Riparian Vegetation Effects on Freeze-Thaw Cycling and Desiccation of Stream Bank Soils

Abstract

Riparian vegetation is frequently used for stream bank stabilization, but the effects of vegetation on stream bank erosion have not been quantified. Subaerial processes, such as desiccation cracking and freeze-thaw cycling, are climate-related phenomena that deliver soil directly to the stream channel and make the banks more vulnerable to flow erosion by reducing soil strength. This study compares the impact of woody and herbaceous vegetation on subaerial processes by examining soil temperature and moisture regimes in vegetated stream banks. Soil temperature and soil water potential were measured at six paired field sites in southwest Virginia for one year. Differences in daily minimum and maximum soil temperatures, daily temperature range, and average daily soil water potential were compared for both summer and winter. Results of this study showed that stream banks with herbaceous vegetation had higher soil temperatures overall and a greater diurnal temperature range, as compared to forested stream banks. These differences decreased during the growing season as the herbaceous vegetation matured. Additionally, increases in daily average soil water potential of 13% to 57% were observed under herbaceous vegetation, as compared to woody vegetation, likely due to the evapotranspiration from the shallow herbaceous root system on the bank face. In contrast to summer conditions, the deciduous forest buffers provided little protection for stream banks during the winter: the forested stream banks experienced diurnal temperature ranges two to three times greater than stream banks under dense herbaceous cover and underwent as many as eight times the number of freeze-thaw cycles. With the absence of a dense canopy, the stream banks under mature forest cover were exposed to solar heating and night time cooling, which increased the diurnal soil temperature range and the occurrence of freeze-thaw cycling. Results of this study suggest that in areas with soils susceptible to desiccation cracking, woody vegetation may provide the best protection against degradation by subaerial processes. In areas with silty soils prone to freeze-thaw cycling, a dense groundcover may provide more protection against soil loss due to freeze-thaw cycling than just deciduous woody vegetation.