Gonzalez-Garcia, S., Mola-Yudego, B., & Murphy, R. J. (2013). Life cycle assessment of potential energy uses for short rotation willow biomass in Sweden. Int J Life Cycle Assessment, 18(4), 783-795. doi:10.1007/s11367-012-0536-2
8. Serapiglia, M.J., K. Cameron, A.J. Stipanovic, L.P. Abrahamson, T.A. Volk and L.B. Smart. 2013. Yield and Woody Biomass Traits of Novel Shrub Willow Hybrids at Two Contrasting Sites. Bioenergy Research 6:533-546
23. Volk, T.A., L.P. Abrahamson, K.D. Cameron, P. Castellano, T. Corbin, E. Fabio, G. Johnson, Y. Kuzovkina-Eischen, M. Labrecque, R. Miller, D. Sidders, L.B. Smart, K. Staver, G.R. Stanosz, and K. Van Rees. 2011. Yields of biomass crops across a range of sites in North Amercia. Aspects of Applied Biology 112:67-74.
This recent thesis also has a good summary of yields across a range of site in the U.S.
Liu, B. 2013. Biomass production of willow short-rotation coppice across sites and determinants of yields for SV1 and SX61. SUNY ESF, Syracuse, NY - http://search.proquest.com/pqdtlocal1005759/docview/1518531991/fulltextPDF/A9CFE048617149FDPQ/1?accountid=26746