The Effects of Reduced Mowing in Roadside Medians on Stormwater Filtration and Invasive Species Management

Abstract

Roadside ecosystems are the pervasive, highly managed areas adjacent to roads that buffer neighboring ecosystems. While frequently studied and managed at significant cost to ensure motorist safety, roadside ecosystems are often undervalued for their ecological functions (e.g., water filtration, carbon storage, wildlife habitat). Reducing roadside mowing can provide native habitat, save money, and reduce habitat fragmentation. However, transitioning to a reduced mowing maintenance regimen raises concerns that invasive plants may proliferate. On roadside ecosystems with different mowing regimens, we tested the hotly debated diversity-stability hypothesis, which posits that high native species diversity may confer resistance to invasion. To test this relationship, we surveyed vegetation at four spatial scales (1000-m2, 100-m2, 10-m2, and 1-m2) using Modified-Whittaker plots in three types of roadside ecosystems - forested, early successional, and frequently mowed grasslands - under three types of vegetation management - Never Mowed (N=5), Reduce Mowed (N=5), and Fully Mowed (N=5). We then compared total, native, introduced (any non-native flora), and invasive (plants with a state conservation status of invasive) plant diversity and percent plant cover. We found that roadside ecosystems with high native plant species diversity tended to have the lowest introduced species richness at all four spatial scales. This evidence supports the diversity-stability hypothesis in these highly disturbed and human-manipulated areas. We also found that Never Mowed sites had higher species richness than Fully Mowed sites (P=0.046), higher native species richness than Fully and Reduce Mowed sites at the two largest spatial scales (1000-m2 P=0.0001, 100-m2 P<0.0001), lower introduced species richness than Fully and Reduced Mowed sites at three spatial scales (100-m2 P=0.003, 10-m2 P=0.003, 1-m2=P<0.0001), and less introduced species percent cover (P=0.0001) when compared to adjacent communities. We found no significant differences between the community composition of Fully Mowed and Reduce Mowed communities. Additionally, we did not observe differences in invasive species richness or percent cover by mowing regimen. In addition to invasive species monitories, we performed stormwater filtration monitoring to insure that this vital ecosystem provided by roadside ecosystems was not negatively affected by this change in maintenance. To determine if changes mowing practices affect filtration of pollutants, we analyzed soil water concentrations of Al, Cd, Cu, Fe, P, Pb, Mg, Mn, Ni, Zn, NO3, and NH4 in Frequently mowed (N=5) and Reduce mowed (N=5) roadside communities in Rhode Island using buried Ion-exchange resin membranes. We found that only iron (Fe) was significantly lower in concentration (p=0.04) than other