Urban wetlands: Hope on the invasion front

Background/Question/Methods

Invasive plant species dominance and floral diversity in headwater wetlands are reflective of local flooding conditions. The stressors of flooding may limit the spatial distribution of invasive plants, particularly in the shaded understory of forested wetlands. However, the extent to which flooding limits the distribution and dominance of invasive plants in forested wetlands remains unexplored. We hypothesize that nonnative-invasive dominance is reflective of local flooding conditions, and that flood timing (seasonality), duration, and depth inhibit the establishment of some invasive plants. Here, hydrologic classification follows National Wetlands Inventory designation. We quantified vegetation structure in six urban wetland patches in New Jersey, two large (>125 ha), two medium (<35 ha), and two small (<12 ha). We further explored the effects of flooding duration and depth in a three-month greenhouse mesocosm experiment quantifying Japanese stiltgrass (Microstegium vimineum) seedling production. Flooding duration or depth was manipulated for the first three weeks of the experiment, after which seedling production in flooded vs. control treatments was assessed.

Results/Conclusions

Our preliminary results support the hypothesis that seasonally flooded plots have a greater floral richness of 48±3 (n=25) species per plot (707 m²) when compared to sites that experience only root zone saturation (n=36, 34±2). Invasive dominance (m²), in particular Japanese stiltgrass, was lower in seasonally flooded plots (1%±0.7) than in saturated plots (9.5%±3.6). Flooding condition was the greatest predictor of invasive species dominance, a trend that we explored further in our mesocosm experiment. We concluded that Japanese stiltgrass seed germination and seedling production are inhibited by flooding. The control treatment (n=7) was the only treatment to produce seedlings, even two months following drawdown of the flooded treatments (4.6±0.6). Less than a week of shallow flooding during the natural germination period of Japanese stiltgrass may significantly reduce recruitment of this invasive annual grass. The field and mesocosm results suggest that even small shifts in flooding may have profound impacts on invasive species dominance and spatial distribution.