The influence of vegetation control on the growth and pulping properties of a eucalyptus grandis x camaldulensis hybrid clone.

Abstract

In order to determine if weed control as practised during the establishment phase of tree growth had a beneficial and long term (over a six to eight year rotation) impact on tree performance, a Eucalyptus hybrid clone (GC304) was planted in a field trial in 1990. The trial was situated in the coastal Zululand region near the KwaZulu-Natal town of Mtunzini. Nine different vegetation management treatments were imposed from establishment. These included a weedy control, a manually weeded treatment, a chemically weeded treatment, a 1.2 m row and 1.2 m inter-row weeding, a 0.5 m radius ring weeding, a complete weeding except for a 0.5 m radius ring around the tree, and the use of two legume cover-crops, Mucuna puriens (cowpea) and Vigna sinensis (velvet bean). Initial improvements in tree performance from these competition control treatments were detected from 60 days after planting, and were maintained over seven growing seasons. This occurred despite the absence of competitive vegetation after the first growing season due to reduced light, following crown canopy closure. There were strong indications that initial competition was mainly for moisture and possibly also for nutrients, rather than competition for light. Initially those trees that had weeds within their immediate vicinity were most affected (weedy control, inter-row weeding and the complete weeding except for a 0.5 m radius around the tree). With time, tree performance was more closely related to an increase in the percentage of the area kept free of weeds. The best performing treatment at felling, the manually weeded control, produced 17.1 % and 38.5 % more merchantable timber than the 1.2 m row weeding and the weedy control, at an increased profit of 8 % and 27 %, respectively. Two forms of competition (interspecific and intraspecific competition) were evident in the weedy control at different stages of tree development in contrast to the one (intraspecific competition) in the manually weeded treatment. Interspecific competition resulted in greater variability between the trees in the weedy control by the time canopy closure had occurred. This differentiation in tree size was further enhanced by asymmetric intraspecific competition once the trees had become established. The onset of intraspecific competition was first detected 995 days after planting for the manually weeded treatment and 1641 days after planting for the weedy control. Of the various competition indices that were tested in order to try and explain this differential growth in terms of individual tree performance, none was able to do so to complete satisfaction. The growth rates of different tree size classes were therefore compared for the weedy control and manually weeded treatment. The diverging slopes of the different stem area classes indicated that the larger trees were growing at the expense of the smaller trees. This type of competition is known as asymmetric intraspecific competition. In addition, a comparison was made between the slopes for the weedy and weedfree treatments for similar stem area classes. No significant difference was detected, indicating that similar size classes in these two treatments grew at similar rates. Trees from three treatments were selected (manually weeded treatment, 1.2 m row weeding treatment and the weedy control) and tested for the wood and pulping properties of density, active alkali consumption, extractable content, screened pulp yield, pulp yield per hectare and fibre length and coarseness. The use of Canonical Variate Analysis to determine if there were differences between the three treatments for the variates measured, indicated that they were significantly different. There was a significantly positive trend of an increase in density, extractable content and active alkali consumption with increased weed control. A possible explanation for this could be that the larger trees of the manually weeded treatment were under more stress (from increased intraspecific competition) during the latter phase of their growth. This was demonstrated by comparing the growth rates for these three treatments. The smaller trees of the 1.2 m row weeding treatment and the weedy control exhibited a lower rate of decline. As no significant difference was detected for screened pulp yield between the treatments, any differences in the pulp yield per hectare values could be attributed to differences in the merchantable volume. There was a 22.6 % and 40.8 % increase in the pulp yield per hectare for the manually weeded treatment in comparison to the 1.2 m row weeding treatment and the weedy control. The planting of cover-crops, although beneficial in terms of weed suppression, caused significant tree suppression. This occurred despite the fact that their initial biomass accumulation was slower than that of the natural weed population. Of the two cover-crops, the use of the velvet bean was not considered suitable due to its vigorous vining habit which affected the growth form of the trees. Subsequent work suggests that if the beneficial qualities of cowpeas are to be realised (that of weed suppression, erosion control and nitrogen fixation), a delay in their planting by three months after establishment of the trees should alleviate any negative impacts on tree growth.