Tree-ring δ¹⁵N as an indicator of nitrogen dynamics in stands with N₂-fixing Alnus rubra.

This article investigates whether tree-ring nitrogen isotope ratios (δ¹⁵N) in the N₂-fixing red alder (Alnus rubra Bong.) reflect long-term soil nitrogen (N) dynamics and whether fixed N influences neighboring coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) in mixed stands. Using a 27-year replacement series trial on Vancouver Island, British Columbia, the study found that forest floor N content and δ¹⁵N increased with higher red alder proportions, while red alder tree-ring δ¹⁵N showed a significant quadratic trend over time—rising for about 15 years before plateauing or declining—suggesting a peak and subsequent down-regulation of N fixation possibly due to shading or nitrate availability. Douglas-fir tree-ring δ¹⁵N remained mostly stable but was elevated in the 50/50 alder-fir mix, indicating some uptake of alder-derived N. The findings support the use of tree-ring δ¹⁵N as a tool to infer site-specific N dynamics under N₂-fixing trees, though the mixing of soil and fixed N sources complicates interpretations, and N accretion was moderate in this relatively dry environment.