Evidence of a negative effect of prolonged drought periods on woody vegetation cover and communities in the Mediterranean region, Israel

Yoni Waitz, Ron Drori and Efrat Sheffer, Robert H. Smith Institute of Plant Science and Genetics in Agriculture, Hebrew University of Jerusalem, Rehovot, Israel

Background/Question/Methods - Climate predictions, based on currently used emission scenarios, indicate that Israel is expected to experience a most severe drying, ~20% reduction in rainfall. Such climatic changes are unprecedented and therefore likely to impact the local flora and fauna. The rapid rates of change in climatic regimes may not allow time for species adaptation and acclimation.
Here, we evaluate the effects of a relatively arid decade (2008-2018) on the cover and diversity of woody perennials (trees, shrubs and vines) across a wide landscape in the Mediterranean region, Israel. We monitored vegetation in 71 sites with various vegetation formations – from sparse shrublands to dense woodlands - along a steep precipitation gradient (350 to 850 mm/year) in two periods - 2008/9 and 2018/9.
Using maximum likelihood and model comparison methods, we assessed the multi-factor effects of a prolonged drought period, coupled with environmental factors on the patterns and trends of the general vegetation cover and of dominant species.
Results/Conclusions - Our field surveys revealed a decline in woody vegetation cover between the two surveys, which was associated with the precipitation gradient. The relative change in the woody cover along the decade was positively correlated with the gradient of mean annual precipitation and with species richness. Additionally, the resistance of the woody vegetation to change was positively correlated with annual precipitation, species richness and the initial woody cover, indicating that rainy and diverse habitats are stable compared to dry and species-poor habitats.
Five out of the seven dominant species of trees and shrubs in both surveys showed a decline in cover between surveys, with no relation to the precipitation gradient or changes in climatic factors, associated with their initial cover. Only one species - Quercus calliprinos, the most dominant oak in the region, exhibited an increase in cover between surveys, positively correlated with the mean precipitation gradient.
Our results imply that ongoing aridity has been influencing the local structure of woody vegetation and the landscape in the Mediterranean region, while a high species richness contributes to the ability of vegetation to cope with aridity stress and persist, potentially resisting climate change scenarios.