potentially relevant to climatic cha nge as they may be indicative of historical shifts in climate and as t hey may influence biophysical aspects of land surface-atmosphere inter actions and alter carbon and nitro en cycles. Traditional explanations offered to account for the historic displacement of grasses by woody plants in many arid and semi-arid ecosystems have centered around chan ges in climatic, livestock grazing and fire regimes. More recently, it has been suggested that the increase in atmospheric CO2 since the ind ustrial revolution has been the driving force. In this paper we evalua te the CO2 enrichment hypotheses and argue that historic, positive cor relations between woody plant expansion and atmospheric CO2 are not ca use and effect.