National forest inventory data reveals differential impacts of deer species on plant growth forms

Through selective herbivory, expanding deer populations can impact the structure and functioning of forests, affecting biodiversity and ecosystem services. Despite decades of research, uncertainty persists around the specific effects on understory plant communities. Here, we investigate impacts of fallow, red, and sika deer on understorey vegetation in 2020 forest plots across Ireland. Relative harvest indices, proxies for deer pressure, were combined with data from Ireland’s National Forest Inventory to test whether paradigms in the literature regarding deer impacts to plant growth forms (ferns and grasses responding positively, herbs and shrubs negatively) are supported. We hypothesise that the nature of impacts depends on ecological traits of deer species, and, that deer are causing detectable shifts in plant community structure, determined by plant palatability/tolerance. We found that plant groups responded differentially for cover, richness, and diversity with notable differences between deer species linked to variation in deer pressure and feeding ecology. Sika deer were the only species that caused the predicted impacts to herbs and shrubs, while impacts on grasses depended on deer feeding ecology rather than abundance related pressure, with ferns being minimally impacted. Deer clearly influenced plant community structure, driving shifts toward assemblages of unpalatable and browse-tolerant plant species. We emphasize caution around generalisations about ‘deer impacts’ to forests and the importance of species-specific effects, while showing that invasive sika deer are strongly affecting understory vegetation in Ireland. An improved understanding of how deer shape plant communities can inform future conservation and forest management.