Productivity and cost of biomass harvesting for energy production in coppice natural stands of Quercus pyrenaica Willd. in central Spain

Abstract

Biomass growing stocks, harvesting productivity and costs of energy production, were studied in thinnings of Quercus pyrenaica. Eleven forest sites were selected in coppices located in Castilla y León (Spain). Three systems were employed for harvesting: a mechanised whole tree harvesting (WTH) system, which included chipping at landing; a fully mechanised cut-to-length (CTL) harvesting system for firewood; and a semi-mechanised CTL system with manual felling and bunching. Three WTH trials had a hauling off 34.7–44.1 oven-dried tonnes (odt) ha−1. In contrast, only 11.6–30.0 odt ha−1 were obtained in the other eight CTL trials. Biomass weight equations for stems and whole trees were fitted. Worker operations were time-studied using software designed by the authors. The WTH system reached maximum productivity rates of 3.9 oven-dried tonnes per productive hour (odt h−1) for the felling and bunching operation and 6.9 odt h−1 for the forwarding operation. The mechanised CTL harvesting method achieved a productivity range of 1.3–0.5 odt h−1 for the harvesting operation. Furthermore, the average forwarding productivity was 7.3 odt h−1. Motor-manual felling and crosscutting in the CTL system accomplished a productivity range of 0.7–1.9 odt h−1. Manual bunching resulted in similar values. The lowest firewood unit cost for trees with 10 cm of diameter at breast height (DBH) was 62.0 € odt−1 using a semi-mechanised CTL system. Under similar conditions, this cost was 85.2 € odt−1 for the mechanised CTL system. Finally, the cost corresponding to chips at landing from WTH sites (average DBH = 10 cm) was 65.3 € odt−1.