The impact of climate change decrease of winter precipitation on the water use efficiency and sustainability of a Mediterranean forest.

Over the past century, climate change has been reflected in altered precipitation regimes worldwide. Recently, Montaldo and Sarigu (2017) showed that Sardinia runoff decreased over the 1975-2010 period, with mean annual values 40% lower than the 1922-1974 period.

The Marganai forest, located in South West Sardinia (Italy), is a European Site of Community Importance (Natura 2000) and case study of the SWATCH European Research Project. The trees are mainly Quercus Ilex and the soil depth varies between 10 cm and 50 cm. Historical precipitation from 16 rain stations (1928-2018) and runoff at the Fluminimaggiore section (basin area of 83 km²) are available. A persistent decrease trend of winter precipitation (Mann-Kendall t of -0.11) impacted runoff, which decreased with an annual t of -0.22, with a potential impact on the protected forest.

With the objective of evaluating the impact of past and future climate change on the forest, we first developed a distributed eco-hydrological model at daily time scale for predicting both hydrologic (soil moisture, runoff, evapotranspiration, ET) and vegetation dynamic variables.

The model has been successfully calibrated for runoff and ET estimation for the 1929 - 2018 period. We have predicted an alarming significant decrease trend of spring ET (t of -0.17), and spring leaf area index (LAI) (t of -0.16). This is consistent with observations in the forest during the autumn 2017, with trees suffering the long 2017 drought.

We then investigated the impact of future climate change scenarios, selected from IPCC climate change scenarios. From the 12 GCMs of Flato et al. (2013), we selected 4 GCMs (HadGEM2-AO, HadGem2-ES, HadGem2-CC and CNRM), that simulate reasonably observed past seasonal precipitation and air temperature changes. The eco-hydrological model predicted a significant further reduction of tree LAI (-31%), a reduction of tree density (-11%), and runoff (-43%). The impact of future climate scenarios is particularly alarming for the sustainability of Marganai protected forest, suggesting the need of new strategies in both forestal and water resources planning and management.