Comparison of Woody Riparian Vegetation Communities Along an Urban-to- Rural Gradient in San Antonio, Texas

Understanding urban impacts is important for restoration of degraded riparian ecosystems. I surveyed woody riparian vegetation communities, including species traits, along an urban-to-rural gradient in San Antonio, and tested whether flood frequency, represented by geomorphic surface elevation, influenced vegetation communities. Impervious surfaces were predicted to alter hydrologic regimes and impact communities. Surveys occurred in eight ephemeral creeks and data were collected for canopy cover, basal area, and stem density. I tested the dissimilarity of vegetation communities between the sites and between geomorphic surfaces using Non-metric Multidimensional Scaling. Environmental fitting tested whether and how land use and flow metrics influenced site grouping and tested whether species traits aligned along an urbanization gradient. Lastly, linear modeling tested the effects of impervious cover on flow metrics, species richness, diversity, and traits. Urban development influenced community composition as measured by basal area and stem density, communities were not influenced by geomorphic surfaces, and there was a significant negative relationship between impervious cover and diversity. Drought and heat tolerance traits aligned with the urban gradient and basal area and stem density of medium water use species declined with increasing watershed impervious cover. Overall, riparian communities followed an urban-to-rural gradient, but were site-dependent based on local land-cover. Sites with higher levels of impervious cover face more frequent floods, which may explain loss of diversity and community composition with increasing urbanization. However, surface elevation did not strongly influence communities and shifts may be caused by greater drought and heat levels in more urban sites.