SBR- Content.pmd - INBO

State of the Basin Report - 20033. Environmental consequences of deforestationThe most direct environmental consequence of deforestation is the loss of the forest biota in thedeforested area. Because forests are almost always more biologically diverse than the systems withwhich they are replaced, this usually results in a local loss of biodiversity, and potentially a reductionin global species diversity. While many of the larger more conspicuous forest species (the “charismaticmegafauna”such as tigers and rhinos which are often associated with calls for biodiversityconservation), tend to have large distributions, many of the small, less spectacular species do not.Insects and small herbaceous plants are far more limited in distribution and it is these species thatare most frequently lost when forest is cleared.An issue of great concern in the LMB and the region as a whole has been the consequences of forestclearance on hydrology and related processes such as flooding, soil erosion and mass soil movement.Much of the literature to 1990 on this issue was reviewed by Bruijnzeel. 40 The outcomes are notsimple and depend greatly on how the forest is removed, the soil type, slope and rainfall patterns ofthe area from which the forest is removed, and the subsequent fate of the area.The way the forest is removed, and even the seasonal timing of the removal, influence the amount ofsoil disturbance and soil compaction. For example, use of heavy equipment tends to compact soil sothat commercial logging operations using bulldozers will cause more compaction than other formsof logging or clearance due to swidden agricultural practices. Similarly compaction occurs more inwet than dry soils so forest clearance during the wet seasons is likely to cause greater compactionand dry season clearing. 41 Compaction and soil disturbance can have important consequences forerosion and water infiltration.When mature forest is cleared, the hydrological consequences depend on the subsequent use of theland. For example, there are a number of studies showing that young, regenerating forest has highertranspiration rates than mature forest so the consequence may be decreased runoff and stream flow.On the other hand if the forest is replaced with grassland or paved areas, runoff may increase andstreamflows will be higher, although the seasonal pattern may alter with wet season flows increasingat the expense of dry season flows.All these hydrological impacts have been documented from controlled studies on relatively smallcatchments. Attempts to demonstrate that they occur on the scale of large river basins have so farnot been successful. In tropical regions such attempts have been made for catchments in Thailand,on the island of Taiwan and the Amazon. 42A suggested reason for the failure to detectthe effects is that the spatial and temporalvariability of rainfall in large tropicalcatchments is too great and masks theimpact of catchment vegetation change.Increasing run off in logged areascan result in erosion, turbidity andsedimentation198