Tree ring δ15N as validation of space-for-time substitution in disturbance studies of forest nitrogen status

Abstract

Forest ecosystem nitrogen (N) response to disturbance has often been examined by space-for-time substitution, but there are few objective tests of the possible variation in disturbance type and intensity across chronosequence sites. We hypothesized that tree ring δ15N, as a record of ecosystem N status, could validate chronosequence assumptions and provide isotopic evidence to corroborate N trends. To test this we measured soil N availability, soil δ15N, and foliar N attributes of overstory Douglas-fir (Pseudotsuga menziesii) and understory western hemlock (Tsuga heterophylla) across three old-growth stands and nine second-growth plantations on southeast Vancouver Island, British Columbia (Canada). Increment cores for wood δ15N were retrieved from three co-dominant Douglas-fir per plot. Bulk soil δ15N was well aligned with both foliar and recent wood δ15N, demonstrating the utility of wood δ15N in monitoring ecosystem N status. Strongly contrasting trends in tree ring δ15N were evident among second-growth stands, with most trees from plantations older than 50 years exhibiting steep declines (3–4‰) in δ15N but with no temporal trends detected for younger plantations. The discrepancy in tree ring δ15N suggests disturbance history varied considerably among second-growth sites, likely because of greater slash loads and hotter broadcast burns on older cutblocks. As a consequence, the pattern of increased soil N availability and foliar N concentration with time since disturbance derived from the chronosequence could not be validated. Tree ring δ15N may provide insights into disturbance intensity, especially fire, and correlations with foliar N concentration could inform the extent of changes in stand nutrition.