Erosion processes in steep terrain—Truths, myths, and uncertainties related to forest management in Southeast Asia

Abstract

To assess the effects of forest management on soil erosion in Southeast Asia, clear distinctions must be made between surface erosion and landslide processes. Although surface erosion is a natural process, it is exacerbated by surface disturbance and compaction that reduce the soil hydraulic conductivity and break down soil aggregates. Management practices and attributes such as roads and trails, agricultural cultivation, fire, land clearing, and recreation all accelerate surface erosion processes due to their disturbance, compaction, and connectivity along hillslopes. Agroforestry practices in Southeast Asia that incorporate cover crops with trees reduce surface erosion by more than an order of magnitude compared to monoculture plantations with no ground cover. Cleared fields tilled up and down steep slopes are highly erodible; passive conservation practices (e.g., contour tillage, strip cropping, reduced tillage; maintaining adequate ground cover) are effective in reducing surface erosion if properly implemented. Poorly designed and managed terraces are not effective in controlling surface erosion and may actually increase mass wasting if they concentrate water. In contrast to surface erosion, shallow, rapid landslides are episodic processes triggered by individual rainfall events or artificial inputs of water; slower, deep-seated landslides initiate or activate after a longer-term accumulation of water. Thus, landslide assessment must be based on long-term observations. Deep rooted trees and shrubs impart a significant cohesive strength into shallow soil mantles and facilitate preferential drainage, thereby reducing the probability of shallow landslides. Conversion of mountain forests to cropland or plantations permanently reduces rooting strength, thus increasing landslide potential, while timber harvesting with subsequent regeneration of secondary forests reduces rooting strength for up to two decades after initial cutting. Roads contribute the largest surface erosion and landslide losses (per unit area disturbed) compared to other land uses. Both landslide and surface erosion fluxes along roads are typically one to more than two orders of magnitude higher compared to undisturbed steepland forests. High storm runoff from roads is caused by the generation of infiltration-excess overland flow on compacted surfaces and the interception of subsurface flow at road cuts; these altered pathways increase surface erosion and accelerate the delivery of storm runoff to streams. Discharge nodes from roads facilitate the connectivity of water and sediment to headwater streams. Trails, although narrower than roads, can contribute significantly to soil loss and storm runoff and are important sediment conduits if directly linked to channels. Thus, the location of roads and paths with respect to the hydrologic network is a critical factor governing the spatial–temporal movement of sediment and water in the tropics.