Tree recruitment in a drought- and herbivory-stressed oak-beech forest: Implications for future species coexistence

Abstract

Mixed oak-beech forests are particularly valuable from an ecological and socioeconomic perspective but are increasingly exposed to anthropogenic impacts. We wonder whether anthropogenic change drivers, such as increased drought and ungulate populations affect optimal regeneration niches, species coexistence, and mixed-species forest dynamics. We compared understorey composition, structure and distribution with the overstorey counterpart in an oak-beech forest located at its southernmost European distribution (Central Spain) with increasing drought and ungulate herbivory for the last decades.Results showed that Ilex aquifolium and Prunus avium had greater representation in the recruitment layer as compared to the understorey layer whereas, for the dominant Quercus spp. and Fagus sylvatica, there was a tendency suggesting an expansion of Fagus at the expense of Quercus. Density of tree recruits showed strong differences across microsites. The greatest density was found under tree and under shrub cover whereas the lowest was found under woody debris. By species, Quercus regenerated better in open and shrub microsites whereas Fagus and Ilex regeneration was higher under tree cover. Large patches of shrub cover favoured Prunus avium recruitment, particularly against ungulate herbivory as it was the most heavily browsed species, followed by Quercus, Ilex and Fagus in a descending order. Herbivory occurrence and intensity on tree recruits strongly varied with the microsite, with greater herbivory levels in open microsites. Woody debris on the ground only protected tree recruits from browsing when they were located under tree cover.Overall, fenced areas with no ungulates showed greater tree species coexistence (28% increase in species richness) and recruitment abundance (81% increase) after 7 years of exclusion. We also found a large increase in tree recruit growth and height:diameter ratio (h:d) for all studied species when fenced. Fagus and Prunus avium showed the greatest growth and h:d increase, much greater than those of Quercus and Ilex. The h:d was identified as a reliable indicator of ungulate herbivory and reference values (under null levels of ungulate herbivory) were provided for each species.We conclude that there is an imperative need for adaptive management strategies that address the joint effects of climate change (e.g. increased drought) and ungulate herbivory in mixed oak-beech forests. The provision of different microsites, including patches of shrub cover in open microsites and woody debris under dense tree cover, will facilitate the availability of suitable regeneration niches for most species and will therefore promote future species coexistence.