Tropical forest structure and understorey determine subsurface flow through biopores formed by plant roots

Erosion and mass wasting processes on mountain slopes can benefit from or be adversely affected by the presence of biopores formed by plant root systems or soil fauna. The relationship between biopores and subsurface flow during rainstorms is poorly understood. Here, we examined the link between subsurface flow and biopores formed through different processes, including soil faunal activity and abundance of fine and coarse roots. As the distribution of biopores is influenced by the type of vegetation present, we investigated the effect of plant diversity (forest with or without understorey vegetation) on the pattern of water infiltration throughout the soil. We hypothesized that increased species diversity would enhance the extension of subsurface flow because biopores would be distributed throughout the soil profile and that more coarse roots would create large biopores, increasing subsurface flow. In situ experiments were conducted on hillslopes with plantations of rubber trees (Hevea brasiliensis) growing on terraces, or with secondary mixed forests, in the tropical zone of Yunnan province, China. Three sites with Ferralsol soils and different vegetation types were examined: (1) plantation with no understorey; (2) clear-cut plantation with understorey; and (3) secondary mixed forest with understorey. Irrigation experiments with Brilliant Blue FCF dyed water were performed upslope of trees at each site and staining patterns resulting from infiltrated dyed water were examined at two different scales. After dye irrigation, soil was removed in 1.0 × 0.8 m slices starting 1 m downslope and soil profiles were photographed for subsequent mapping of dyed areas in the profile (macroscale). Each profile was then divided into a 0.1 × 0.1 m grid (microscale) and burrows formed by macrofauna and fine and coarse root densities were measured. At the macroscale, the greatest lateral extension in subsurface flow occurred in the natural forest and the least in the rubber tree plantation with no understorey vegetation. At the microscale, and in all types of vegetation, fine roots significantly increased the incidence of subsurface flow compared to coarse roots and macrofauna activity. We conclude that in tropical Ferralsols, fine roots, and hence understorey vegetation, play a positive role in promoting subsurface flow and therefore reducing water erosion and mass wasting processes. Thus, planting mixtures that include a diversity of species and strata could significantly improve soil conservation.