Modelling the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation

During the past century, annual mean temperature has increased by 0.75 degr ees C and precipitation has shown marked variation throughout the Mediterra nean basin. These historical climate changes may have had significant, but presently undefined, impacts on the productivity and structure of sclerophy llous shrubland, an important vegetation type in the region. We used a vege tation model for this functional type to examine climate change impacts, an d their interaction with the concurrent historical rise in atmospheric CO2. Using only climate and soil texture as data inputs, model. predictions sho wed good agreement with observations of seasonal and regional variation in leaf and canopy physiology, net primary productivity (NPP), leaf area index (LAI) and soil water. Model simulations for shrubland sites indicated that potential NPP has risen by 25% and LAI by 7% during the past century, alth ough the absolute increase in LAI was small. Sensitivity analysis suggested that the increase in atmospheric CO2 since 1900 was the primary cause of t hese changes, and that simulated climate change alone had negative impacts on both NPP and LAI. Effects of rising CO2 were mediated by significant inc reases in the efficiency of water-use in NPP throughout the region, as a co nsequence of the direct effect of CO2 on leaf gas exchange. This increase i n efficiency compensated for limitation of NPP by drought, except in areas where drought was most severe. However, while water was used more efficient ly, total canopy water loss rose slightly or remained unaffected in model s imulations, because increases in LAI with CO2 counteracted the effects of r educed stomatal conductance on transpiration. Model simulations for the Med iterranean region indicate that the recent rise in atmospheric CO2 may alre ady have had significant impacts on productivity, structure and water relat ions of sclerophyllous shrub vegetation, which tended to offset the detrime ntal effects of climate change in the region.