Response of Eucalyptus regnans F. Muell. to thinning and urea fertilization in New Zealand

A 7-year-old Eucalyptus regnans plantation on the North Island of New Zealand was urea fertilized (230 and 460 kg N ha^-1) and thinned to 350 and 150 trees ha^-1 (TPH) from an initial stand density of 1200 TPH. Tree diameter, height, and crown height were measured periodically for the next 2.5 years. Crop trees (the 100 TPH with the largest diameter at final measurement) responded to both thinning and fertilization with greater diameter growth at the intermediate levels of both treatments. Thinning increased diameter growth slightly more than did fertilization. There was no interaction of thinning and fertilization on diameter growth. Thinning tended to decrease height growth while fertilization had a small positive effect on height growth. Live crown ratio was increased by both thinning and fertilization. Effects on stand basal area and volume increment were similar to those for diameter. Foliar nutrient analyses showed that the highest level of fertilization lowered concentrations of non-fertilizer elements and substantially altered ratios of nitrogen to the other elements. This possibly caused the lack of response at that level of fertilization. Radical thinning to final crop stocking (150 TPH) at an early age did not offer any growth advantage over less extreme thinning (350 TPH). The heaviest rate of fertilization (460 kg N ha^-1) at any level of thinning did not increase growth. Therefore, more moderate rates of thinning and fertilization are recommended in young E. regnans plantations.