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

<p>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 N_2-fixing tree species are lacking. We examined whether tree-ring δ¹⁵N of N₂-fixing red alder (<em>Alnus rubra</em>) 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 (<em>Pseudotsuga menziesii</em>) in five proportions (0/100, 11/89, 25/75, 50/50, and 100/0, respectively). A<span>n 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 </span>δ¹⁵N<span> under denser red alder treatments. </span><span>Red alder had a consistent quadratic fit in tree-ring </span><span>δ</span>^{<span>15</span>}<span>N over time, with a net increase of </span><span>1.5</span><span>‰</span><span>, on average, before declining slightly. Douglas-fir tree-ring </span><span>δ</span>^{<span>15</span>}<span>N, in contrast, was largely unchanged over time (in 3 of 4 plots) but significantly enriched in the 50/50 mix. The minor differences in current litter N content and leaf </span><span>δ</span><span>15</span><span>N between alder and Douglas-fir suggests the declining trend in alder tree-ring </span><span>δ</span>^{<span>15</span>}<span>N could coincide with lower N-fixation rates, either by down-regulation via nitrate availability or loss in alder vigour with shading and drought. We suggest tree-ring </span><span>δ</span>^{<span>15</span>}<span>N can provide insights into the abiotic constraints and facultative/obligate nature of N fixation for N₂-fixing trees.</span></p>