COMPARISON OF GROWTH AND NUTRITION OF YOUNG MONOCULTURES AND MIXED STANDS OF EUCALYPTUS-GLOBULUS AND ACACIA-MEARNSII

The long-term sustainable production of fast-growing single-species pl antations is now being questioned because of their effects on soil fer tility. As an alternative to monocultures, growing mixed stands with N -fixing trees as an essential component of the mixture was tested in t his trial. Eucalyptus globulus and Acacia mearnsii were planted in fiv e different proportions (100E, 75E:25A, 50E:50A, 25E:75A, 100A) at two densities (3.3 m x 2 m and 3.3 m x 3 m) at Cann River in East Gippsla nd, Australia. Effects of species interactions on tree growth and conc entrations of N and P in plant tissues and on soil N mineralization we re assessed. Height and volume increments of individual eucalypt trees were positively affected by the presence of acacia. There was an indi cation at age 33 months that the 50E:50A treatment had the greatest he ight at the higher planting density. This was attributed to better N s tatus of eucalypts as indicated by the N concentration of live and sen escent leaves and of fine roots. The presumed transfer of N from acaci as to eucalypts during the early stages of plantation development prob ably resulted mainly from below-ground turnover of roots and nodules, as above-ground litter decomposition had not then commenced. The study showed no conclusive evidence of any change in P cycling in mixed sta nds at 33 months after planting, suggesting the need for inputs of P a s fertilizer to remove any P deficiency and enhance N fixation by acac ia. The trial will in due course provide data which will enable a bett er understanding of the potential long-term nutritional benefits and c osts of using an acacia in mixture with eucalypts. (C) 1997 Elsevier S cience B.V.