Long-term responses of rainforest erosional systems at different spatial scales to selective logging and climatic change

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.

Effects of different land-use on suspended sediment dynamics in Sabah (Malaysian Borneo) – a view at the event and annual timescales

“…The shorter tree, shrub and herbaceous cover of LF3 intercept raindrops just before hitting the soil thereby reducing the landing velocity and erosive power of the rainfall. A similar phenomenon of reduced erosion rates has been recorded elsewhere in Sabah by erosion monitoring on heavily disturbed terrain in phase with dense nearground vegetation during long-term recovery (Walsh et al, 2011). Also, Negishi et al (2006) noted the role of fern cover in reducing erosion from abandoned logging roads in peninsular Malaysia.…”

“…The dominant geology of the LF2, LF3, OP and PF catchments comprises Oligocene to Middle Miocene rocks of the Kuamut (and Kalabakan) Formations, which consist of a melange of sedimentary and volcanic rocks, including slump breccia and interbedded mudstones, tuffs, tuffaceous sandstones, shale, conglomerate, chert and limestones (Walsh et al, 2011). The VJR catchment, however, is underlain by Cretaceous to Early Tertiary igneous and metamorphic rocks (mainly gabbro, dolerite, serpentinite, peridotite, dunite and pyroxenite).…”

“…Although there have been many studies on the effects of logging and alternative land-uses on suspended sediment yield (Douglas, 1999;Douglas et al, 1992Douglas et al, , 1999Walsh et al, 2011), impacts of repeat logging and forest rehabilitation have received little attention. Likewise, impacts of oil palm conversion and associated management practices on suspended sediment in different types of terrain have not been assessed.…”

“…Likewise, impacts of oil palm conversion and associated management practices on suspended sediment in different types of terrain have not been assessed. Roads are known to play a major role in generating sediment and (if poorly aligned) causing landslides in post-logging terrain (Gomi et al, 2006;Negishi et al, 2008;Walsh et al, 2011;Sidle and Ziegler, 2012), but their role in oil palm plantations remains unevaluated. This area of study is important because of downstream effects on sedimentation, water supply, river ecology and flooding.…”

Conclusion: applying South East Asia Rainforest Research Programme science to land-use management policy and practice in a changing landscape and climate

“…(d) Hydrological and erosional ecosystem service benefits of rainforest A number of key policy messages are clear from the hydrological work of Walsh et al [11]. The historical record of sedimentation rate for the upper Segama reconstructed using caesium-137 and lead-210 showed the striking differences in erosional consequences of logging terrain of varying steepness.…”