Natural regeneration of Pinus pinaster facilitates Quercus ilex survival and growth under severe deer browsing pressure

Maritime pine (Pinus pinaster) stands in Italy have recently been invaded by maritime pine bast scale (Matsucoccus feytaudi), resulting in widespread tree mortality. Clearcutting has been applied to progressively remove infected maritime pine, concurrent with planting of holm oak (Quercus ilex) seedlings. Establishment of holm oak regeneration has, however, been severely constrained by fallow deer (Dama dama) browsing pressure. We hypothesized that the ability of planted holm oak to grow above the browse line was closely correlated with protective cover provided by natural regeneration. This study was carried out in a coastal protected area in central Italy, where overall performance of planted holm oak was assessed under browsing pressure in relation to the relative distance of seedlings to surrounding vegetation. To interpret the results, four categories of holm oak were identified: (i) dead plants, (ii) sprouts from stump, (iii) repeatedly browsed plants without growth, and (iv) plants that were able to develop at least one tall shoot safe from browsing. The occurrence of the latter was positively related to increasing canopy cover and plant height of surrounding vegetation, especially maritime pine. Density, height, canopy cover, and crown radius of maritime pine, as well as its relative distance from holm oak all promoted the survival and growth of target plants. Complete protection around holm oak was not required for facilitation, but the presence of crown and branches occupying the space above the target plants was crucial. Maritime pine saplings primarily served as a mechanical barrier to browsing and thereby favored the development of holm oak shoots. Site-specific, long-term monitoring studies on competition from and facilitation by surrounding vegetation are required, before implementing under-planting strategies.