Abstract
A trial was initiated in 1990 to determine the impact of early vegetation competition on the growth of a Eucalyptus grandis × E. camaldulensis hybrid (GC304) at a subtropical site in KwaZulu-Natal, South Africa. Treatments included a non-weeded control, a manually weeded treatment, a chemically weeded treatment, a 1.2m row and 1.2m inter-row weeding, a 0.5m radius ring weeding, a complete weeding except for a 0.5m radius ring around the tree, and the use of two legume cover-crops, Mucuna puriens (L.) DC. and Vigna unguiculata (L.) Walp. Growth responses to treatments and economic profit were assessed when the trial was felled at seven years of age. Initial growth benefits (first detected from 60 days after planting) were maintained until the trial was felled. The manually weeded treatment produced 62% more merchantable timber, at an increased profit of 30%, than the non-weeded control, thereby demonstrating the potential gains that can be obtained through vegetation control. The non-weeded control, inter-row weeding and the cover-crops experienced interspecific competition during tree establishment and yielded trees of similar height but smaller taper when compared with other treatments. This work shows that volume estimates, derived by using only top height as a predictor variable, may be problematic for non-weeded stands due to the adverse effect of tree suppression on taper and hence timber volume.
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