Impacts of mass movement erosion on land productivity: a review

Wherever people gain their livelihood in mountains and steeplands, the productive capacity of the soils they use is likely to be affected by mass movement erosion. The impacts of mass movement erosion on land productivity are significant but under-rated in the scientific literature. Impacts on cropping are here reported from 15 countries in south and southeast Asia, east Africa, the Caribbean and Melanesia, but accounts are generalized or anecdotal, and do not quantify crop loss or damage attributable to mass movement separately from that due to surface or fluvial erosion. Impacts on pastoral grazing have been studied in New Zealand, where production losses of up to 80% at field scale, and up to 20% at farm scale, have been measured. Studies in the Pacific Northwest coastal forests of North America show plantation forest wood volume declines by 35-50% on eroded sites. Mass movement impacts on production from tropical forests or agroforestry appear to be as yet undocumented. The reasons for lack of documentation are, first, that most soil erosion-productivity research has been done on gently sloping cropland, which is subject to surface rather than mass movement erosion. Secondly, geomorphological research in steeplands has dealt with mass movement as a hazard to human life, settlements and infrastructure -with limited identification of its contribution to sediment loads in rivers, and disregarding its impact on land productivity. We suggest there are many other countries where significant impacts are likely to occur, and that erosion-productivity studies should pay more attention to this type of erosion. Studies should not be restricted to cropland, but also extend to grazing land, plantation forestry, agro-forestry and traditional uses of natural forest as mass movement appears to affect all these forms of land-based production, particularly in densely populated steeplands whether tropical or temperate. Topics needing study are the documentation and costing of productivity losses, ways to reduce mass movement impacts on productivity, and ways to enhance recovery of soil on eroded areas (e.g., revegetation with fertility-building shrubs and legumes).