Sediment sources and delivery following plantation harvesting in a weathered volcanic terrain, Coromandel Peninsula, North Island, New Zealand

Abstract

Sediment generation and vegetation recovery was measured over a 2-year post-harvest period in a 36-ha catchment of exotic forest located in andesitic terrain, Whangapoua Forest (36.46°S, 175.36°E), Coromandel, New Zealand. Slopewash, soil scraping (on-slope removal of the regolith by the repeated dragging of logs), and storm-initiated landsliding were identified as the principal sediment-generating processes. Slopewash and vegetation recovery rates were measured using field-based plots located on sites of shallow- and deep-disturbance and a regression relationship was established between sedimentation rate (accumulation (g)/day.mm rain.m2) and per cent vegetation cover for both plot types. At the basin scale, slopewash was calculated using the plot-based rates times the total area of deep- and shallow-disturbance sites as identified from a ground-based, transect survey and using sequential aerial photography. Sediment production, by soil scraping and landsliding, was determined by multiplying mean scar depth by the total affected area. In the first post-harvest year deep-disturbance sites generated 92% of total slopewash produced from both disturbance classes combined, and in year 2, slopewash halved. Half of the first post-harvest year’s slopewash-derived sediment was generated within the first 7 months following the completion of harvesting and before the application of desiccant. Thereafter, on deep-disturbance sites, slopewash rates declined further as sites became hardened against the generation of new sediment (i.e. sites became sediment limited). In contrast, during both the initial post-harvest recovery period and the post-desiccation period, the decline in sediment production on shallow-disturbance sites was more a consequence of site recolonisation. Sediment generated and redistributed by scalping and by landsliding occurred at the time of the respective events and coincided with the early part of the first post-harvest year. Collectively, soil scraping, slopewash, and landslides generated 1864 t (52 t/ha) of sediment, 88% of which remained on-slope. Of the sediment delivered to streams (228 t), landslides contributed 72%, soil scraping 26%, and slopewash 2%. For this harvested basin a single, storm-initiated, landsliding event was the most important hillslope process responsible for the generation of sediment and its delivery to streams, and slopewash was the least important.