RESPONSE TO NUTRIENTS IN EUCALYPTUS-GRANDIS .1. BIOMASS ACCUMULATION

Fertilizer and irrigation treatments (with and without) were applied in factorial combination to four seedlots of Eucalyptus grandis planted near Gympie, Queensland. Treatments aimed to ensure that growth was not limited by lack of nutrients (in fertilized plots) or water (in irrigated plots) during the first 3 years after planting. After 3 years, fertilizer treatment had significantly and substantially increased growth in mean height and basal area but there had been no response to irrigation and no interaction between fertilizer and irrigation. Over the four seedlots, trees in fertilized plots were two and a half times taller and had five times more basal area than those in unfertilized plots. More detailed growth estimates of the best seedlot (a first generation seed orchard from Coffs Harbour) showed fertilizer treatment had substantially increased leaf area, stemwood volume and all components of above-ground biomass. After 3 years, fertilizer treatment had increased stemwood mass from 6.9 to 41.4 t ha-1. Mean annual increment in stemwood volume in fertilized plots at age 3 years had reached 34.2 m3 ha-1 year-1, which is the highest growth rate yet reported for any plantation in Australia at such a young age. Above-ground net primary production was directly related to leaf area index and in fertilized plots a productivity of 30 t ha-1 year-1 was associated with a leaf area index of 4.5 at 2 years of age. Tree growth rate in unfertilized plots in this study was severely inhibited by low soil nutrient supply, particularly during the period prior to canopy closure. Higher productivity following application of fertilizer resulted from more rapid expansion of leaf area during the first 12 months while tree crowns were developing, and maintenance of a higher leaf area index thereafter. The possible mechanisms responsible for increased growth and greater leaf area are discussed.