The influence of various methods of plantation residue management on replanted<i>Acacia mearnsii</i>growth

Abstract

Summary Commercially planted Acacia mearnsii (black wattle) plays an important role in the South African forestry industry as a source of timber, pulpwood and tannin. Typical re-establishment practices have, in the past, involved the planting of seedlings into compartments where the post-harvest plantation residues have been burned. With the recent emphasis on long- term site sustainability, alternative establishment practices have been sought and tested with the aim of minimizing site and soil disturbance without adversely affecting timber yield. A field trial was initiated in 1994 with Acacia mearnsii at the Bloemendal Field Experiment Station situated in the KwaZulu-Natal Midlands. Four different residue management treatments were tested in combination with or without weed control. Tree growth and performance were compared where the residue was burned, the residue remained in situ or where the residue was broken-down with a tractor-mounted slash mulcher or coulter ripper. Tree performance was not affected by the different harvest residue treatments where the weeds had been controlled, indicating the potential for alternative establishment methods. Tree performance measured at 2.3 years of age showed a highly significant interaction (p<0.01) between residue management and weed control. There was a reduction in growth in all the treatments where the weeds remained on the site, with the burned and weedy treatment performing the worst. The reason for this difference was associated with the stimulation of wattle seed germination by burning which resulted in a highly competitive weed community in these plots when compared to the treatments that were not burned. Weeding resulted in a significant improvement in tree foliar levels of P and K, two of the elements which have been shown to enhance wattle performance. This underscores the importance of good weed control since inadequate weed control results in competition for those nutrients in greatest demand. Tree variability was found to be a function of the presence of weeds and the method of residue management. More variable tree growth was detected in the weedy treatment plots as well as in those treatments where the plantation residues had not been burned. Initial results indicated that browsing by small antelope was affected by the different treatments, with significantly (p<0.01) less browsing in areas where harvesting residues had been retained rather than burned. There was also less browsing in the weedy treatments than the weed free treatments.