Effects of urbanization on herbaceous forest vegetation: the relative impacts of soil, geography, forest composition, human access, and an invasive shrub

Abstract

We studied how degree of urbanization affected forest-floor herbs in deciduous forest along an urbanization gradient from west to east of Cincinnati, OH. We measured species diversity, richness, and abundance of herbs in 16 30 × 30 m plots at two Urban, two Exurban, and two Wildland sites. Because the invasive shrub Amur honeysuckle (Lonicera maackii) negatively affects richness and abundance of native herbs, half of these plots contained honeysuckle, except at the Wildland sites where honeysuckle was absent. We used General Linear Models or Generalized Linear Mixed Models to determine the effect of edaphic, geographic, forest composition, human effects, and honeysuckle variables on herbs and used model comparison techniques to identify those variables that significantly affected herbs. Human effects (e.g., proximity to roads) and geography (e.g., aspect, slope) were the most important factors affecting herb richness and abundance, and geography (e.g., elevation) was the most important factor affecting herb diversity. Honeysuckle (measured as diameter of primary stem) had no effect on diversity or richness of herbs, but positively affected herb abundance. Herb diversity did not vary significantly along the urbanization gradient, but higher herb richness and abundance in Exurban and Wildland sites along the urbanization gradient were associated with higher tree diversity, richness, and abundance, shallower slopes, greater distance to roads, and smaller honeysuckle shrubs.