Recognizing the importance of tropical forests in limiting rainfall-induced debris flows

Worldwide concern for continuing loss of montane forest cover in the tropics usually focuses on adverse ecological consequences. Less recognized, but equally important to inhabitants of these affected regions, is an increasing susceptibility to rainfall-induced debris flows and their associated impacts. The same high rainfall rates that sustain tropical forest cover can often serve as the triggering mechanism for debris flows. The natural rate of debris flow occurrence on steep slopes subject to episodic, intense rainfall is dependent on the stabilizing effect of tropical forests. Either loss or significant reduction in forest cover can weaken this natural defense. Information from postdisaster observations and research on the November 1988 storm event in southern Thailand provides a case study illustrating the potential impacts of increased debris flow susceptibility resulting from conversion of forest cover to rubber tree crops. Development resulting in the loss of tropical forest cover may be accompanied by local increase in population, property development, and infrastructure. Consequently, the potentially disastrous consequences of increased debris flow occurrence are amplified by the greater vulnerability of local populations. Preserving the tropical forest cover is an obvious and often difficult means of retaining this natural protection. Effective policy should capitalize on the values of tropical forests as part of the strategy for retaining adequate forest cover. Policy should also seek to avoid creating pressures that foster forest removal or their conversion to other types of land cover in steep terrain. Areas where tropical forests were converted to other cover types can be restored to secondary forests to avoid a permanent state of increased debris flow susceptibility. Restoration of secondary tropical forests can successfully re-establish the forest characteristics that limit debris flow occurrence. Experience in Central America and the Caribbean demonstrates that successful restoration is possible but requires a significant commitment of both time and resources. In addition to the cost and technical difficulties involved, the increased susceptibility to debris flow occurrence persists through many years until successful restoration is achieved. Both retention of existing tropical forests and restoration of forest cover where loss has occurred are often justified by the reduced risk of debris flow impacts to vulnerable populations and infrastructure. © 2012 Springer-Verlag (outside the USA)