Post‐disturbance conifer tree‐ring δ ¹⁵ N reflects openness of the nitrogen cycle across temperate coastal rainforests

Abstract Post‐disturbance losses in nitrogen (N) may diminish forest productivity, and soils with inherently ‘open’ N cycles are considered the most vulnerable to leaching losses of . Monitoring ongoing N depletion from soil profiles is challenging, but tree‐ring δ 15 N of regenerating stands may offer an effective method for assessing site‐specific, long‐term soil N dynamics. Evidence to date is mixed, however, and includes increasing, unchanging or decreasing tree‐ring δ 15 N in young stands following stand‐level disturbances, possibly because of contrasting soil N availability among study sites. In addition, a consensus on post‐disturbance N trajectories is hampered by the inconsistent patterns in tree‐ring δ 15 N found between tree species of differing mycorrhizal association. We compared tree‐ring δ 15 N of two conifer species ( Picea sitchensis with ectomycorrhizal fungi and Thuja plicata with arbuscular mycorrhiza) from a replicated silviculture trial across temperate rainforests of Vancouver Island (Canada). A natural gradient in soil N status across the six sites, driven largely by topography and parent materials, was demonstrated by in situ increases in N mineralization and nitrification rates with declining C:N ratios for both organic horizons and mineral soils. Five decades after timber harvest, the overall trend in tree‐ring δ 15 N was positive, indicating a loss of nitrate from the system, but among individual plots the slope of δ 15 N ranged from nearly 0 to 0.13. We found the gains in tree‐ring δ 15 N with time were consistent between mycorrhizal types and escalated (up to 6‰) with increasing N mineralization, although less so on flat terrain with seasonal water tables. The most recent sapwood was also enriched in 15 N on soils with higher N mineralization rates, perhaps slightly more so for T . plicata than P . sitchensis . Synthesis . The alignment of tree‐ring δ 15 N with soil N cycles may be especially strong in regenerating forests because of ontogeny effects, including the expansion of rooting depth and increases in N resorption efficiency with stand age. Sharp increases in tree‐ring δ 15 N underscore the vulnerability of low C:N soils with open N cycles to post‐disturbance N losses, and highlight how multiple, frequent harvesting cycles may risk substantial N depletion from these productive rainforest ecosystems.

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