Short rotation woody crops are woody tree species that have been bred and selected to have extremely high rates of growth, allowing them to be harvested after a short growing period. They are an ideal feedstock component for biomass supply chains. While many hardwood species have full management rotations that may approach 50 years or more in length, short rotation woody crops can be harvested every three to seven years. While short in rotation, these trees make up for it with high yields. Short rotation trees can be utilized for the production of liquid fuels, such as ethanol, diesel, and jet fuels. They can also be used for the production of heat, steam, and/or electricity. Due to their rapid growth, their wood is valued for fiber and not strong lumber or other products.
Hybrid poplar (Populus spp.) and willow (Salix spp.) are the two species most commonly adapted to short rotation management. Poplars are typically grown in the mid-Atlantic, Southeast, and Northwestern portions of the US. Willow is a more cold-tolerant species and is the species of choice in the upper Midwest and Northeastern parts of the country. Both types of hardwoods are coppice species. Coppicing occurs when the tree is harvested and a new sprouts grow from the existing root system. Having trees that exhibit this trait allows for multiple harvests following a single planting.
Hybrid poplars are normally crosses between native cottonwoods and are close relatives to aspens. The trees can grow throughout the U.S. where sufficient water is available. They are planted using cuttings or “sticks” harvested from a tree nursery. These cuttings very quickly sprout roots and grow very quickly. Hybrid poplars have short production cycles, with harvesting for energy occurring 5-7 years after planting. After a cutting, the root system of each tree coppices, producing new stems. These trees typically remain productive for three harvest cycles before replanting, a period of 15-20 years. Yields range from 4 to 10 dry tons of wood/acre/year depending on the geographic region where they are being produced.
Willow, while a tree, exhibits shrubby growth patterns with multiple stems per plant. It is normally planted using cuttings or “whips.” Whips are unrooted cuttings taken from mature willow trees. Harvesting can begin fairly quickly after planting, with the first harvest occurring after three growing seasons. Harvesting is typically done with forage harvesters that are adapted using a special cutting head that produces small chips directly in the field. After harvest, the tree root systems coppice, growing new stems for biomass production. Repeated harvesting can occur for over twenty years before replanting is required. Yields range from 3-5 dry tons per acres per year. Historically, willow has been used for natural windbreaks and snow fencing in northern states. Expanding its uses has generated significant interest in breeding new varieties of willow that can yield much higher quantities of biomass than standard varieties. Some of these trials exhibit yield increases of over 30%!
Contact Genera Energy today to learn more about how short rotation hardwoods and other energy crops can make your project successful.
By Sam Jackson, Ph.D., VP of Business Development
