Data for: Tree-ring 15N isotope of red alder

Nitrogen (N) accretion rates under N2-fixing tree species can vary with site condition and possibly decline over time with down-regulation of N fixation. Tree-ring δ15N may depict these site-specific, long-term patterns in N dynamics, but field trials with N2-fixing tree species are lacking. We examined whether tree-ring δ15N of N2-fixing red alder (Alnus rubra) would mirror N accretion rates and δ15N of soils. We sampled a 27-year-old replacement series trial on southeastern Vancouver Island with red alder and coastal Douglas-fir (Pseudotsuga menziesii) in five proportions (0/100, 11/89, 25/75, 50/50, and 100/0, respectively). An escalation in forest floor N content was evident with an increasing proportion of red alder, equivalent to a difference of approximately 750 kg N ha-1 between 100% Douglas-fir vs. 100% alder. The forest floor horizon was also enriched in δ15N under denser red alder treatments. Red alder had a consistent quadratic fit in tree-ring δ15N over time, with a net inc...