Variation in traits related to photosynthesis and water-use were examined within and between geographic sources (provenances) of black cottonwood in two range-wide common garden experiments in British Columbia, Canada. In the first experiment, CO2 assimilation (A), stomatal conductance (g(s)), instantaneous intrinsic water use efficiency (WUEi), stomatal density, specific leaf area, growth height, and foliar N were measured on five 2-year-old trees of 20 clones from five widely separated provenances (i.e., 4 clones per source). Leaf disks were analysed for stable carbon isotope composition (delta C-13) to provide a more long-term measure of WUE. Photosynthetic rate per unit leaf nitrogen was used as a measure of nitrogen use efficiency (NUE). A differed between (pa < 0.001), but not within provenances, and increased with latitude of origin (R-2 = 0.70). NUE and WUEi also varied between (p = 0.034 and p = 0.039, respectively), but not within provenances. In contrast, no variation among provenances was detected for delta C-13, but there were strong differences between clones within provenances (pa < 0.001). Variation in A was well correlated with foliar nitrogen, g(s), and stomatal density and adaxial:abaxial distribution ratio; hence, WUEi, delta C-13 and NUE were mostly unrelated to latitude or associated climate variables. Species-wide patterns in stomatal density and distribution were confirmed in the second experiment which utilized 140 clones. Stomatal density on the adaxial (but not the abaxial) leaf surface was strongly correlated with latitude (pa < 0.001). We speculate that northern provenances may have inherently higher A and g(s) to compensate for shorter growing seasons.
