Faster growth of 'Eucalyptus grandis' and 'Eucalyptus pilularis' in mixed-species stands than monocultures

Abstract

'Eucalyptus' plantations cover more than 20 Mha worldwide and are almost exclusively monospecific. However, in native forests 'Eucalyptus' species often grow in mixtures. Mixed species stands of trees can be more productive than monospecific stands but despite the implications of this effect, for plantations and native 'Eucalyptus' forests, the effects of mixing eucalypts has received little attention. The aim of this study was to examine whether two species that naturally coexist ('Eucalyptus grandis' W. Hill and 'Eucalyptus pilularis' Sm.) grow faster in mixtures than their respective monocultures. Monocultures and 1:1 mixtures, 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. This study also suggests that interactions between co-occurring 'Eucalyptus' species in natural forests might actually facilitate individual tree growth, in addition to simply enabling co-existence.