Urban grassland restorations have reduced plant fitness but not pollinator limitation

Abstract

Native grasslands worldwide have been greatly reduced due to anthropogenic activities. Grassland restoration efforts strive to increase biodiversity and ecosystem services of these habitats, but often fall short of management goals. This is especially true for small-scale restoration efforts in urban areas, which face unique challenges including increased surface temperatures, increased pollution, decreased habitat connectivity, and decreased pollinator availability. In 2019–2020, we conducted a seed-addition field experiment in four restoration sites near Louisville, KY USA to examine how urbanization influenced survival, growth, and reproduction of a common native forb, Chamaecrista fasciculata. We also conducted observations twice per week to characterize the active pollinator community in each site. We found that plants in urban restoration sites had reduced growth, flowering period, and reproduction compared to those in rural restoration sites. However, pollination rates (visits/flower) in urban sites did not differ from those in rural sites, and urban sites even tended to have greater pollinator effectiveness (seeds/flower) than rural sites. Our results suggest that pollination services are adequate in urban restorations, but that other factors associated with urbanization, such as increased drought stress, can reduce growth and fitness of native plants. Further work is needed to better characterize environmental stresses associated with urban grassland restoration in order to minimize fitness effects on target plant species.