Variation in leaf physiology of Salix arctica within and across ecosystems in the High Arctic: test of a dual Delta C-13 and Delta O-18) conceptual model

Leaf carbon isotope discrimination (Delta C-13) varies with the balance between net photosynthesis (A) and stomatal conductance (g(s)). Inferences that can be made with Delta C-13 are limited, as changes could reflect variation in A and/or g (s) . Investigators have suggested that leaf delta O-18 enrichment above source water (Delta O-18) may enable differentiation between sources of variation in Delta C-13, as leaf Delta O-18 varies with transpiration rate (E), which is closely correlated with g (s) when leaves experience similar leaf to air vapor pressure differences. We examined leaf gas exchange of Salix arctica at eight sites with similar air temperatures and relative humidities but divergent soil temperatures and soil water contents near Pituffik, Greenland (76 degrees N, 38 degrees W). We found negative correlations at the site level between g (s) and Delta O-18 in bulk leaf tissue (r(2) = 0.62, slope = -17.9 degrees/mol H2O m(-2) s(-1), P = 0.02) and leaf alpha-cellulose (r(2) = 0.83, slope = -11.5 parts per thousand mol H2O m(-2) s(-1), P < 0.01), consistent with the notion that leaf water enrichment declines with increasing E. We also found negative correlations at the site-level between intrinsic water-use efficiency (iWUE) and Delta C-13 in bulk leaf tissue (r(2) = 0.65, slope = -0.08 C degrees/mu mol CO2 /mol H2O, P = 0.02) and leaf alpha-cellulose (r(2) = 0.50, slope = -0.05 parts per thousand/[mu mol CO2 /mol H2O], P = 0.05). When increasing Delta C-13 was driven by increasing g (s) alone, we found negative slopes between Delta C-13 and Delta O-18 for bulk leaf tissue (-0.664) and leaf alpha-cellulose (-1.135). When both g (s) and A (max) increased, we found steeper negative slopes between Delta C-13 and Delta O-18 for bulk leaf tissue (-2.307) and leaf alpha-cellulose (-1.296). Our results suggest that the dual isotope approach is capable of revealing the qualitative contributions of g (s) and A (max) to Delta C-13 at the site level. In our study, bulk leaf tissue was a better medium than leaf alpha-cellulose for application of the dual isotope approach.