This Article Has been Published in BioEnergy Research and may be found at the following link http://link.springer.com/article/10.1007/s12155-015-9617-y Minor differences may exist between the drafts associated with the galley proof corrections 1 Bark content of two shrub willow cultivars grown at two sites and relationships with centroid bark content 1 and stem diameter 2 Spencer Eich 1 , Timothy A. Volk 1 , and Mark H. Eisenbies (corresponding author) 1 3 1 State University of New York, College of Environmental Science and Forestry. 1 Forestry Dr., Syracuse, NY 4 13210, USA 5 Contact information for corresponding author - mheisenb@esf.edu, Ph. 315-470-4850, Fax 315-470-6934 6 Abstract 7 Understanding the characteristics of short rotation woody crops (SRWC), like shrub willow, that affect 8 feedstock quality and its variability are a priority as this source of biomass is expanded. Because of its relationship 9 to ash content the allotment of bark and wood within the stem are often highlighted as a factor impacting feedstock 10 quality. Bark to wood ratios are frequently determined in willow by measuring this ratio for the centroid and 11 assuming it represents the entire stem. The objectives of this study were to determine how site and genetic factors 12 influenced the allometry of whole stem bark content on a dry weight basis (WSB%) and if the centroid bark 13 percentage on a dry weight basis (CB%) adequately represents WSB%. A multiple linear regression approach was 14 used to model WSB% and centroid bark content (CB%) using diameter, cultivar and site factors. Five candidate 15 models were evaluated ranging from parsimonious to complex. The simplest models estimated WSB% from CB% 16 (R 2 = 0.76) and stem diameter (R 2 = 0.44). The most complex model included all factors and had an R 2 of 0.90. Two 17 key relationships demonstrated by several models are that (1) CB% adequately predicts, but underestimates WSB%, 18 particularly for larger stems, and (2) WSB% increases as diameter decreases. However, empirical models can be 19 enhanced with the inclusion of diameter, site, and cultivar information. The overall approaches can be useful for 20 making relative comparisons between cultivars and sites. In a three year old stand of willow stems less than 20 mm 21 had high WSB%, the maximum being 23.8%, but accounted for only 5 to 15% of the total biomass. Large diameter 22 stems accounted for the majority of the total and bark biomass but the WSB% in large diameter stems is as low as 23 11.5%. WSB% ranged from 12.9 to 14% across the two cultivars at the stand level. 24 25 Keywords - Short Rotation Woody Crops; Shrub Willow; Bark Content; Feedstock Quality; Bark Allometry 26 27