Post‐disturbance conifer tree‐ring δ15N reflects openness of the nitrogen cycle across temperate coastal rainforests

Abstract

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 δ15N 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 δ15N 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 δ15N found between tree species of differing mycorrhizal association. We compared tree‐ring δ15N 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 δ15N was positive, indicating a loss of nitrate from the system, but among individual plots the slope of δ15N ranged from nearly 0 to 0.13. We found the gains in tree‐ring δ15N 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 15N on soils with higher N mineralization rates, perhaps slightly more so for T. plicata than P. sitchensis. Synthesis. The alignment of tree‐ring δ15N 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 δ15N 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.