No abstract

Long-term (21-30 years) erosional responses of rainforest terrain in the Upper Segama catchment, Sabah, to selective logging are assessed at slope, small and large catchment scales. In the 0.44 km 2 Baru catchment, slope erosion measurements over 1990-2010 and sediment fingerprinting indicate that sediment sources 21 years after logging in 1989 are mainly road-linked, including fresh landslips and gullying of scars and toe deposits of 1994 -1996 landslides. Analysis and modelling of 5 -15 min stream-suspended sediment and discharge data demonstrate a reduction in stormsediment response between 1996 and 2009, but not yet to pre-logging levels. An unmixing model using bed-sediment geochemical data indicates that 49 per cent of the 216 t km 22 a 21 2009 sediment yield comes from 10 per cent of its area affected by road-linked landslides. Fallout 210 Pb and 137 Cs values from a lateral bench core indicate that sedimentation rates in the 721 km 2 Upper Segama catchment less than doubled with initially highly selective, low-slope logging in the 1980s, but rose 7 -13 times when steep terrain was logged in 1992 -1993 and 1999-2000. The need to keep steeplands under forest is emphasized if landsliding associated with current and predicted rises in extreme rainstorm magnitude-frequency is to be reduced in scale.

This paper synthesizes the results of a 15-year study (1988-2003) assessing the changes in slope and catchment erosion and sediment sources within the Segama catchment in Sabah, Malaysia, that was selectively-logged using a combination of tractor and high-lead logging techniques between December 1988 and June 1989 and then left to regenerate naturally. Comparisons are drawn with date on slope erosion rates and sediment sources in adjacent primary forest catchments.

Detailed information on post-logging sediment dynamics in tropical catchments is required for modelling downstream impacts on communities and ecosystems. Sediment tracing methods, which are potentially useful in extending to the large catchment scale and longer time scales, are tested in primary and selectively logged rainforest catchments of Sabah, Borneo. Selected nutrient (P and N) and trace metal (Ni and Zn) concentrations are shown to discriminate surface, shallow subsurface and deep subsurface sediment sources. Analysis of channel-stored fine-sediment samples and use of an unmixing model allow the relative importance of these vertical sediment sources to be estimated and erosion processes to be inferred for catchments of contrasting size.