Eastern skunk cabbage (Symplocarpus foetidus), a common native wetland plant, shows reduced size and seedling abundance near invasive Japanese knotweed (Reynoutria japonica) 1

Abstract

Nonnative plants may shift community composition and reduce diversity through competition with native neighbors, or invasion patterns may reflect edaphic gradients that create suitable habitat for colonization. We tested the impacts of invasive Reynoutria japonica (Japanese knotweed) on performance of a co-occurring native plant, Symplocarpus foetidus (eastern skunk cabbage), to determine if invasion has potential to change community composition by altering population-level processes. We tested two hypotheses: (1) Reynoutria japonica invasion is correlated with reduced population performance in S. foetidus; (2) Reynoutria japonica invasion is correlated with environmental variation, and contrasting habitat preferences allow R. japonica and S. foetidus to coexist. Sampling took place in an urban riparian site in Pittsburgh, PA, USA. Data were collected from 26 1-m2 plots with R. japonica present (n = 13) or absent (n = 13). We recorded pH, texture, and moisture. Size and flowering of all S. foetidus individuals and R. japonica cover and abundance were recorded in each plot. Symplocarpus foetidus size was significantly reduced in invaded plots, and both size and seedling abundance were significantly negatively associated with R. japonica cover. Abundance was negatively associated with percent clay and positively associated with soil moisture; seedling abundance was also predicted by lower soil pH. In contrast, R. japonica was most abundant and had highest cover with high pH and lower percent sand. Flowering of S. foetidus did not differ significantly between invaded and uninvaded plots. We conclude that R. japonica invasion both suppresses S. foetidus and correlates with environmental heterogeneity. Reduced performance of S. foetidus in invaded plots suggests competitive suppression. While R. japonica negatively impacted S. foetidus, distinct microhabitat preferences may allow them to co-occur.