Browsing impact on Mediterranean forests: The role of introduced deer and implications for conservation

Herbivory may threaten forest conservation, depleting habitat quality in protected areas, yet effects of herbivore population densities on Mediterranean forests remain poorly understood. Holm oak Quercus ilex forests are the most widely-distributed in the Mediterranean, being carbon sinks and protected under the EU-directive, despite unfavourable status. We investigated how browsing pressure by introduced fallow deer Dama dama and native roe deer Capreolus capreolus affected habitat quality of Holm oak forests in protected areas. Typical plants' cover increased forest patch palatability to deer, escalating their browsing pressure. Thus, deer impact may target sentinel species of this habitat. However, only spatiotemporal increases in fallow deer densities boosted browsing at the community- and sentinel species-levels, depleting up to half of the browse responsible for plant regeneration when at moderate-high density. This result highlights stronger ecological impacts of introduced deer compared to their native counterparts. We found fallow deer inhabiting ∼20 % EU-protected Holm-oak forests, which raises potential bioregion-wide threats to Mediterranean forest conservation. Precipitation did not dilute deer browsing via enhanced plant resprouting; therefore, reversing current Mediterranean aridification would benefit forests only if deer densities are mitigated. Fallow deer affected browsing at multiple spatial-scales, recommending coordinated management actions at the appropriate extent to reduce impact. Throughout our study, the decline in fallow deer density, possibly triggered by wolf Canis lupus recolonisation, resulted in browsing collapse, suggesting that top-down control by apex predators may entail forest restoration. Our findings contribute practical solutions to foster conservation of Mediterranean forests under browsing pressure, offering insights into ecosystem resilience.