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 activity during the 1980s, caused by climatic warming. However, whether this was due to artificial trends in the satellite data remains in question. We used a mechanistic model of vegetation growth and a database of observed climate to test whether the observed increase in NDVI could have been caused by changes in canopy structure driven by changes in climate. The model reproduced the longterm upward trend in maximum seasonal NDVI between 1981 and 1991, indicating that a change in vegetation structure could feasibly explain the satellite observations. The model indicated that the NDVI trend was caused by a 12% increase in leaf area index (LAI), mainly owing to changes in precipitation and rising atmospheric CO2. By contrast, climatic warming during 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% increase in transpiration. From this analysis, we conclude that trends in historical satellite observations of NDVI have a plausible biological basis.