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

Fertilizer and irrigation treatments (with and without) were applied i n factorial combination to four seedlots of Eucalyptus grandis planted near Gympie, Queensland. Treatments aimed to ensure that growth was n ot limited by lack of nutrients (in fertilized plots) or water (in irr igated plots) during the first 3 years after planting. After 3 years, fertilizer treatment had significantly and substantially increased gro wth in mean height and basal area but there had been no response to ir rigation and no interaction between fertilizer and irrigation. Over th e four seedlots, trees in fertilized plots were two and a half times t aller and had five times more basal area than those in unfertilized pl ots. More detailed growth estimates of the best seedlot (a first gener ation 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 incr eased 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 pl antation in Australia at such a young age. Above-ground net primary pr oduction was directly related to leaf area index and in fertilized plo ts 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, par ticularly during the period prior to canopy closure. Higher productivi ty following application of fertilizer resulted from more rapid expans ion of leaf area during the first 12 months while tree crowns were dev eloping, and maintenance of a higher leaf area index thereafter. The p ossible mechanisms responsible for increased growth and greater leaf a rea are discussed.