Yield And Biomass Quality Of Shrub Willow Hybrids In Differing Rotation Lengths And Spacing Designs

To study the effects of planting design and harvest rotation on shrub willow bioenergy crops, 10 shrub willow genotypes were established in 2009 in side-by-side randomized complete block trials, one with a double-row design and the other a single-row design. Both double-row and single-row designs were established with the same stand density, similar to 14,000 plants ha(-1). The single-row design was harvested four times on a 2-year rotation from 2009 to 2017. The double-row plots were harvested twice on a 4-year rotation. Biomass yield, growth parameters, wood composition, specific gravity, and calorific values were evaluated. Mean annualized yield over 8 years from the single-row design was 9.98 Mg ha(-1) yr(-1) whereas mean annualized yield for the double-row spacing design was 9.32 dry Mg ha(-1) yr(-1). Cumulative biomass production averaged across all genotypes from four biennial, single-row harvests resulted in 79.9 Mg ha(-1) compared with two quadrennial double-row harvests of 74.5 Mg ha(-1). A triploid hybrid, LA970253, produced the greatest mean annual yield in the single-row design of 14.0 dry Mg ha(-1) yr(-1), followed by another triploid, 'Fabius', which had the greatest yield in the double-row design of 12.5 dry Mg ha(-1) yr(-1). In both planting designs, hemicellulose and cellulose content increased over time, while lignin and ash content decreased. Calorific values ranged from 17.62 to 19.02 MJ kg(-1) with no significant differences between designs or genotypes. The results from this study suggest that some genotypes respond better to shorter rotations, but over multiple rotations there is no difference in total yield.