Inter-tree competitive processes during early growth of an experimental plantation of Eucalyptus pilularis in sub-tropical Australia

Individual tree diameters, heights and crown dimensions were measured over four growth periods, from 2.5 to 6.6 years of age, in a plantation experiment with blackbutt (Eucalyptus pilularis Smith) on a highly productive site in sub-tropical, eastern Australia. The experiment compared several stocking densities at planting (ranging over 816–1667 stems ha⁻¹) and rectangularities of tree espacement (ranging over 1–6). The effects of inter-tree competitive processes on individual tree diameter growth rates were examined using both a partial differential diffusion equation to describe the change with time of the frequency distribution of diameters and by examining which tree size classes were dominating growth. Over all four growth periods, it appeared that inter-tree competition for the light and soil resources necessary for growth was largely symmetric. That is, whilst larger trees would have been occupying more of the above- and below-ground space from which to obtain those resources than smaller trees, all trees would have been able to obtain quantities of each in amounts proportional to their sizes and grow at rates accordingly. However, even at these young ages, a tendency for larger trees to display growth rates disproportionately large relative to their sizes gave early evidence of asymmetric inter-tree competition developing; that involves taller trees denying light to shorter trees through shading, the reverse of which may not occur. These inter-tree competitive interactions became more pronounced both as tree sizes increased with age and when trees were planted more closely to each other. Asymmetric competition is expected to be a major determinant of individual tree size development for a large part of the subsequent growth of the forest. The results are consistent with the forest showing a transition from the first to the second phase of the Binkley et al. model of individual tree growth and development in forest stands.