Applying PROMOD spatially across Tasmania with sensitivity analysis to screen for prospective Eucalyptus globulus plantation sites

When screening areas as prospective plantation sites two criteria are of interest: the expected yield at that site and the uncertainty associated with this yield expectation. This paper describes a methodology to enable spatial estimates of uncertainty to be attached to predictions of tree growth in relation to variations in soil depth, nutrient status, and drainage. An environmental sensitivity analysis was applied using the growth model PROMOD to produce site productivity and suitability maps for Eucalyptus globulus (Labill.) on a 1000 m regular grid of Tasmania. Nine productivity surfaces were computed, one for each of three assumed soil-water holding capacities (for soil depths of 0.5, 1.0, and 1.5 m — the common classes in Tasmania), combined with three permutations of soil-nutrient rating (the expected rating and one class above and one below this value), and incorporating potential waterlogging estimates. The mean and coefficient of variation was calculated for each 1000 m grid cell in the nine subsequent productivity surfaces. A final plantation suitability grid was then mapped to show areas of the State with predicted high (>25 m³ ha⁻¹per year), moderate (15–25 m³ ha⁻¹ per year), and low productivity (<15 m³ ha⁻¹ per year), each with corresponding high variability (coefficient of variation (CV>30%)) and low variability classes (CV<30%). As no areas had both, high productivity and high variability, only five plantation suitability classes were mapped.Those areas indicated as high productivity/low variation sites have a high probability of being successful E. globulus plantations. Low productivity sites with either high or low variation are very unlikely to be favourable for this species. Moderate productivity sites with low variation are likely to be viable potential plantation sites with relatively low risk, particularly with nutrient amelioration. Moderate productivity sites with high variation might warrant further field investigation to determine if there are major environmental factors or combinations of these, such as soil depth (water holding capacity), nutrient status, or drainage that are actually going to be limiting reasonable growth at these sites.