Novel insights into the spatial dynamics of english Holly (Ilex aquifolium) invasion in an urban forest ecosystem in the Pacific Northwest of the united States

Invasive plant species threaten forest ecosystems by outcompeting native flora for essential resources such as sunlight, water, and nutrients. One such invader, English holly (Ilex aquifolium), a shade-tolerant evergreen shrub or tree native to Eurasia and northern Africa, has become increasingly problematic in the Pacific Northwest of the United States. This study investigates the environmental factors influencing I. aquifolium distribution and spread using species distribution modeling (maximum entropy) and spatial analysis (Ripley’s K function) in an urban forest near Seattle, Washington. Results indicate that evergreen forests support holly establishment more than deciduous vegetation. Temperature emerged as a strong positive predictor of holly presence, suggesting that climate change could accelerate its invasion. In contrast, soil nutrient levels showed a slight negative association with holly occurrence, potentially indicating either holly’s tolerance for nutrient-poor soils or a disadvantage in nutrient-rich environments. Additionally, holly may deplete soil nutrients through uptake or by altering soil chemistry. Further, the clustered pattern of holly dispersion is likely driven by a combination of short-distance seed dispersal and vegetative spread. By identifying the ecological conditions that favor holly’s spread, this study provides insights to inform management strategies aimed at mitigating its impact on native biodiversity, forest succession, and ecosystem services in urban forests in the Pacific Northwest of the United States.