Supplementary data are available online at: www.aob.oxfordjournals.org and consist of the following. Table S1: soil physico-chemical properties during the second growing season in the eight plantation sites; Figure S1: total height growth of two white spruce seed sources at the end of the second growing season (H2) plotted against site mean July temperature (MJT), soil total nitrogen and soil C:N ratio. Figure S2: Nmass and SLA of plants of two white spruce seed sources growing at eight plantation sites; Figure S3: Aopt and Rd10 plotted against Nmass for the two white spruce seed sources; raw data on An response to Ci and temperature.