Mixtures of eucalypts

The eucalypt genus dominates many of Australia’s forests and in most cases these forests are mixtures of several different eucalypt species. The aim of this project was to examine whether two eucalypts that are often grown in monospecific plantations might actually grow faster in mixtures. Monocultures and 1:1 mixtures of Eucalyptus pilularis and Eucalyptus grandis, and stands of two initial planting densities (1250 and 2500 trees ha^-1), were used to compare the productivity and stand structures of mixtures and monocultures and to quantify inter- and intra-specific competition. Interactions between these Eucalyptus species increased their relative yields in mixtures by 10-30%. This was associated with changes in stand structure where the diameter distributions of both species became less positively skewed (lower proportions of small trees). Mixing effects were relatively consistent as the stands developed suggesting that if this complementarity effect is used in plantations there is considerable silvicultural flexibility, such that these mixed plantations could be used on both shorter rotations for biomass or pulp-logs as well as longer rotations for solid wood products. The results also suggested that interactions between co-occurring Eucalyptus species in natural forests might actually facilitate individual tree growth, in addition to simply enabling co-existence.

A mixed-species plantation of three different eucalypt species in north-eastern New South Wales, Australia

Forrester, D.I., Smith, R.G.B. (2012). Faster growth of Eucalyptus grandis and Eucalyptus pilularis in mixed-species stands than monocultures. Forest Ecology and Management 286, 81-86. doi:10.1016/j.foreco.2012.08.037