Vertical canopy gradients in δ13C correspond with leaf nitrogen content in a mixed-species conifer forest

Stable carbon isotope composition varies markedly between sun and shade leaves, with sun leaves being invariably more enriched (i.e., they contain more(13)C). Several hypotheses have emerged to explain this pattern, but controversy remains as to which mechanism is most general. We measured vertical gradients in stable carbon isotope composition (delta C-13) in more than 200 trees of nine conifer species growing in mixed-species forests in the Northern Rocky Mountains, USA. For all species except western larch, delta C-13 decreased from top to bottom of the canopy. We found that delta C-13 was strongly correlated with nitrogen per unit leaf area (N (area)), which is a measure of photosynthetic capacity. Usually weaker correlations were found between delta C-13 and leaf mass per area, nitrogen per unit leaf mass, height from the ground, or depth in the canopy, and these correlations were more variable between trees than for N (area). Gradients of delta C-13 (per meter canopy depth) were steeper in small trees than in tall trees, indicating that a recent explanation of delta C-13 gradients in terms of drought stress of upper canopy leaves is unlikely to apply in our study area. The strong relationship between N (area) and delta C-13 here reported is consistent with the general finding that leaves or species with higher photosynthetic capacity tend to maintain lower CO2 concentrations inside leaves. We conclude that photosynthetic capacity is a strong determinant of delta C-13 in vertical canopy profiles, and must be accounted for when interpreting delta C-13 values in conifer forests.