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New Zealand Journal of Forestry (1997) 42(2): 29–34
©New Zealand Institute of Forestry

Research article
Vegetation recovery and indicative sediment generation rates by sheetwash erosion from hauler-logged settings at Mangatu Forest

M. Marden and D. Rowan

Soil disturbance, vegetation recovery and sediment generation by sheetwash erosion on hauler-logged settings were evaluated at Mangatu Forest. Three disturbance indices were recognised: undisturbed, shallow disturbance and deep disturbance. The latter is here referred to as areas of exposed mineral soil (EMS). Groundcover vegetation recovery was slowest on sites of EMS. Based on plot-sized areas (9 m2), EMS per cent cover averaged 31%, two years after clearfelling. Setting-based measurements showed that overall groundcover recovery was rapid, and resulted in an 83% reduction in areas of EMS within two years of clearfelling.
Mean rate of sediment generation from areas of EMS averaged 11 (SE 3.5) kg/m2for the first year and 4 (SE 2.4) kg/m2 during the second year after logging. Annual rates of sediment generation from areas of EMS on logged setting were 8 (SE 2.5) tonnes/ha in year one and one (SE 0.5) tonne/ha in year two. Most sediment mobilised from areas of EMS was relocated on slopes immediately downslope of source areas, where it was effectively entrapped by surviving groundcover vegetation and slash and in microtopographic hollows. Only those riparian areas of EMS with a direct connection to a watercourse contributed much sediment to stream channels, from which 2 (SE 0.6) tonnes of sediment per hectare of logged setting was generated in the first year after logging, declining to 0.2 (SE 0.1) tonnes/ha within two years. Sediment generated by sheetwash erosion from logged settings one year after clearfelling was at least several orders of magnitude less than that generated from non-forestry-related sources, especially gullies; it was not therefore particularly significant, given the high background rates of sediment production within Mangatu Forest. Similar amounts of forest-related sediment entering streams where background rates of sediment production are low would, however, be regarded as unacceptable. It would be advantageous for the forest industry to minimise soil disturbance and consequent sediment production from hauler-logged settings, particularly within 10 m of stream channels.

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