Shrub Willow (Salix) Biomass Crop Performance on Five Sites Over Two Rotations in Michigan, USA and the Implications of Adequate Field Testing to Commercial Producers

Abstract

Fifteen varieties of willow (Salix) hybrids were observed in a replicated study on five diverse sites in Michigan during the establishment year and over two subsequent 3-year rotations. Sixty-one percent of the total variation in yield observed was due to environmental factors, 11% was due to genetic factors, and the remainder was unexplained. Biomass yield over 6 years ranged from 50.5 oven-dry Mg ha−1 at one site to 22.9 oven-dry Mg ha−1 at another. Warmer and wetter sites tended to produce more biomass than colder drier sites, but correlations between yield and other edaphic and climatic factors were less clear. High-yielding varieties tended to be taller, but survival and number of stems per stool were uncorrelated with yield. A cohort of elite varieties selected based on test-wide performance produced up to 26% more biomass than randomly chosen varieties. Cohorts of elite varieties selected based on performance in local tests did better, producing up to 31% more biomass than randomly chosen varieties. Because of ranking changes, selections made after two rotations outperformed those made after only one rotation by as much as 9%. Adequately tested planting stock has the potential to increase the financial return to a willow energy farmer by nearly $100 ha−1 year−1. This will multiply rapidly as willow is planted on some of the 700 million hectares of retired cropland in the USA. The nominal cost of breeding and field testing willow energy crops can be easily justified as we proceed to the envisioned billion-ton bioeconomy.