Influence of Land Use and Site Characteristics on Invasive Plant Abundance in the Quinebaug Highlands of Southern New England

Northeastern Naturalist, 2004 by Lundgren, Marjorie R, Small, Christine J, Dreyer, Glenn D

Abstract-Invasive exotic plants have been identified as one of the major threats to ecosystem function and biodiversity. This study examined the distribution and abundance (cover and frequency) of invasive plants in natural habitats of the Quinebaug Highlands forest block (13,760 ha) of northern Connecticut and southern Massachusetts in relation to current and historical land use and site conditions. Multiple regression and GIS analyses were used to identify areas of high infestation and factors most useful in predicting invasions. Celastrus orbiculatus, Rosa multiflora, and Berberis thunbergii were the most frequent invasives. Past land use was the strongest predictor of invasive cover (r^sup 2^ = 0.219) and richness (r^sup 2^ = 0.303; p

Introduction

Invasive exotic plants are recognized as one of the major threats to ecosystem function and biological diversity through competition with and suppression and displacement of native species (Bratton 1982, Sala et al. 2000, Wilcove et al. 1998). By competing with and suppressing other plants, invasive plant species can greatly reduce the structural and compositional complexity of invaded ecosystems (Luken and Thieret 1996). Effects of invasive species range from simple competitive replacement of one or more species to loss or reduction of whole guilds or strata, with consequent losses in species diversity. Dramatic changes or even complete disruption to community structure and successional trajectories may occur in extreme cases (Fike and Niering 1999, Woods 1997).

Because there are few ecosystems in the world, terrestrial or aquatic, that have not been affected by invasions, the negative effects of invasive exotic species are a global concern (Heywood 1989). Even areas set aside and managed to protect biodiversity are in danger of becoming refuges for these exotic species (Ehrenfeld 1997). Although not all exotic species are invasive, it is estimated that there are approximately 5000 exotic plant species that have escaped and become naturalized in natural U.S. ecosystems, compared to [asymptotically =]17,000 native plant species ([asymptotically =] 23% non-native; Morse et al. 1995). In southern New England states such as Connecticut and Massachusetts, the non-native plant species are particularly problematic, representing [asymptotically =] 35% and 45% of the flora, respectively (Mehrhoff 2000). Forty-two percent of the plants and animals that are listed as threatened or endangered under the Endangered Species Act are considered to be at risk primarily because of competition with these exotic species (Wilcove et al. 1998). Armstrong (1995) estimates that in other regions of the world, up to 80% of endangered plant and animal species are threatened due to pressures imposed by exotic species.

Although habitat disturbance plays an integral role in structuring many plant communities, some types or combinations of disturbance may increase the potential for invasion by non-native species (Hobbs and Huenneke 1992). Many exotic plants were originally introduced because of their resistance to harsh environmental conditions and their ability to colonize disturbed areas. These attributes, however, frequently allow invasive species to competitively exclude native species. The woody vine kudzu (Pueraria lobata Willd.), introduced from Japan for erosion control and as ground cover, now encroaches over thousands of hectares of fields and forests every year in southeastern and south central North America (Rossman 2001). Russian olive (Elaeagnus angustifolia L.) and autumn olive (E. umbeliata Thunb.), two introduced species planted heavily along highways for their salt tolerance, are now spreading rapidly off highways, invading fields and forest edges (Dirr 1998).

The New England landscape has undergone dramatic changes over the past few hundred years primarily due to anthropogenic impacts, mostly involving the transition from forested to agricultural land (Bellemare et al. 2002, Foster 1992, Niering 1998). These historical anthropogenic changes in land use have had lasting impacts on current plant community structure and species diversity. Foster (1992) describes the legacy of anthropogenic land use, particularly the massive deforestation of the New England landscape for agriculture and pasture by European colonists in the mid-1800s, relative to natural disturbances. Despite subsequent forest regeneration throughout much of the region, Foster's studies have linked historical land use to long-term changes in vegetation and environmental relationships, shifts in dominant canopy species, and reductions in community diversity. Niering (1998) further suggests a correlation between exotic species invasions and these historical anthropogenic impacts on northeastern forests.

A number of studies in the northeastern U.S. have focused on the effects of individual invasive exotic plants (e.g., Dreyer et al. 1987, Silander and Klepeis 1999, Van Clef and Stiles 2001, Webb et al. 2000). For example, such studies have demonstrated the widespread distribution of Japanese barberry (Berberis thunbergii [DC]) in the understory of closed-canopy forests throughout the Northeast (Ehrenfeld 1997) and the ability of Asiatic bittersweet (Celastrus orbiculatus [Thunb.] and Murray) to suppress native species and alter pathways of vegetation development in early successional forests of southern New England (Fike and Niering 1999). However, very few studies have examined the distribution of multiple invasive exotic species and their impacts at the community level.