Post-disturbance conifer tree-ring δ15N 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 (Formula presented.). 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.