Production and cost of harvesting, processing, and transporting small-diameter (≤5 inches) trees for energy.

Dense, small-diameter stands generally require thinning from below to improve fire-tolerance. The resulting forest biomass can be used for energy production. The cost of harvesting, processing, and transporting small-diameter trees often exceeds revenues due to high costs associated with harvesting and transportation and low market values for forest biomass. Productivity and cost were evaluated in a whole-tree harvesting system on four fuel-reduction thinning treatment units in Arizona. Thinning required removal of trees less than 5.0 inches in diameter at breast height (DBH). Time studies were applied to evaluate the harvesting productivities and costs. Sensitivity analyses were performed to test the effects of different variables on costs. Simulations along with break-even analysis were used to examine the economic feasibility of using forest biomass for energy. Harvest productivity for each machine in the system ranged from 2.31 to 39.32 bone dry tons per productive machine hour (BDT/PMH). Harvest system costs including transportation for 36 miles or less averaged $55.27 per bone dry ton. Hauling cost represented the largest component (47.24%) of the total cost. Close to market operations, reduced off-highway hauling, shortened skidding distance, increased harvest tree size, and improvement in system balance could significantly reduce cost. Below the market value of $40 per bone dry ton for hog fuel, breaking even or realizing profit would remain difficult. Other values, such as reducing fire risks, preventing smoke pollutions, and creating renewable energy sources, should increase the attractiveness of harvesting forest biomass for energy.