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

Abstract

Abstract1. 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 δ15N can be related to site histories (site disturbance, soil N dynamics and plant development), we measured soil and foliar δ15N 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 δ15N 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 δ15N (5·5 and 4·3‰ for 10‐year‐old Redcedar and Salal, respectively), and there was no relationship between foliar δ15N and measured rooting depth, demonstrating that rooting depths cannot be used to explain foliar δ15N variation among coexisting woody taxa.5. Foliar and soil δ15N declined with site age and with a presumed change from ‘open’ to ‘closed’ N cycling; the 15N‐depleting effects of mycorrhizal N transformations contributed to the observed δ15N decline.