The root of the problem

Authors: Piers Maclaren
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2011, Issue N.Z.J.For. 56(4) 2012, pp 2-3, Jan 2012
Publisher: New Zealand Institute of Forestry

Abstract: Massive December floods in Golden Bay. Over half a metre of rain in 28 hours. Steep slopes. Crumbly granite soils. Heart-rending loss of the houses, gardens and roads built on fans and flood plains. Forestry cops a lot of the blame. So what’s new? Let’s recap. One third of New Zealand averages an uplift of more than a millimetre per year, from tectonic movement. Obviously, the altitude of our peaks does not change that much – erosion ensures a rough equilibrium. We have steep slopes (perhaps 28° median) because, in part, they are protected by indigenous forest. So how do trees minimise erosion? Our superseded Catchment Boards – mostly agriculturally trained – had some funny ideas. They believed that the most important factor was that trees dried out the ground. But when we get half a metre of rain on saturated soil and the whole landscape is a liquid mush, the minuscule contribution of interception by vegetation is irrelevant. Catchment Boards compounded their crazy ideas by planting deciduous poplars (even though most rainfall occurs during winter, work that out) and advocated wide spacings to allow understorey grazing. Grass has always been King.
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