Vegetation classification and invasive species distribution in natural areas of southern New England1

Invasive plant species pose serious threats to ecosystem function and community diversity, dominating many natural systems through suppression of, competition with, and replacement of native species. This study examines the distribution of invasive alien plant species relative to vegetation, site characteristics, and disturbance indicators in a relatively unfragmented matrix forest block (∼8,000 ha) in southern New England and provides the first quantitative description of the forested vegetation of this region. Within 139 - 10 × 10 m sample plots, percent ground coverage was estimated for each vascular plant species and basal area determined for all woody species reaching breast height (1.4 m). Site conditions, including topography, soil moisture and fertility, and evidence of current and historical site disturbance, were assessed to identify factors influencing invasive plant abundance. Fifteen relatively distinct vegetation types were identified using NMS ordination and cluster analysis, including wetland forest, terrestrial open woodland, and terrestrial closed canopy vegetation types. Of the 209 species occurring in sample plots, only 10 (4.8%) were invasive. Most abundant were Berberis thunbergii, Celastrus orbiculatus, and Rosa multiflora, each occurring in 5% or fewer plots. Moist sites were invaded most frequently, especially those with high species richness, lower canopy cover, and those dominated by Acer rubrum or Pinus strobus. Site disturbance, particularly roads, trails, and former land use, also was correlated with invasive abundance. The low frequency of invasive plant species in our study area offers an exceptional opportunity to identify ecosystem characteristics inhibitory to invasive plant species and the opportunity to preserve a series of relatively unfragmented and uninvaded natural habitats in this region.