A review of modeling the effects of vegetation on large wood recruitment processes in mountain catchments

Abstract

Large wood (LW) is an important element in riparian and mountain ecosystems and influences geomorphic controls, particularly in torrents and mountain rivers. Despite several advantages, LW can also exacerbate flood damage near infrastructure due to logjams or backwater rise and is often treated as a potential hazard. In an attempt to reduce such problems, channel slopes and banks are often clear cut in practice. This leads to a debate between the opinions of different scientists as well as stream and forest managers, where it is difficult to find a compromise between optimizing risk mitigation and eco-morphological functionality. Regarding risk mitigation due to LW, situations where the positive effects of vegetation succumb to negative effects should be identified to distinguish if shrubs or small trees (DBH < 10 cm) are more suitable to maintain streambank and hillslope stability. However, drastically removing vegetation can severely influence the ecosystem which should be avoided. In the case where large trees (DBH > 10 cm) have the potential to reduce the magnitude and frequency of recruitment processes, practices for forest management need to be optimized. Based on literature research, this article summarizes state of the art knowledge of vegetation effects on LW recruitment processes. In doing so, it focuses on three main recruitment processes: hydraulic bank erosion, geotechnical bank erosion and hillslope failure. Hydraulic bank erosion is responsible for delivering high volumes of sediment as well as LW in mountain catchments by removing streambank material through excess shear stress. The positive effects of vegetation can be quantified and implemented in modeling approaches through the adjustment of discharge-specific resistance coefficients or by using stochastic approaches. Geotechnical bank erosion and hillslope failure contribute to LW recruitment through failure. Root reinforcement is predominately the most important effect on how vegetation stabilizes streambanks and hillslopes. Based on the information mentioned above, a flow chart was formulated that uses specific criteria to define conditions in which forest management should be performed to mitigate potential LW recruitment without drastically removing all trees.