Spatial patterns of invasion by exotic plants in a forested landscape

Few landscapes are immune to invasion by exotic plant species. The forestedlandscape in the western Cascade Range of Oregon appears to have some barriers toinvasion, but the extensive road network provides a corridor and habitat for a suite ofexotic species to enter and become established. This study examined how biologicalfactors and physical factors interact to produce the observed spatial pattern of invasion inthe forested landscape, particularly along roads and streams. Results from a seed bankstudy indicated that dispersal barriers may be preventing movement of some exotics fromthe road into mature forest. Exotics that were found in the seed bank within mature foresthave a high potential for dispersal, especially by wind. However, no exotic plants werefound in the existing vegetation within the mature stands, suggesting that environmentalbarriers were preventing their establishment.
Surveys at 1-km and 100-km scales showed that the most frequent species alsohad 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 ofthe "lag phase" after introduction, and various biological factors may have a stronginfluence on the spatial patterns of invasion.
Exotic plant distribution at the 1-km scale varied among four habitat types thatrepresent different levels of disturbance. The number and frequency of exotic specieswere much higher along high-use and low-use roads than along abandoned roads orstreams. This pattern may reflect the more favorable light conditions along currently usedroads, and the lower light levels along abandoned roads and streams may be barriers toinvasion for many exotics. However, confounding factors such as competitive exclusionby natives and opportunities for dispersal may also be operating. Spatial patterns ofexotics along the 100-km road network provide additional evidence that environmentalbarriers influence the pattern of invasion. Spatial analysis results provided evidence ofpatchiness and spatial gradients for some target species along some roads. Theheterogeneous arrangement of patches on the landscape suggests that the process ofinvasion is generating multiple, discrete "foci" from which further invasion can occur.