Farm forestry for economic and environmental sustainability - A new decision support system for farm foresters

The objective of this programme is to provide farm foresters and others with knowledge about sustainable and profitable land use with commercial tree crops. Initially the study examined the role of trees in stabilising soils when planted on eroding hill country pasture. Three species - radiata pine, Douglas-fir, and poplar - were compared for effects in reducing erosion. Tree size, tree stocking per hectare, root tensile strength, and rate of decay of roots after harvest were found to be important. Erosion control becomes effective once a stand of trees reached the equivalent of 30 tonnes/ha of radiata pine root biomass. Silviculture, and choice of species, determines to what extent this critical threshold is achieved. Decision support software, developed in this programme as "calculators', estimate the 'Equivalent Farming Gross Margin' of crops of radiata pine, and Douglas-fir, compared to the livestock previously grazing the land. The calculators, which run under Microsoft EXCELTM, also allow the identification of the most profitable silviculture for the tree crop, and generate yield tables. The calculators are being applied in two case studies; the first involves calibrating the radiata pine calculator for Hawkes Bay and the Wairarapa against actual tree data. Calibration of the calculator for bare land is also being studied. The second study will investigate how well tree crops can compete financially with pastoral farming systems in the Lake Taupo catchment, where there are increasing concerns about nutrient in-flows from pastoral farming. Further enhancements of the calculators are planned which will permit calibration against younger stands, and evaluation of a much wider range of silviculture. Keywords: farm forestry, erosion, root biomass, root strength, financial return, IRR, PNW, farming gross margin, Pinus radiata, Douglas-fir, poplar