Vegetation diversity and plant traits affect throughfall partitioning and subsequent splash erosion in managed woodlands

Soil erosion is a serious environmental problem in many parts of the world, especially in ecosystems with high anthropogenic influences. Even if forest stands generally mitigate soil losses, important rates of sediment transport were measured in woodlands in relation with natural and anthropogenic disturbances. Forests provide a multi-storey canopy layer which largely influences rain throughfall patterns as well as a covering layer on the forest floor which protects the soil against direct raindrop impact. Both layers provide different storage capacities and modify the water flow as well as topsoil erosivity. So far, only little research was conducted on how soil erosion control is affected by tree diversity and individual species characteristics under forest stands. Furthermore, ecohydrological processes within the protective leaf litter cover and pioneer non-vascular vegetation developing after disturbances are often not clear.

Here, we summarize results on effects of species diversity, species identity, functional traits of both the tree and the soil covering vegetation layer on soil erosion in subtropical and temperate forest ecosystems with disturbances caused by timber harvesting. We focus on interrill soil erosion determined by micro-scale runoff plots under natural and simulated rainfall and throughfall kinetic energy (TKE) of raindrops measured with splash cups.

Results show that neighbourhood diversity increases TKE, and tree species richness can partly affect sediment discharge, runoff and TKE, although this effect will presumably become more visible after an early successional forest stage. Species identity strongly influences initial soil erosion processes under forest and erosion-promoting and -mitigating species can be clearly identified. That also applies to the leaf litter cover, where single leaf species show varying influences on sediment discharge. Therefore, the appropriate choice of tree species during the establishment of reforestations plays a major role for erosion control. Interestingly, within the soil covering leaf litter layer, the presence of meso- and macrofauna increases soil erosion and thus effects of this fauna group must be considered in erosion experiments. Moreover, species-specific functional traits of trees affect soil erosion rates. High crown cover and leaf area index reduce soil erosion, whereas it is enhanced by increasing tree height. TKE is effectively minimized by low LAI, low tree height, simple pinnate leaves, dentate leaf margins, a high number of branches and a low crown base height. Finally, bryophyte-dominated biological soil crusts (BSCs) importantly mitigate sediment delivery and runoff generation in mesic forest environments and this effect varies tremendously with species specific bryophyte traits. It can be concluded that the ability of BSCs to quickly colonise soil surfaces after disturbance are of particular importance for soil erosion control in early-stage subtropical and temperate forests.