Nonnative English Holly (Ilex aquifolium L.) Associated with Reduced Cover and Diversity of Common Native Understory Plant Species

English holly (Ilex aquifolium L.) is a nonnative shade-tolerant evergreen shrub or small tree invading forests of the Pacific Northwest region of the United States. The species is rapidly increasing and has no ecological analogue in the native forests it is invading; however, little scientific documentation exists regarding its effects on native vegetation. We quantified the cover of native woody and evergreen herbaceous plant species underneath and adjacent to large I. aquifolium trees in a native forest in Washington State. At 20 study trees we recorded estimates of native understory vegetation cover in four 1 m2 plots (n = 80) arranged along a transect designed to capture hypothesized shade effects of I. aquifolium. Relative to control plots (south of canopy edge), total native plant cover was strongly reduced underneath I. aquifolium canopy (71% mean reduction) and moderately reduced in the shade of I. aquifolium (north of canopy edge, 37% reduction). Diversity (as measured by species richness and Shannon index) was also strongly reduced under I. aquifolium. Species that were reduced under I. aquifolium included the ecologically and economically significant Gaultheria shallon and Polystichum munitum. Dense year-round shade from I. aquifolium appears to be a key mechanism driving these patterns, facilitated by species traits including an evergreen habit and greater height than native understory competitors. Our findings suggest that I. aquifolium can greatly reduce or exclude native vegetation and, if left unmanaged, may pose a serious threat to biodiversity, successional development, and ecological function of native forests in the region.