Impact of water stress on Mediterranean oak savanna grasslands productivity

Seminatural grasslands located in Mediterranean areas support a remarkable species diversity and provide multiple ecosystem services, including pasture and forage production and high levels of carbon sequestration. The presence of sparse oak trees in these grassland ecosystems creates a variety of microclimates that contributes to increasing the diversity of plant communities. The phenology and production of the grassland are highly controlled by the water availability, hence the large inter-annual variability in production is directly linked to the variability of the Mediterranean climate. In this work, we have analyzed the links between grassland production and water stress using satellite data to assess the evapotranspiration and biomass accumulation from 2001 to 2018 over the oak savanna grasslands of a region of Southern Spain. 

A surface energy balance model, SEBS (Surface Energy Balance System), has been applied to estimate evapotranspiration (ET) at monthly scale and 0.05-degree pixel size for the study period, using MODIS data and a global atmospheric reanalysis dataset. The anomalies of the ratio of ET to reference ET were used as an indicator of agricultural drought at the monthly and annual scales, and to characterize the main drought events that occurred in this period.

The biomass production was estimated using an adaptation of the Monteith crop production model, based on the relationship between plant growth and incident solar radiation. This adaptation pays special attention to the presence of a tree layer with a variable density as part of the ecosystem that influences spectral data, and to the empirical estimation of the light use efficiency for these seminatural grasslands using biomass field measurements. For this model, the scale of application was 250 m and 16 days.

The close links between grassland production and drought events at the regional scale and over ten selected farms has been analyzed, taking into account the need for external livestock feeding at both scales during these dry events.