Abstract: Oak woodlands in the eastern Mediterranean region support a large diversity of plants and animals and are often dominated by the evergreen tree Quercus calliprinos. However, these communities are threatened by climate change-induced heat and drought, especially at the dry edge of their distribution. Livestock grazing is the most common land management tool in many dryland regions. If applied at moderate rates, grazing prevents canopy closure and the loss of the rich herbaceous vegetation. Research on grazing mainly focuses on biodiversity and herbaceous biomass production, and much less on the consequences for woody vegetation. Grazing, mainly by cattle, can impact trees by reducing competition with herbaceous vegetation and by altering soil functions, i.e. chemical, biological and physical processes in the soil. Currently, we have a very limited understanding of grazing effects on the performance of trees growing under dry conditions and in a changing climate. The overarching objective of this study was to evaluate the impact of climate and grazing on oak woodlands at the edge of the desert. We specifically aimed to assess water relations, nutrient cycling, soil functions and tree growth as affected by diverse climate and grazing conditions. We hypothesized that cattle grazing partly mitigates drought effects on the growth of oak trees by improving tree water and nutrient status, and by increasing soil faunal and microbial activity and nutrient cycling. The methodological approach included a grazing exclusion experiment in the driest region, and a multiple-site experiment with various grazing intensities along a small climate gradient.
Our results showed that oaks in the driest as compared to more moist woodlands were smaller and grew at low density, had low structural flexibility, exhibited the highest degree of drought stress, and had the lowest nutritional status. However, grazing consistently improved the water status and increased the growth rate of the oak trees in the driest region. Soil microbial activity was high and the decomposer arthropod community composition was changed by grazing, but only in those locations under oak trees that were most disturbed by cattle. Litter decomposition was reduced by grazing, likely by reducing litter quality, and grazing did not affect soil nutrient cycling and tree nutrient status. This study showed that cattle grazing can mitigate the negative impacts of climate change-induced drought on oak trees and might advance the conservation of oak woodlands at the edge of the desert.