Impact of fertilization on tree-ring δ15N and δ13C in beech stands: A retrospective analysis

Abstract

We studied the effects of two fertilization treatments (N and NPKCa) on wood nitrogen (N) isotope composition (δ15N), water-use efficiency (WUE) estimated by carbon isotope composition (δ13C) analyses, and ring width of trees in 80-year-old beech (Fagus sylvatica L.) stands in the forest of Fougères, western France. Four replicates were fertilized in two successive years (1973 and 1974), 20 years before core sampling. Unfertilized control trees displayed a decreasing δ 15N trend with time. The N and NPKCa treatments both increased δ15N compared with the control treatment. Wood extraction by organic solvents enhanced the δ15N signal. Thus, N addition to the beech ecosystem, even in moderate amounts, could be traced back in tree-ring δ15N, suggesting that wood N isotope analysis is a promising tool for studying the long-term effects of N deposition on forests. Although WUE decreased for about 6 years after N fertilization, WUE in NPKCa-treated trees did not differ significantly from that in control trees. Results were similar whether based on cellulose or total wood δ13C analysis, suggesting that extraction of cellulose is not necessary when studying fertilization impacts on WUE. The NPKCa treatment had a large impact on radial growth, causing a significant long-lasting increase of 29% compared with the control treatment. Nitrogen alone did not change radial growth significantly. © 2005 Heron Publishing.