Riparian Woody Plant Diversity, Composition, and Structure across an Urban-Rural Land Use Gradient in the Piedmont of Georgia, US

Abstract

Riparian forests are increasingly threatened by urban expansion and land use change worldwide. Understanding the impacts of urbanization on riparian forests is critical because riparian areas provide a variety of important ecological services and are biological hotspots for species diversity. The overall goal of this work was to describe the relationships among landscape characteristics and woody plant diversity, structure, species composition, and plant functional traits of small order riparian corridors along an urban-rural land use gradient in the Georgia Piedmont, US. The objectives were to: (1) examine the influence of land use and urbanization indices on riparian woody plant species diversity and composition, (2) quantify changes in riparian forest structure across an urban-rural gradient, (3) elucidate changes in woody plant functional traits along an urban-rural gradient, and (4) compare trends in diversity, composition, structure, and trait charateristics in the mature forest stand and forest tree regeneration. This work demonstrates that changes in species diversity, composition, and structure are occurring in response to land use and the surrounding landscape matrix. Non-native invasive species appear to be driving many of the changes, specifically the shrub, Ligustrum sinense. Species richness was positively correlated to rural landscape characteristics and negatively related to urban characteristics. Shannon diversity was negatively associated with dominance of non-native species, especially for the forest regeneration layer. Urban sites were characterized by high richness of non-native species and several pioneer species. Developing sites were dominated by the non-native shrub, Ligustrum sinense, and several native overstory trees, mainly Acer negundo. While agricultural and managed forest sites were composed of ubiquitous species, the unmanaged forest type exhibited a structurally distinct midstory. Midstory tree biomass was positively related to forest cover and negatively related to impervious surface cover and shrub biomass was positively related to patch density. Urban and agriculture sites showed signs of recruitment failure. Species functional traits also varied across the gradient. Specifically, differences in leaf type, plant form, flood and shade tolerance, seed dispersal, pollination, growth rate, rooting characteristics, and life span were found. Flood tolerance was strongly reduced in the regeneration layer in urban riparian areas suggesting a potential shift in hydrologic function. Results from this study highlight the impact of urbanization on riparian forest plant biodiversity and structure.