Edge type defines alien plant species invasions along Pinus contorta burned, highway and clearcut forest edges

Abstract

Forest edges have been long recognized as the first landscape elements to be invaded by alien plant species in forest ecosystems. However, little is known about the role of forest edge type in invasive species patterns. In the Northern Rocky Mountains of the United States, disturbance-caused forest edges are a common feature in the landscape with anthropogenic (e.g. roadside, clearcut) and natural (e.g. burned, windthrown) edge types. In this paper we examine patterns of alien species invasion and native community structure across three forest edge types in Pinus contorta forests including contrasts between undeveloped landscapes in Yellowstone National Park (YNP), and developed (roaded and logged) portions of the adjacent Gallatin National Forest (GNF). Six sets of transects were located in each of five study sites, including clearcut, bum and highway edge types. When bum and highways edges were compared between GNF and YNP, landscape matrix did not have a significant effect on either alien species richness or cover. In both landscapes, highway edges had higher alien species richness and cover than burn edges. For burn and highway edges, alien species richness and cover were significantly related to edge type, distance from edge and the interaction between both variables. In Gallatin NF, alien species were concentrated along highway edges, while burn and clearcut edges are significantly less invaded. Overall, alien species richness was negatively correlated with native species richness in plots with at least one alien species (R-2 = 0.30, p < 0.001). We were able to explain from 23 to 68% of the variation in alien species richness from simple measures of vegetation structure within each of the edge types. Our study suggests that at least for high elevation conifer ecosystems such as West Yellowstone, alien plant invasion into interior forest is significantly enhanced by disturbance edges only along roadsides. Further studies are needed to determine causal mechanisms that explain the high degree of invisibility of roadside edges. Our results highlight the need for careful planning and management of roads, as they may become a primary conduit of alien plant invasions.