Spatial patterns of invasion by exotic plants in a forested landscape Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4x51hn08c

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  • Few landscapes are immune to invasion by exotic plant species. The forested landscape in the western Cascade Range of Oregon appears to have some barriers to invasion, but the extensive road network provides a corridor and habitat for a suite of exotic species to enter and become established. This study examined how biological factors and physical factors interact to produce the observed spatial pattern of invasion in the forested landscape, particularly along roads and streams. Results from a seed bank study indicated that dispersal barriers may be preventing movement of some exotics from the road into mature forest. Exotics that were found in the seed bank within mature forest have a high potential for dispersal, especially by wind. However, no exotic plants were found in the existing vegetation within the mature stands, suggesting that environmental barriers were preventing their establishment. Surveys at 1-km and 100-km scales showed that the most frequent species also had a high potential for dispersal. However, dispersal potential did not explain all of the observed spatial patterns. Confounding factors such as time since introduction, length of the "lag phase" after introduction, and various biological factors may have a strong influence on the spatial patterns of invasion. Exotic plant distribution at the 1-km scale varied among four habitat types that represent different levels of disturbance. The number and frequency of exotic species were much higher along high-use and low-use roads than along abandoned roads or streams. This pattern may reflect the more favorable light conditions along currently used roads, and the lower light levels along abandoned roads and streams may be barriers to invasion for many exotics. However, confounding factors such as competitive exclusion by natives and opportunities for dispersal may also be operating. Spatial patterns of exotics along the 100-km road network provide additional evidence that environmental barriers influence the pattern of invasion. Spatial analysis results provided evidence of patchiness and spatial gradients for some target species along some roads. The heterogeneous arrangement of patches on the landscape suggests that the process of invasion is generating multiple, discrete "foci" from which further invasion can occur.
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