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

1. Post-disturbance losses in nitrogen (N) may diminish forest productivity, and soils with inherently ‘open’ N cycles (high nitrification rates) are considered the most vulnerable to leaching losses of NO3-.  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 fertility among study sites.  In addition, a consensus on post-disturbance N trajectories is hampered by the sometimes inconsistent patterns in tree-ring δ15N found between tree species of differing mycorrhizal association.

2. 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 fertility across the six sites, driven largely by topography and parent materials, was confirmed by an in situ increase in N mineralization and nitrification rates with declining C:N ratios for both organic horizons and mineral soils. 

3. Five decades after logging, the overall trend in tree-ring δ15N was positive, but among individual plots there was a wide range in δ15N slopes, ranging from nearly 0 to 0.13.  We found the gains in tree-ring δ15N over time were consistent between mycorrhizal types and escalated sharply (up to 6‰) with increasing N mineralization rates, although less so on flat terrain with seasonal water tables.  The most recent sapwood was also enriched in 15N as soil N mineralization rates increased, perhaps slightly more so for T. plicata than P. sitchensis. 

4. Synthesis. The correspondence of tree-ring δ15N with soil fertility may be especially strong in regenerating forests because of tree ontogeny effects, including the expansion of rooting depth and differences in N resorption efficiency with stand age.  Sharp increases in tree-ring d15N underscore the vulnerability of low C:N soils with open N cycles to post-disturbance N losses, and highlight how repeated, frequent harvest cycles may risk substantial N depletion from these productive rainforest ecosystems.