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An index for assessing plant species invasion into riparian vegetation in eastern Oregon.
Magee, Teresa*,1, Ringold, Paul2, Bollman, Michael1, 1 Dynamac Corporation, Corvallis, Oregon2 United States Environmental Protection Agency, NHEERL-WED, Corvallis, Oregon
ABSTRACT- Many riparian areas are invaded by alien plant species that affect native species composition, community dynamics, and ecosystem properties. Thus, a key indicator of vegetation condition is the extent of invasion, with higher relative abundance of alien species indicating lower vegetation integrity. However, some alien species spread more aggressively or have greater influence on ecosystem function than others. To address this issue, we present a preliminary Invasion Index (II) that incorporates both abundance and a ranking of the invasion potential (Ri) for each alien species occurring in a region or species assemblage of interest. The R value was determined by screening each alien for traits that have been associated with invasiveness including life history attributes (n=8), ecological tolerances consistent with riparian habitat of the study area (n=8), and ability to alter ecosystem processes (n=7). Presence of traits for each species was determined by review of literature and published floras. We surveyed vegetation along 1 km reaches of 31 low order streams in eastern Oregon spanning an elevation range from 670 to 1695 m. The II was calculated for groups of sample units representing each stream reach, species assemblages, and coarser structural classes based on dominant vegetation layer (closed conifer, open conifer, broadleaf tree, and herb/shrub). Initial results indicate that drier, low elevation riparian areas or communities were more strongly influenced by alien species than those at moister, higher elevation locations. Closed forest had significantly (p<0.05) lower II values than open forest, shrub or herb dominated vegetation. We anticipate that the II will be useful in identifying relationships between vegetation status and environmental attributes, and aid in prioritizing individual stream reaches, types of stream reaches, or watersheds for conservation or restoration efforts.
KEY WORDS: invasion index, riparian vegetation, alien plant species, vegetation condition