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

1. About 10 years after establishment, plantations of Western Redcedar (Thu ja plicata Donn ex D. Don) on northern Vancouver Island, British Columbia b ecome nutrient deficient and chlorotic, grow slowly, and are susceptible to invasion by the ericaceous shrub Salal (Gaultherin shallon Pursh.). 2. To test the hypothesis that delta(15)N can be related to site histories (site disturbance, soil N dynamics and plant development), we measured soil and foliar delta(15)N in the summer of 1992 in 3-year-old (nutrient-suffic ient) and 10-year-old (nutrient-deficient) plantations and in old-growth st ands. The foliar and soil delta(15)N values of the plantations and old-grow th forests were different and closely reflected site histories. Salal invas ion 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 decayi ng litter) and forced Salal (ecto- and ericoid mycorrhizal) into the humus layer, where it was in direct competition with Redcedar, thereby disadvanta ging arbuscular mycorrhizal/ non-mycorrhizal Redcedar in its nutrient acqui sition during a period when N and P are severely limited. 4. There was a large seasonal range of foliar delta(15)N (5.5 and 4.3 parts per thousand for 10-year-old Redcedar and Salal, respectively), and there was no relationship between foliar delta(15)N and measured rooting depth, d emonstrating that rooting depths cannot be used to explain foliar delta(15) N variation among coexisting woody taxa. 5. Foliar and soil delta(15)N declined with site age and with a presumed ch ange from 'open' to 'closed' N cycling; the N-15-depleting effects of mycor rhizal N transformations contributed to the observed delta(15)N decline.