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New Zealand Journal of Forestry (1996) 40(4): 29–33
©New Zealand Institute of Forestry

Research article
Historical slip erosion in catchments under pressure and radiata pine forest, Hawke's Bay hill country

P. Franson and R. Brownlie

As part of research on the effects of erosion associated with the harvesting and re-establishment of plantation forests, historical levels of slip erosion and their probable causes were determined in two similar-sized catchments. Slips and vegetation cover were digitally mapped from a series of aerial photographs (1943-1994) using an analytical stereoplotter connected to a Geographic Information System (GIS) that facilitated spatial analyses. High levels of storm-induced slip erosion were recorded for land use under pasture in 1943 and 1988. In 1943, slips occupied 1.3% of the total catchment area which was mainly in pasture. Comparison of slip records enabled estimation of slip recovery rates (0.7 ha per year) by pasture plants and consideration of the effect of earlier major storm events on information recorded at a specific date. Extensive conversion of indigenous scrub cover to pasture land since the 1880s, a large earthquake in 1931, and two large storms in 1938 were significant influences on the relatively high level of erosion. However, establishment of radiata pine forest in Pakuratahi Catchment in the early 1970s subsequently mitigated the erosive impact of Cyclone Bola in 1988. Overlaying of consecutive slip records enabled calculation of the proportion of slips that were new and those associated with an earlier event. In the future, the impact of storm-induced erosion on harvested and re-established sites can be compared with adjacent pasture land and the historical slip records.
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