Environmental Consequences of the Demise in Swidden Cultivation in Montane Mainland Southeast Asia: Hydrology and Geomorphology

The hydrological and geomorphological impacts of traditional swidden cultivation in Montane Mainland Southeast Asia are virtually inconsequential, whereas the impacts associated with intensified replacement agricultural systems are often much more substantial. Negative perceptions toward swiddening in general by governments in the region beginning half a decade ago have largely been based on cases of forest conversion and land degradation associated with (a) intensified swidden systems, characterized by shortened fallow and extended cropping periods and/or (b) the widespread cultivation of opium for cash after the Second World War. Neither of these practices should be viewed as traditional, subsistence-based swiddening. Other types of intensive agriculture systems are now replacing swiddening throughout the region, including semi-permanent and permanent cash cropping, monoculture plantations, and greenhouse complexes. The negative impacts associated with these systems include changes in streamflow response, increased surface erosion, a higher probability of landslides, and the declination in stream water quality. Unlike the case for traditional swiddening, these impacts result because of several factors: (1) large portions of upland catchments are cultivated simultaneously; (2) accelerated hydraulic and tillage erosion occurs on plots that are cultivated repetitively with limited or no fallowing to allow recovery of key soil properties, including infiltration; (3) concentrated overland flow and erosion sources are often directly connected with the stream network; (4) root strength is reduced on permanently converted hillslopes; (5) surface and ground water extraction is frequently used for irrigation; and (6) and pesticides and herbicides are used. Furthermore, the commercial success of these systems relies on the existence of dense networks of roads, which are linear landscape features renowned for disrupting hydrological and geomorphological systems. A new conservation focus is needed to reduce the impacts of these intensified upland agricultural practices.