Riparian vegetation is frequently used for stream bank stabilization, but the effects of vegetation on subaerial processes have not been quantified. Subaerial processes, such as soil desiccation and freeze-thaw cycling, are climate-related phenomena that deliver soil directly to the stream and make the banks more vulnerable to fluvial 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 water tension were measured at six paired field sites in southwestern Virginia, USA, for one year. Results showed that stream banks with herbaceous vegetation had higher soil temperatures and a greater diurnal temperature range during the summer compared to forested stream banks. Daily average summer soil water tension was 13 to 57 per cent higher under herbaceous vegetation than under woody vegetation, probably due to evapotranspiration from the shallow herbaceous root system on the bank.
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. During the winter, the stream banks under the deciduous forests were exposed to solar heating and night time cooling, which increased the diurnal soil temperature range and the occurrence of freeze-thaw cycling.
Study results also indicated that freeze-thaw cycling and soil desiccation were greater on the upper stream bank due to thermal and moisture regulation of the lower bank by the stream. Therefore, subaerial erosion and soil weakening may be greater on the upper stream banks. Additional research is needed on the influence of subaerial processes on both subaerial and fluvial erosion.