As will be evident in Section 14.4, there are a large number of established and emerging equipment options for harvesting and in-woods preprocessing of woody biomass. The level of preprocessing that occurs and the point at which it occurs in the supply chain have important impacts on supply chain efficiency because transportation costs, whether from stump to landing, landing to depot, or over long distances by rail or barge, are affected by the energy and mass density of the material. In general, supply chains that reduce the parti­cle size, ash content, and moisture content of woody biomass close to the harvest location have the greatest transportation efficiency. This is because more densely packed densified, dried biomass contains the highest energy content per unit volume or mass (BTUs m-3,

BTU ton-1). To address this characteristic of the woody supply chain, a number of special­ized harvesters and forwarders have evolved that process, comminute, and densify biomass in the woods, immediately after harvest, to varying degrees. These include, for example, slash bundler forwarders, self-feeding chipper-forwarders, and even mobile depot units that dry, grind, and densify regional woody biomass supply at tactical scales (e. g., 3-4 years within a draw area) before moving to another location. Some of the more common har­vesting and combined harvesting and processing equipment types currently available are described briefly and generally in Section 14.6.

Most woody biomass currently used or being actively studied in the context of biofuels and bioenergy development is derived from three major source categories: dedicated short — rotation woody crops (SRWCs), thinning materials, and logging residues.