New Zealand Journal of Forestry (1994) 39(3): 18–25
©New Zealand Institute of Forestry
The effect of plantation forestry on water yeild in New Zealand
This paper discusses the hydrological consequences of converting land in native forest, scrub, tussock grassland, and pasture to plantation forestry, and the impacts of harvesting and re-establishing a tree crop. Forests influence water yield and associated streamflow responses through increased canopy interception of rainfall. Thus afforestation of pasture may reduce water yield by 30-50% five-ten years after planting. For tussock grasslands the reduction is between 25 and 30%. A similar percentage reduction can be expected in low flows. Storm quickflows and flood peaks can fall by over 50%. Silvicultural practices, such as understorey control and spreading the time of planting, have the potential to augment water yield.
Forest harvesting in moderate-to-high rainfall areas can cause a 60-80% increase in water yield for three-five years after clearfelling. Yields should return to pre-harvesting levels within six-eight years, depending on the silvicultural regime adopted. Mean flood peaks can rise by up to 50%. However, the hydrological impact must be viewed within the context of the area harvested compared with the total forest area. If the former is small in comparison, any local increases in water yield and flood response may be quickly attenuated