Livestock Grazing Alters Seasonal Water-Use Strategies of Mediterranean Oaks

A dry climate and intense grazing by livestock are dominant drivers of vegetation structure and ecosystem processes in Mediterranean woodlands. Ecological studies predict contrasting effects of grazing on tree drought stress, reflecting a balance between browsing damage and reductions in competition for water with the herbaceous layer.

We investigated the combined effects of grazing and drought on the water status of Quercus coccifera, the dominant evergreen oak of the Eastern Mediterranean region. We set up an ecophysiological experiment in Southern Israel, at the dry edge of distribution of oak woodland ecosystems. We compared mature oak trees exposed to continuous cattle grazing with four nearby individuals protected from grazing by fencing. Trees have been monitored using high-frequency sensors measuring sap flow, stem water content, and radial stem growth, complemented by continuous meteorological observations and soil water content. We analysed the effects of grazing, season, and their interaction effects using linear mixed-effects models. Furthermore, we applied structural equation modelling to disentangle direct and indirect relationships between climatic drivers and ecophysiological variables.

All ecophysiological variables exhibited strong seasonal patterns, with a significant buffering effect of grazing on tree water use in the dry season. Grazed trees maintained higher stem water content and higher transpiration rates relative to ungrazed trees during periods of high atmospheric and soil drought. The climatic control on tree water deficit and sap flow differed between seasons, with vapor pressure deficit dominating during the wet season and radiation controlling water fluxes during the dry season. Stem water content functioned as an internal water reservoir, buffering tree water deficit in winter and sustaining transpiration during summer drought.

Our results suggest that grazing can buffer drought stress during periods of low water availability, likely by reducing competition for soil water with the herbaceous layer. By improving the tree water status, grazing may increase tree resilience under drought stress and mitigate climate change effects.