Knowledge of the components of above-ground biomass of low-quality stems harvested to produce biofuel pellets is important, since bark has higher ash contents (a key quality parameter for the pellets) than wood. Therefore, single-tree biomass functions by Marklund (1988. Rep. 45. Department of Forest Survey, Swedish University of Agricultural Sciences, Umea, Sweden.) were evaluated using a sample population of 1612 Scots pine ( Pinus sylvestris L.), Norway spruce ( Pices abies (L.) Karst.), and birch ( Betula pendula Roth and Betula pubescens Ehrh.) trees and three methods to model bark proportions. Bark percentages calculated subtractively using functions for stem biomass over and under bark showed anomalous patterns, especially for pine and birch. However, additive use of biomass functions for bark biomass and stem biomass under bark resulted in similar patterns to bark percentages traditionally calculated from bark biomass and stem biomass over bark. Published ash contents of bark and stemwood indicate that pine, birch, and spruce stems with breast height (1.3 m) diameters of 4–40, 4–30, and >19 cm, respectively, have <0.7% ash contents (the current limit for the highest quality fuel pellets). However, if the highest recorded ash contents are used, only pine stems meet this criterion. Thus, material of different species and stem dimensions may need to be carefully mixed when whole-stem biomass is used as feedstock for pelletizing.
Improving the robustness of biomass functions: from empirical to functional approaches
