A climate analysis of the current and potential future Eucalyptus nitens and E. globulus plantation estate on Tasmanian State forest

Plantations of Eucalyptus globulus and E. nitens have been developed on Tasmanian State forest, with their combined area currently totalling 50,246 ha. The climatic envelope for this plantation estate was described by spatial interpolation of long-term climatic averages from an array of weather stations, using as parameters the mean minimum temperature of the coolest month, and effective rainfall (rainfall minus evaporation). Reanalysis of existing trial data suggested that growth reduction in E. globulus would begin to occur on sites with a mean minimum temperature of the coolest month below 1.8°C, and this threshold was thus chosen to segregate warmer areas of Tasmania suitable for planting either E. globulus or E. nitens, from cooler areas suitable only for planting E. nitens. This segregation based on temperature predicts that 53% by area of the current E. nitens and E. globulus plantation estate on State forest (26,607 ha) is suitable to plant either E. globulus or E. nitens. However, only 29% of this area is currently planted to E. globulus, a proportion which could potentially be increased to take advantage of the superior wood properties of E. globulus. There was strong overall agreement between segregation based on the 1.8°C temperature threshold, and segregation based on the previous altitudinal cut-off for E. globulus of 350 m, with 87% of the State forest eucalypt plantation estate being classified similarly under both systems. However, there were significant differences among State forest Districts in the relationship between the mean minimum temperature of the coolest month and altitude. The altitude corresponding to the 1.8°C temperature threshold was substantially below 350 metres in Districts at a more southerly latitude or with plantations situated at greater distances from the coast. Two-thirds of the disparity between the temperature and altitudinal segregation (4,204 ha) was due to sites colder than the 1.8°C temperature threshold occurring at altitudes below 350 m. Currently 49% of this plantation area is under E. globulus, although the sites could be more suitable for E. nitens.