The theoretical temporal variability associated with three biomass supply options is shown in Figure 14.1, representing conversion to densified biomass from multiple rotations of a dedicated short-rotation woody crop, two intermediate thinnings from a stand grown primarily for sawlog production, and logging residues utilized only during final harvest in a sawlog production system.

From Figure 14.1, it should be evident that there is an interaction of temporal and spatial variability at play in utilizing woody biomass from forestry activities that may be less relevant for agricultural crops. In particular, woody biomass from stand thinning operations and logging residues from an intermediate or final harvest may be spaced as much as an entire rotation length (25-100 years) apart at any fixed point on the landscape. Thus, in order for woody materials from logging residues to adequately supply annual demand for a depot or conversion facility, spatial rotation of management activities between the stands that make up an estate ownership or management area is needed. Accurate characterization of the frequency of treatments performed, types of woody biomass available, spatial pattern, and transportation network associated with projected annual utilization within a draw region is critical for long-term supply planning.

Two common ways to manage long-term supply planning in well-regulated, managed forests are area control and volume control. Strict area or volume control are most eas­ily applied in even-aged silvicultural systems growing a single cohort of trees from the

regeneration phase to final harvest, it is the harvesting of the primary sawlog crops that result in logging residues utilized for woody biomass. Area control is realized when, for a given estate area of size A hectares and a stand rotation length of N years, A/N hectares are harvested each year. Volume control refers to the case in which a fixed target sawlog vol­ume, (V + G)/N, is harvested over the rotation length, N, from all standing timber volume (V) plus growth (G) over that time period. For example, in the inland northwestern United States, it is assumed that the yield of useable woody biomass from terminal harvest logging residues falls between 0.5 and 1.5 bone dry tons (BDT) per 1000 US board feet, or 2.4 m3, of sawtimber volume. Depending on regional variability, a typical mature stand might have between 15 000 and 25 000 U. S. board feet (15-25 MBF) per acre (0.4 ha) or more. At moderate residue concentration, in a productive and mature stand in the inland northwest, approximately 25 BDT of logging residues might be available for every 0.4 hectares of sawlog volume harvested, or 61.75 BDT per hectare. Thus, yields from harvesting potential available woody biomass are considerably larger, more spatially variable, and less frequent than yields from agricultural crops on a per unit area basis.