PS 40-21 - Recovery of diverse flora after control of Japanese knotweed (Reynoutria japonica, s.l.) by goat grazing and repeated cutting

Background/Question/Methods

Japanese knotweed (Reynoutria japonica, s.l.) has been controlled via goat grazing and then repeated cutting on .53 ha for 6 years prior to measuring vegetation recovery. What species would colonize the site after suppression of the prior knotweed monoculture? How would species distributions relate to each other and to soil conditions? After the treatment Japanese knotweed was still a dominant species, but its biomass declined dramatically across the site, allowing other species to colonize the treated area. We recorded species identity and cover, and soil characteristics, e.g. texture, depth and available water capacity, along a transect.

Results/Conclusions

Both native species (e.g. clearweed, Virginia creeper, New York fern, skunk cabbage, jewelweed, white snakeroot and tearthumb) and non-native species (e.g. wineberry, Indian strawberry, garlic mustard, narrowleaf bittercress and Canadian thistle) colonized the treated area. Japanese knotweed cover was positively correlated with percent clay and available water capacity, and negatively correlated with soil depth, indicating that success of knotweed control depends on environmental conditions. We used multivariate analysis (DCA) to explore distribution of newly established species across environmental gradients. Variables that were significantly (p< 0.05) correlated with species distribution were percent bare soil in sampling quadrats and percent clay. Species assemblages reflected differences in soil texture and soil moisture, with a pattern of non-native species occupying areas of shallow soil, in contrast to native species that were more prevalent in deeper soil. We plan reintroduction of more native herbaceous and woody perennials to further diversify the area and compete with knotweed, using soil features and sun exposure to guide these efforts.