Effects of Changing the Supply of Nitrogen and Phosphorus on Growth and Interactions between 'Eucalyptus globulus' and 'Acacia mearnsii' in a Pot trial

Significant increases in aboveground biomass production have been observed in mixed plantations of 'Eucalyptus globulus' and 'Acacia mearnsii' when compared to monocultures. However, this positive growth response may be enhanced or lost with changes in resource availability. Therefore this study examined the effect of the commonly limiting resources soil N, P and moisture on the growth of 'E. globulus' and 'A. mearnsii' mixtures in a pot trial. Pots containing two 'E. globulus' plants, two 'A. mearnsii' plants or one of each species were treated with high and low levels of N and P fertiliser. After 50 weeks, 'E. globulus' plants grew more aboveground biomass in mixtures than monocultures. 'A. mearnsii' were larger in mixtures only at low N, where both species were similar in size and the combined aboveground biomass of both species in mixture was greater than that of monocultures. At high N and both high and low levels of P fertiliser 'E. globulus' appeared to dominate and suppress 'A. mearnsii'. In these treatments, the faster growth of 'E. globulus' in mixture did not compensate the reduced growth of 'A. mearnsii', so mixtures were less productive than (or not significantly different from) E. globulus monocultures. The greater competitiveness of 'E. globulus' in these situations may have resulted from its higher N and P use efficiency and greater growth response to N and P fertilisers compared to 'A. mearnsii'. This trial indicates that the complex interactions between species in mixtures, and thus the success of mixed plantations, can be strongly influenced by site factors such as the availability of N and P.