Mediterranean ecosystems under threat: impact of recurrent extreme drought events and shrub invasion on savannah-type cork oak forests in Portugal

Extreme drought events and plant invasion are concurrently threatening water-limited cork oak ecosystems on the Iberian Peninsula. Frequency of extreme droughts have increased in the Mediterranean in the past years with detrimental effects on many ecosystems. Such events increase the vulnerability of ecosystems by decreasing soil water, reducing fitness and growth of trees, eventually resulting in increased tree mortality. Shrub invasion can further increase competition for soil water, exaggerating tree vulnerability and decreasing tree resilience. Yet, experimental evidence on the cumulative effects of plant invasion in addition to recurrent extreme drought events is still scarce and inconclusive.

We aim to gain a comprehensive knowledge of the underlying mechanisms of recurrent extreme droughts, shrub invasion and their synergistic effects in an open savannah-type cork oak ecosystem invaded by the native shrub Cistus ladanifer in Portugal. We installed replicates of rainout shelters (30-45% reduction) in invaded and uninvaded cork oak stands in December 2017, complemented by control plots with natural precipitation. Measurements of water and carbon fluxes of the trees (n = 9) and the shrubs (n = 9) in each treatment will allow to unravel species-specific responses and competition effects under recurrent droughts.

The hydrological year 2018 was dominated by large rainfalls in spring and above average annual precipitation. No clear treatment effects were observed due to sufficient water supply, indicated by minimum pre-dawn leaf water potentials (Ψ_PD) of the cork oak trees of only -1.5 ± 0.1 MPa. However, a distinct species-specific behaviour was evident with maximum sap flux densities of the shrubs being over four-fold higher (8.8 ± 0.8 m³ m^-2 day^-1) than those of the trees (2.1 ± 0.2 m³ m^-2 day^-1). In 2019, winter and spring precipitation were strongly reduced, revealing negative combined effects of drought and plant invasion during the dry-down period. Invaded trees exposed to the experimental drought had lower minimum Ψ_PD and average sap flux densities reduced by 50% (0.5 ± 0.1 m³ m^-2 day^-1), compared to control trees (1.0 ± 0.1 m³ m^-2 day^-1). The ongoing investigations target further determination of stress tolerance and critical physiological thresholds for both species and the entire ecosystem.