Interspecific competition affects early growth of a Eucalyptus grandis x E-camaldulensis hybrid clone in Zululand, South Africa

摘要

To determine the effects of the onset and development of vegetation competition on tree performance, a Eucalyptus hybrid clone (GC304) was planted in a field trial in Zululand, South Africa. Nine vegetation management treatments, imposed from planting, included a weedy control treatment, a manually weeded treatment, a chemically weeded treatment (glyphosate), a 1.2m row and a 1.2m inter-row weeding, a 0.5m radius ring weeding, a complete weeding except for a 0.5m radius ring around the tree (no ring weeding), and the use of two legume cover-crops, Mucuna puriens (L.) DC. (velvet bean) and Vigna unguiculata (L.) Walp. (cowpea). The different treatments applied during establishment resulted in the differential growth of the trees as determined by measurements of tree height and crown diameter. This occurred from as early as 60 days after planting. The degree of competition could be directly related to the type of vegetation (cover-crops or naturally occurring weeds) and its proximity to the tree. The predominant vegetation on this site, yellow nutsedge (Cyperus esculentus L.), was able to colonise the site rapidly, causing severe and early competition. There were strong indications that this initial competition was mainly for moisture and possibly also for nutrients, rather than competition for light. Initially, trees in those treatments that had vegetation within their immediate vicinity were most affected (weedy control, inter-row weeding and no ring weeding). With time, tree performance was more closely related to an increase in the percentage of the area kept free of vegetation. At 180 days after planting the ranking of the top five treatments in relation to the area kept free of vegetation was: manually weeded treatment (100% of area free of vegetation) > chemically weeded treatment (100% of area free of vegetation) > no ring weeding (90% of area free of vegetation) > row weeding (40% of area free of vegetation) > ring weeding (10% of area free of vegetation). The planting of cover-crops, although beneficial in terms of the suppression of competing vegetation, also 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 a velvet bean cover-crop was not considered suitable due to its vigorous vining habit which adversely affected growth form.