Leaf-level resource use for evergreen and deciduous conifers along a resource availability gradient

1. We compared leaf-level carbon, nitrogen and water use for a deciduous (Larix occidentalis Nutt.) and sympatric evergreen (Pseudotsuga menziesii, Beissn., Franco, or Pinus contorta Engelm.) conifer along a resource availability gradient spanning the natural range of L. occidentalis in western Montana, USA. 2. We hypothesized that leaf photosynthesis (A), respiration (r), specific leaf area (SLA) and foliar nitrogen concentration (N) would be higher for deciduous than sympatric evergreen conifers in mixed stands, and that these interspecies differences would increase from high to low resource availability. We also hypothesized that leaf-level nitrogen and water-use efficiency would be higher for the co-occurring evergreen conifer than L. occidentalis. 3. In general, mass-based photosynthesis (A(m)) was significantly higher for L. occidentalis than co-occurring evergreen conifers in the drier sites, but A(m) was similar for evergreen and deciduous conifers at the mesic site. 4. Mass-based foliar nitrogen concentration (N(m)) was positively correlated to SLA for all species combined across the gradient (R² = 0.64), but the relationship was very weak (R² = 0·08-0·34) for evergreen and deciduous species separately. Mass-based A(m) and r(m) were poorly correlated to N(m) for all species combined across the gradient (R² = 0·28 and 0·04, respectively). 5. For each site-species combination, daily maximum A(m) was negatively correlated to vapour pressure deficit (VPD) (R² = 0·36-0·59), but was poorly correlated to twig predawn water potential (R² < 0·04). 6. Instantaneous nitrogen-use efficiency (NUE(i); A(m) divided by N(m)) and water-use efficiency (δ¹³C) increased significantly (P = 0·05) from high to low resource availability for both evergreen and deciduous conifers, except for NUE(i) in L. occidentalis.