Hydrological impacts of converting native forests and grasslands to pine plantations, South Island, New Zealand

Abstract

Changes in water yield, flood hydrology, and low flows caused by replacing indigenous forests and grasslands with commercial softwoods have been investigated in New Zealand since the mid-1970s. The long-term results of two of these studies are discussed here. The first deals with the conversion of mixed evergreen forest to pine plantation in the northwestern South Island. After a 2-year calibration period one catchment was left as the control (DC2) and the other two catchments were harvested in 1981, one by skidder (DC1) and the other by hauler (DC4), and planted in pines shortly thereafter. For the first 4 years after harvesting the average annual difference in water yields between DC1 and DC2 was 352 mm (69%), and between DC4 and DC2 it was 463 mm (90%), which equates to an annual increase of 312 mm (61%) and 344 mm (68%), respectively, when compared with the calibration period. Planting the harvested areas caused the water yield from both catchments to return to pre-harvesting levels within 8 years, and an estimated reduction in runoff of 340 mm within 5 years at DC4. Mean flood peaks increased after harvesting, especially for small and medium storms on the skidder-logged catchment (75–100%). The response of the storm quickflows to harvesting was similar but much more subdued. Low flows also increased after harvesting. Tree growth brought storm peak flows, quickflows, and low flows back to the levels of those in the original beech forest within 10 years. The second study examines the impact of converting tussock grasslands to pine plantations using data collected from two catchments in the eastern uplands of southern New Zealand. After a 3-year calibration period (1980–1982) one catchment was planted in pines over 67% of its area and the other was left in tussock. By 1989 the difference in annual water yield from the planted catchment was 130 mm, and between 1991 and 1994 it averaged 260 mm (27% of total runoff from the control). Differences in low flows (represented by the minimum annual 7-day mean) showed a similar trend, and suggest that in dry periods, afforestation of tussock grasslands can reduce water yields by 0.18 mm day −1. Higher interception losses from increased canopy evaporation is believed to be the main reason for the reduction in water yield. After 10–12 years of tree growth mean flood peaks had fallen between 55 and 65%, and quickflows had decreased by about 50%.