Hydrologic sustainability of a mediterranean tree-grass ecosystem under climate change

In Mediterranean dryland ecosystems climate change is occurring with an increase in air temperature and a decrease (mainly in wet seasons) of precipitation, which are key for grass and tree growth. We investigated an attractive Sardinian case study with a typical tree-grass ecosystem, where wild olives and seasonal grass species grow on thin surface soil layer overlaying a fractured rock sublayer. A very long-term database with almost 60 years of data is available, with micrometeorological and meteorological measurements, and remote sensing data, providing a unique opportunity to analyze the response of tree-grass ecosystems to historical climate and land cover changes. We proposed an ecohydrological model that was able to reproduce the soil, vegetation, and atmosphere interactions and dynamics, and their long-term evolution. The model accurately predicted the long-term dynamics of the tree cover fraction, which was drastically reduced (∼0.10) by a human-induced fire about 50 years ago, and restored naturally in almost 20 years, reaching the equilibrium value (∼0.33). The Sardinian tree-grass ecosystem suffered a historically significant reduction in rain and a significant increase in air temperature in the last century. The predicted future scenarios are even more severe, with a further decrease of mean annual precipitation (MAP) of up to ∼400 mm, and an increase of air temperature of +4 °C, which will cause a reduction of the tree cover fraction of up to 0.10 and a strong decrease of the tree LAI. At present, the developed tree cover percentage of the Sardinian site is sustainable with the historical MAP (>600 mm/y), thanks to the tree hydraulic redistribution contribution to transpiration (up to 80 %). In the predicted future scenarios, the increase of dry conditions with a wetness index (precipitation/potential evaporation) below 0.005 will increase the hydraulic redistribution contribution, reaching 91 % of tree transpiration, which, however, will be not enough to support tree growth and maintenance. The soil is predicted to become drier, with less grass and vegetation in general, with consequences for the landscape aspect, becoming more and more a savanna-like ecosystem.