Research article Soil impacts of afforestation in the high country
Afforestation of South Island high-country grassland soils with exotic conifer species influences the amounts of plant nutrients in topsoils, nutrient availability and soil acidity. The effects are dependent on soil type and tree stand age. Under young stands levels of total nitrogen (N) and phosphorus (P) and Bray-2 extractable potassium (K) and magnesium (Mg) are generally lower than under adjoining grasslands - an effect attributed to greater nutrient uptake by trees. Under older stands total N and P and extractable cation levels are sometimes lower, but may be similar or higher than under grassland. Afforestation stimulates mineralisation of organic matter in topsoils, leading to increased levels of mineralisable (potentially available) N and of sulphate sulphur (S) in hygrous high-country yellow-brown earth soils, and to increased availability of P in all soil groups examined. Under mature pine stands on drier Mackenzie Basin soils mineralisable N is commonly lower than under grassland. Vegetation growing under young pine stands prior to canopy closure contains markedly higher concentrations ofN, P, and S in foliage than similar vegetation in adjoining unplanted grassland. Such vegetation may also contain higher concentrations ofK and Mg, which is not consistent with results of soil analyses. Soil pH is reduced by afforestation. Productivity of pasture established after harvest of plantations at three sites was found to be 1.5 to 14 times greater than that in adjoining grassland. Access to organically bound nutrients means that growth of conifers is less likely than pasture growth to be limited by N, P or S supply. Improved availability of nutrients to understorey species may increase the potential for grazing in high-country forests.
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