Performance of Eucalyptus dunnii as influenced by vegetation control when felled at nine years in KwaZulu-Natal, South Africa

Abstract

In 1997, a trial was initiated to determine the impact of eight vegetation control treatments on the growth of Eucalyptus dunnii Maiden at a warm-temperate site in the KwaZulu-Natal Midlands, South Africa. The treatments included a Weedy and Weed-free check, a 1.2 m and a 2 m Row weeding, a Broadleaves treatment (only grasses controlled), a Grasses treatment (only broadleaves controlled), and the planting of sweet potato cuttings at 20 cm intervals, 1.5 m from the tree rows (16 667 cuttings ha^–1). After planting, manual weeding was carried out to allow for the establishment of the sweet potato as a cover-crop for the suppression of weeds, after which the vines were either allowed to develop unrestricted across the plots (Sweet potato) or kept 0.6 m from around each tree (Sweet potato 1.2 m). The treatments, imposed from the time of planting, were maintained until canopy closure (14.5 months). Tree growth was monitored throughout the rotation and this, together with the cost of the various weeding operations during re-establishment, was used to make comparisons between the treatments. The dominant weed species on the site, Acacia mearnsii De Wild. (black wattle), resulted in the early (from three months after planting) and sustained suppression of tree growth in those treatments where it occurred (Weedy check and Broadleaves). This resulted in a significant reduction of 54% and 41%, respectively, in merchantable volume of these treatments when compared to the Weed-free check at rotation end (9.2 years). No significant differences in tree volume were detected between the Weed-free check and the five other treatments, although there were differences associated with their weeding costs (ranking for costs as follows: Sweet potato 1.2 m > Sweet potato > Weed-free check > 2 m Row weeding > 1.2 m Row weeding). In principle, the planting of sweet potatoes to aid as an inter- and cover-crop was successful (weed suppression, whilst not causing a reduction in tree volume), but innovative, cost-effective ways will need to be tested before they can be considered as a viable alternative to the use of herbicides. In contrast, the 1.2 m and 2 m Row weeding treatments provided a cost-effective alternative to the Weed-free check, together with the added benefit of reduced herbicide use.

Keywords: competition; cover-crop; Ipomoea batatas; merchantable volume; vegetation management

Southern Forests 2008, 70(3): 183–191