Biological mechanisms underlying recent increases in the NDVI of Mediterranean shrublands

Long-term satellite observations of normalized difference vegetation index (NDVI) for Mediterranean shrublands suggest an increase in vegetation activ ity during the 1980s, caused by climatic warming. However, whether this was due to artificial trends in the satellite data remains in question. We use d a mechanistic model of vegetation growth and a database of observed clima te to test whether the observed increase in NDVI could have been caused by changes in canopy structure driven by changes in climate. The model reprodu ced the longterm upward trend in maximum seasonal NDVI between 1981 and 199 1, indicating that a change in vegetation structure could feasibly explain the satellite observations. The model indicated that the NDVI trend was cau sed by a 12% increase in leaf area index (LAI), mainly owing to changes in precipitation and rising atmospheric CO2. By contrast, climatic warming dur ing the 1980s exerted little control over this variation in LAI. Simulated trends in canopy structure exerted significant impacts on canopy function, being associated with a 15% rise in net primary productivity and a 30% incr ease in transpiration. From this analysis, we conclude that trends in histo rical satellite observations of NDVI have a plausible biological basis.