Eight nonnative plants in western Oregon forests: associations with environment and management.

Nonnative plants have tremendous ecological and economic impacts on plant communities globally, but comprehensive data on the distribution and ecological relationships of individual species is often scarce or nonexistent. The objective of this study was to assess the influence of vegetation type, climate, topography, and management history on the distribution and abundance of eight selected nonnative plant taxa in forests in western Oregon. These eight taxa were selected as being reliably detected by a multi-resource inventory of 1127 systematically-placed plots on non-federal forest lands from 1995 to 1997 by the USFS Forest Inventory and Analysis (FIA) program. One or more of the eight nonnative taxa studied were found on 20% of the sampled subplots in the study area, but relatively few stands were dominated by them. Overall abundance of nonnative taxa was likely much greater, because few composites and graminoids were identified to species in this general-purpose inventory. Distribution of most taxa was more closely associated with low density of overstory trees than with climate. Nonnative taxa were significantly more abundant in stands that had been recently clearcut or thinned than in stands that had not. Frequencies of several taxa decreased with elevation, which may reflect proximity to source populations and intensive land use rather than any climatic constraints. Although the greatest potential for displacement of native forest species appears to be in early-successional communities, the potential for spread of some shade-tolerant evergreen shrubs also seems high.