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 tri...