Regional vegetation management standards for commercial pine plantations in South Africa

Abstract

During the 1990s a number of trials were implemented in the summer rainfall region of South Africa to determine both the short- and long-term impacts of competing vegetation on pine growth. Trial sites were selected across different physiographic regions such that a range of altitudinal, climatic and environmental gradients were represented. Although the understanding gained from these trials allowed for the development of vegetation management standards, their operational and economic viability need to be tested on a commercial basis. Four pine trials were thus initiated to test the applicability of these standards when utilised on a commercial scale. Two of the trials were situated at lower-altitude sites (900 m and 1 000 m above sea level [asl]), one at a mid-altitude site (1 267 m asl), and one at a higher-altitude site (1 650 m asl). Several vegetation management treatments developed to suit the predicted vegetation load at each site were implemented and varied according to either weeding intensity (high, moderate and minimal), or the area around the tree that was kept free of vegetation (no vegetation control, a ring/row weeding and complete vegetation control). Tree growth up to 3.5 years was linked to the level of vegetation management in each treatment and its associated total cost for that time period, allowing for the development of different cost–benefit comparisons. The intensity of vegetation management required to produce significant growth benefits decreased with increasing altitude, as did the area that needed to be kept free from competing vegetation. In contrast to the higher-altitude site, where tree growth did not benefit from vegetation management, there was a 18, 25 and 112% increase in diameter growth for the high-intensity vegetation management operations relative to minimal or no vegetation control at the three mid- and lower-altitude sites. Besides demonstrating the commercial applicability of research results, this series of trials has shown that site-dependent vegetation management is viable on a commercial scale and will allow the South African forest industry to adjust their weed control budgets on a regional scale (linked to altitude).

Keywords: across-site comparisons, altitude, cost–benefit comparisons, site specific, weed control, weeding intensity

Southern Forests 2009, 71(3): 187–199