The role of spatial and temporal scale in colonization and spread of invasive shrubs in early successional habitats

Early successional habitats are becoming increasingly scarce in the northeastern United States and may require additional management to support the diverse groups of plants and animals that are dependent on them. Because these habitats require some form of disturbance to maintain them, they may be especially vulnerable to exotic plant invasions. We examined the vulnerability to invasion of 44 early successional sites in southeastern New Hampshire using a two-tiered approach. Colonization (Tier 1, based on presence/absence) and establishment and spread (Tier 2, based on percent coverage of an occupied site) of invasive shrubs among patches of early successional habitat were related to landscape and local habitat features. Landscape elements (e.g., amount of disturbed lands within 1 km of the invaded patch) and features described at a local scale (e.g., soil pH and clay content) were associated with colonization, whereas spread of invasive shrubs at a patch of early successional habitat was described exclusively by local features. Prevalence of agricultural fields (historic and present day) was the most influential feature affecting both colonization and spread of invasive shrubs. Results of this study provided an approach for identifying early successional habitats that may be especially prone to exotic shrub invasions. Although our models explained only a portion of the vulnerability of a site to invasion, we suggest that an evaluation of factors that operate at different spatial scales provided new insight on the habitat characteristics that influence the range expansion of invasive plants.