Foliar δ N variations with stand ages in temperate secondary forest ecosystems, Northeast China.

Natural abundance of 15N changes strongly with gradients in soil or environmental conditions across large spatial and temporal scales, but variation in δ15N with stand age at intermediate scales is poorly understood. We analyzed soil and foliar δ15N along a secondary forest chronosequence in Northeast China in mid-growing seasons 2008 and 2009, to address two questions: how does foliar δ15N vary with stand age; and are the variations driven by compositional differences in species among stands, or by consistent changes in δ15N with plant growth forms, and within species. The results showed that community-level foliar δ15N depleted as stand age increased, and these responses were remarkably consistent within three contrasting plant growth forms (herbs, shrubs, and trees), and within individual species. In spite of the three plant growth forms sharing similar responses to the stand age, tree species showed smaller variations in foliar δ15N along stand ages than herbs and shrubs. Soil δ15N also significantly depleted with increasing stand age, which may drive the variations of foliar δ15N. In addition, mycorrhizal fungi discrimination against 15N may also partly affect the patterns of foliar δ15N along stand ages. The results clearly indicate that differences in foliar δ15N among different stand ages are dominantly driven by the δ15N variations at the species levels, which reflects the variations of soil δ15N and mycorrhizal association intensity and association type, not by compositional difference in species among stands. [ABSTRACT FROM AUTHOR]