Aridification determines changes in forest growth in Pinus halepensis forests under semiarid Mediterranean climate conditions

Using a set of 16 Landsat TM and Landsat ETM+ images from 1984 to 2006, we tested whether climate trends in the last three decades differentially controlled the vegetal activity of eight Pinus halepensis forests located across a marked bioclimatic gradient. Our results show spatial differences in trends in the normalized difference vegetation index (NDVI) between 1984 and 2006, which were highly related to the spatial distribution of aridity. There was a strong correlation between the change in the NDVI values and the climatic water balance in each forest (R =0.86, p =0.006). A large increment of the NDVI was observed in forests located in the most humid areas, whereas those located in the most arid areas showed a slight decrease in the NDVI. We used a forest growth simulation model (GOTILWA+) to estimate the leaf area index (LAI) in each of the eight analyzed forests, and found similar results to those obtained for the NDVI. Significant correlation was found between the spatial pattern of NDVI and LAI trends between 1984 and 2006 (R =0.82, p =0.01). The climate evolution in the last four decades explains the observed and modeled changes in the NDVI and the LAI. In agreement with other studies, we showed that the general response of the forests to more favorable conditions (in terms of temperature increment and CO₂ fertilization) is increased leaf activity and biomass. Nevertheless, in forests located in the most arid areas the positive trend observed in the potential evapotranspiration rates increased water stress, and had a negative effect on forest growth. Given the future predictions of warming and declining precipitation from global climate models for the Mediterranean region, an increase in stress conditions affecting these forests is expected, which will affect their growth and survival, mainly in the most arid areas.