A comparison of biomass production and machine system productivity using three harvesting methods in a conifer first thinning.

Abstract

A thinning trial took place in a 19-year-old Sitka spruce (Picea sitchensis (Bong.) Carr) plantation in the south east of Ireland where three methods of harvesting were performed using the same harvester and forwarder machine system: cut to length harvesting (CTL), integrated harvesting (INT), and whole tree harvesting (WT). The objectives of the trial were to evaluate the log volume and biomass mobilised using each method, the productivity of the machines, and quality parameters of the biomass as a fuel. Using the CTL method, 56.5 m3 ha-1 of pulpwood and 6.4 m3 ha-1 of small sawlog were harvested. This equated to 23.6 odt ha-1 of roundwood material. A greater amount of biomass was mobilised using the INT and WT harvesting methods than the CTL method. On average, compared to the CTL method, 81.8% more biomass was mobilised by the INT method and 93.6% more biomass was removed by the WT method. Wood fuel supply chain (harvesting, forwarding, and chipping) cost to the forest roadside was highest for CTL, and was 23% lower for INT, and 33% lower for WT. The results suggest that the potential biomass mobilisation from first thinnings may be greater than current estimates suggest, while harvesting costs may be further reduced by employing specialised methods. However, the trial was confined to one site, and therefore as the results are site specific they need to be interpreted with caution.