Site history affects soil and plant ¹⁵N natural abundances (δ¹⁵N) in forests of northern Vancouver Island, British Columbia

Abstract 1. About 10 years after establishment, plantations of Western Redcedar ( Thuja plicata Donn ex D. Don) on northern Vancouver Island, British Columbia become nutrient deficient and chlorotic, grow slowly, and are susceptible to invasion by the ericaceous shrub Salal ( Gaultheria shallon Pursh.). 2. To test the hypothesis that δ 15 N can be related to site histories (site disturbance, soil N dynamics and plant development), we measured soil and foliar δ 15 N in the summer of 1992 in 3‐year‐old (nutrient‐sufficient) and 10‐year‐old (nutrient‐deficient) plantations and in old‐growth stands. The foliar and soil δ 15 N values of the plantations and old‐growth forests were different and closely reflected site histories. Salal invasion and nutrient deficiency interacted to depress the growth of Redcedar in 10‐year‐old plantations. 3. Site preparation destroyed the top soil organic layers (fresh and decaying litter) and forced Salal (ecto‐ and ericoid mycorrhizal) into the humus layer, where it was in direct competition with Redcedar, thereby disadvantaging arbuscular mycorrhizal/non‐mycorrhizal Redcedar in its nutrient acquisition during a period when N and P are severely limited. 4. There was a large seasonal range of foliar δ 15 N (5·5 and 4·3‰ for 10‐year‐old Redcedar and Salal, respectively), and there was no relationship between foliar δ 15 N and measured rooting depth, demonstrating that rooting depths cannot be used to explain foliar δ 15 N variation among coexisting woody taxa. 5. Foliar and soil δ 15 N declined with site age and with a presumed change from ‘open’ to ‘closed’ N cycling; the 15 N‐depleting effects of mycorrhizal N transformations contributed to the observed δ 15 N decline.