Altitude trends in conifer leaf morphology and stable carbon isotope composition

The natural ratio of stable carbon isotopes (δ¹³C) was compared to leaf structural and chemical characteristics in evergreen conifers in the north-central Rockies, United States. We sought a general model that would explain variation in δ¹³C across altitudinal gradients. Because variation in δ¹³C is attributed to the shifts between supply and demand for carbon dioxide within the leaf, we measured structural and chemical variables related to supply and demand. We measured stomatal density, which is related to CO₂ supply to the chloroplasts, and leaf nitrogen content, which is related to CO₂ demand. Leaf mass per area was measured as an intermediate between supply and demand. Models were tested on four evergreen conifers: Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii, and Pinus contorta, which were sampled across 1800 m of altitude. We found significant variation among species in the rate of δ¹³C increase with altitude, ranging from 0.91‰ km^-1 for A. lasiocarpa to 2.68‰ km^-1 for Pinus contorta. Leaf structure and chemistry also varied with altitude: stomatal density decreased, leaf mass per area increased, but leaf nitrogen content (per unit area) was constant. The regressions on altitude were particularly robust in Pinus contorta. Variables were derived to describe the balance between supply and demand; these variables were stomata per gram of nitrogen and stomata per gram of leaf mass. Both derived variables should be positively related to internal CO₂ supply and thus negatively related to δ¹³C. As expected, both derived variables were negatively correlated with δ¹³C. In fact, the regression on stomatal density per gram was the best fit in the study (r ²=0.72, P<0.0001); however, the relationships were species specific. The only general relationship observed was between δ¹³C and LMA: δ¹³C (‰)=-32.972+ 0.0173×LMA (r ²=0.45, P<0.0001). We conclude that species specificity of the isotopic shift indicates that evergreen conifers demonstrate varying degrees of functional plasticity across environmental gradients, while the observed convergence of δ¹³C with LMA suggests that internal resistance may be the key to understanding inter-specific isotopic variation across altitude.