Prediction of timber harvesting productivity for semi-mechanised systems in Viphya forest plantations, Malawi

Abstract

At least 200,000 m3 of timber are harvested annually using semi-mechanised harvesting systems (SMS) on the Viphya forest plantations in Malawi. Although these systems have long been used on the Viphya, no investigation on their productivity has so far been reported. The absence of local productivity models created uncertainty about the importance of sitebased factors that influence timber harvesting productivity of these systems on the Viphya. Secondly, there is paucity of information regarding the appropriate timber harvesting systems for production maximisation and cost minimisation. This study aimed to develop prediction models for estimating the productivity and costs of semi-mechanised and simulated mechanised timber harvesting systems on the Viphya forest plantations. The study was conducted in Pinus kesiya stands at Kalungulu and Champhoyo forest stations of the Viphya forest plantations. A work study approach was followed to capture harvesting time and volume data. Stepwise multiple regressions were used to develop felling time models for a chainsaw over tree size, inter-tree distance, slope, ground condition, brush density, and ground roughness; and skidding time models over distance, slope, ground condition, ground roughness and volume skidded per cycle for a grapple skidder. Models were statistically validated. Secondary work study data for semi-mechanised systems were simulated for mechanised productivity based on local site factors. The study had shown that diameter at breast height and inter-tree distance were important factors that best explained felling time prediction models in Pinus kesiya stands on the Viphya forest plantations. Similarly, distance from stump to the roadside landing was the most important factor in addition to volume load, slope and ground conditions that determined grapple skidding time. Mechanised systems appear to be more advantageous than semi-mechanised systems. The former are associated with lower operating costs and inventories with relatively high production rates. Therefore, mechanised systems could help to optimise timber harvesting productivity on the Viphya. Further studies should be conducted to determine the effect of different ground conditions and roughness on skidding productivity.