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The relatively high cost of willow (Salix spp.) biomass feedstock compared with fossil fuels makes it imperative to reduce production costs by increasing yields and reducing input costs. This can be addressed by harnessing the genetic potential of willow based on an understanding of the variation in key traits and how they interact to influence biomass production. Variation in 11 morphological traits among 30 willow clones at the end of a 3-year rotation was analyzed to determine its relationship to biomass production. We found significant differences (p ≤ 0.001) among the 30 clones in all traits. Across all clones, several significant trait correlations were detected. Cluster analysis revealed that high-yielding clones could be separated into two distinct "functional types" characterized by alternate growth strategies. One set of clones was characterized by a large number of small diameter stems, relatively low leaf area index and specific leaf area, but high foliar nitrogen and wood specific gravity. The other was characterized by a small number of large diameter stems, high leaf area index and specific leaf area, but low foliar nitrogen and wood specific gravity. The differences in strategies were apparent at the species level as well. These results suggest that multiple ideotypes need to be incorporated into willow breeding and selection efforts and in the management of willow biomass crops.

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