Effects of stock plant age and fertiliser application at planting on growth and form of clonal Acacia hybrid

Some Acacia hybrid (A. mangium × A. auriculiformis) plantations in Vietnam are prone to heavy branching and poor stem form. The age of the stock plants used to produce cuttings for clonal propagation and/or high rates of fertiliser application at planting have been suggested as causal factors. Their effects on growth, stem form and branch size of three Acacia hybrid clones were examined in south Vietnam. Experimental treatments were: (1) fertiliser: F-0 (no fertiliser) and F-H (high fertiliser, i.e. a mix of superphosphate and NPK (nitrogen–phosphorus–potassium) fertiliser in the planting hole at rates equivalent to 16, 45 and 8 g tree^-1 of N, P and K respectively) and (2) planting stock raised from cuttings taken from stock plants aged 1 and 4 years. Stock plant age had no effect on growth and form traits, nor were there differences between clones. Fertiliser increased growth during the first three years, but by age 4 years, had no significant effect on stem diameter or stand volume. The high dose of fertiliser at planting significantly increased the proportions of trees requiring singling and form pruning, the diameter of the largest branch in the 1–2 m stem height interval, and the severity of stem bending and breakage. High doses of fertiliser at planting should be avoided for Acacia hybrid plantations.