Past and Future Climate Change Impacts on the Sustainability of a Wild Olive–based Heterogenous Ecosystem in Sardinia

Mediterranean ecosystems are commonly heterogeneous savanna-like ecosystems, with contrasting plant functional types (PFTs, e.g., grass and woody vegetation) competing for the water use. At the same time the structure and function of the vegetation regulates the exchange of mass, energy and momentum across the biosphere-atmosphere interface, influencing strongly the soil water budget. Mediterranean regions suffer water scarcity due to (in part) natural influences, i.e., climate changes. Future climate scenarios are predicting further warmer conditions, increasing the uncertainty on the future of the water resources system of these regions.

The objective is to investigate the role of the PFT vegetation dynamics on the soil water budget of a typical water-limited Mediterranean ecosystem in Sardinia, Italy, for both past and future climate conditions. The Sardinian site is characterized by strong heterogeneity, with wild olive trees coexisting dynamically with grass and bare soil, and a long database of land surface data is available from 2003, when an eddy covariance based tower was installed for estimating evapotranspiration, CO2 exchanges and energy fluxes. Sap flow of the wild olives (both in the trunk and in the roots), soil moisture, and leaf area index (LAI) are also measured. In water-limited conditions trees survive absorbing water from underlying fractured bedrocks through roots (hydraulic redistribution, HR). An ecohydrological model, based on the coupling of a land surface model (LSM) and a vegetation dynamic model (VDM) predicted the soil water balance, the tree and grass LAI, HR, and the dynamics of this sensitive ecosystem. The model was successfully tested for the case study, demonstrating model high performance for the wide range of eco-hydrologic conditions.

Interestingly, from 2003 tree cover increased reaching an almost constant LAI (around 4) after six years, but the tree cover dramatically decreased in the last 4 years due to a dramatic drought in 2017, which significantly affected the tree sustainability. Indeed, the winter precipitation decreased in Sardinia, with a concomitant increase of air temperature during the spring and summer seasons.  Future climate scenarios predicted a further increase of air temperature and, therefore, of vapor pressure deficit (VPD), and a decrease of winter precipitation with a concurrent increase of rain extremes. We used the future climate scenarios predicted by Global climate models (GCM) in the Fifth Assessment report of the Intergovernmental Panel on Climate Change (IPCC). Hydro-meteorological scenarios are generated using a weather stochastic generator that allows simulation of hydrometeorological variables from GCM future scenarios. The use of the calibrated VDM-LSM allow to predict soil water balance and vegetation dynamics for the generated hydrometeorological scenarios. Results demonstrate that tree dynamics are strongly influenced by the inter-annual variability of atmospheric forcing, with tree density changing according to seasonal rainfall. At the same time the tree dynamics affected the soil water balance. We demonstrated that future warmer scenarios will impact wild olive trees, which could be not able to adapt to the increasing droughts. The decrease of tree cover will affect water resources and carbon balance of the heterogenous Mediterranean ecosystem.