Harvesting debris or logging residues such as limbs, roots, tops and Forest underwood are future feedstock alternatives for fuel wood pellets.

Forest underwood is comprised of all non-merchantable biomass (unsuitable for high-value wood products, such as saw timber, pulpwood, etc.). Forest underwood biomass typically consists of shrubs and small-diameter (approximately five inches or lower diameter at breast height), underdeveloped, and non-merchantable trees. To prevent catastrophic fires, Forest underwood is usually burned under strict regulations (prescriptions). Instead, Forest underwood biomass could be used as a renewable resource for conversion to a variety of fuels and energy applications, including wood pellets. These feedstock sources could become more profitable with the introduction of fuel reduction contracts between industry, government, and residents in areas affected by forest fires (Neary and Zieroth 2007). Forest underwood as feedstock for wood pellets provides a venue for using invasive species such as bamboo, Chinese tallow, kudzu, and cogon grass. However, Forest underwood feedstock may contain significant amounts of bark, needles, dirt, leaves and rocks that can cause substantial production inefficiencies and substandard pellets. Also, the removal of Forest underwood biomass could deplete the forest ecosystems of nutrients. Considerably more research and development is necessary to assess the sustainability of these emerging feedstock sources and to improve their combusting characteristics.