Growth and Production

Poplar Grown as a Crop for Bioenergy

Hybrid poplar can be grown for bioenergy as a short-rotation woody crop (SRWC) on a succession of coppice production cycles. Coppicing is a silvicultural method in which the poplars are cut near the ground at harvest. In the spring, new stems sprout from the root stock to regenerate the stand. Coppicing works well for bioenergy systems where the goal is to maximize the biomass, rather than produce tall straight trees for sawn wood products. Poplar grown as a bioenergy crop is harvested every three years using a single-pass system that cuts and chips the trees as the harvester moves down the crop row. The trees will re-sprout and will be harvested again on a continuous cycle that can be repeated every three years, six or seven times before the grower would need to replant the trees. The poplar rotation thus lasts for 18-21 years.

The poplar bioenergy cropping system is very different from the poplar systems that were developed over the past several decades for pulp and paper, veneer, and sawn wood markets. Poplar plantations for these other markets are grown on 8 to 15 year rotations, harvested using conventional logging equipment and regenerated by replanting after harvest. In contrast, the poplars being developed for biofuel production will be grown as a woody perennial agricultural crop on three-year coppice production cycles.

In the Pacific Northwest, hybrid poplars will typically grow 5-12 feet per year depending on the quality of the site and management intensity. Biomass yields will likely range from 5-9 bone dry tons (BDT) per acre per year. The growth and yield of poplars are influenced by multiple environmental conditions including climate, nutrient and water availability, and weed competition. Ongoing research will improve poplar growth and production systems for bioenergy feedstocks.

At AHB’s “Poplar for Biofuels” demonstration sites, the hybrid poplars were planted as cuttings in the spring of 2012 and 2013 at a density of 1452 trees per acre with 10 feet between the crop rows and 3 feet separating the trees within the rows. Crop care includes weed management and monitoring for pests and disease. After the first two growing seasons, each site was harvested, producing promising yields of biomass. Additional harvests will occur on two or three year cycles.