Effects of climate, tree age, dominance and growth on δ15N in young pinewoods

Needles, annual rings from basal stem discs and bark of three dominant and three suppressed Pinus pinaster from a 12-year-old pine stand (naturally regenerated after a wildfire) were analysed to study the effects of climate, tree age, dominance, and growth on tree delta N-15. Foliar-N concentration in dominant pines (0.780-1.474% N) suggested that soil N availability was sufficient, a circumstance that allowed isotopic discrimination by plants and (greater) differences in delta N-15 among trees. The delta N-15 decreases in the order wood (-0.20 to +6.12aEuro degrees), bark (-1.84 to +1.85aEuro degrees) and needles (-2.13 to +0.77aEuro degrees). In all trees, before dominance establishment (years 1-8), the N stored in each ring displayed a decreasing delta N-15 tendency as the tree grows, which is mainly due to a more closed N cycle or an increasing importance of N sources with lower delta N-15. After dominance establishment (years 9-12), wood delta N-15 values were higher in suppressed than in dominant trees (2.62 and 1.46aEuro degrees, respectively; P < 0.01) while the reverse was true for needles and bark; simultaneously, the absolute amount of N stored by suppressed pines in successive rings decreased, suggesting a lower soil N assimilation. These results could be explained by lignification acting as major N source for needles in suppressed pines because products released and reallocated during lignification are N-15-depleted compared with the source. According to principal component analysis, wood delta N-15 appears associated with wood N concentration and precipitation during the growing season, but clearly opposed to age, basal area increment and mean temperature in spring and summer.