Physical Characterisation and Quantification of Total Above Ground Biomass Derived from First Thinnings for Wood Fuel Consumption in Ireland
Comprehensive knowledge of wood fuel properties assists in the optimisation of operations concerned with the harvesting, seasoning, processing and conversion of wood to energy. This study investigated the physical properties of wood fuel. These properties included moisture content and basic density. The field work also allowed for the quantification of above ground biomass partitions. The species investigated were alder (Alnus glutinosa), ash (Fraxinus excelsior L.), birch (Betula spp.), lodgepole pine (Pinus contorta Dougl.), Norway spruce (Picea abies (L.) Karst.), and Sitka spruce (Picea sitchensis (Bong.) Carr.). The primary variation sources investigated were spatiotemporal. This included sampling at different times of the year (dormancy, flushing and the growing season) in three different locations on the basis of latitudinal zones (South East, Midlands and North West of Ireland or 52oN, 53oN and 54oN). The age of stands where sampling took place ranged from 12-17 years old.
The findings for moisture content, basic density and biomass have been accumulated from a large dataset (~ 4934 subsamples). Moisture content and basic density ranged between 38-65% and 344-498 kg∙m-3 respectively in the stem wood sections between the six species. Ash was the only species that had a distinguishable temporal trend in stem wood moisture content. Ash stem wood moisture content was lower during dormancy and higher during flushing and the growing season. Moisture content in the branch wood sections ranged from 44-54% between the six species. In most cases, branch wood moisture content reduced significantly for all six species at flushing in comparison to dormancy. With all six species, basic density reduced with an increase in latitude. The quantification of above ground biomass found an increase of 43-53% in the biomass that can be potentially harvested from whole trees, in contrast to harvesting only merchantable stem sections. An evaluation of stand biomass estimation methods using biomass expansion factors (BEFs) to convert standing volume per hectare to biomass in oven dry tonnes and energy content per hectare was also made. BEFs ranged from 1.9-2.3 between the six species. However the utility of the BEFs from this study must be exercised with caution, due to the narrow ages and sizes of the trees sampled, in addition to including above ground biomass components only.