Forest ageing: An unexpected driver of beech leaf litter quality variability in European forests with strong consequences on soil processes

The role of forest age as a potential driver of intraspecific variation in leaf litter quality, that is a key plant trait determining ecosystem functioning, has largely been neglected. Using a set of fully replicated pure beech (Fagus sylvatica) forest stands differing in age (15, 65, 95 and 130 years), we quantified the forest stand age related variability of twelve leaf litter quality traits. Litter Mg, N and K showed significantly higher concentrations in litter from 15-yrs-old stands and decreased with increasing stand age. Mn was the only nutrient analyzed that was highest in the oldest stands. Hemicellulose and cellulose were lowest, and lignin and lignin/N ratio were highest in stands of intermediate age. The amount of N within the litter lignin fraction was highest in the 95-yrs-old stands (51% of total N) and lowest in the oldest stands (34% of total N). The amount of N associated within the hemicellulose fraction (<3%) showed the opposite pattern along the forest stand age gradient compared to lignin. Using Partial Least Squares regressions, we showed that litter N, C/N, lignin/N, K, Mn and Mg were the most important predictors of litter decomposition along the chronosequence. In contrast the proportions of C fractions and the amount of N within these C fractions were the most significant variables explaining the variation in final litter N content after one year of decomposition. N mineralization in ground litter was highly related to the proportion of total N within lignin and humus N mineralization was mostly explained by Mn and the lignin/N ratio. We showed that forest age is an important driver of litter quality variation and contributed considerably to the overall variation of F. sylvatica leaf litter quality traits observed from a reviewed data of published studies conducted at the continental scale. Furthermore, intraspecific litter quality variation greatly impacted belowground processes. Accounting for forest age related litter trait variation, and for the crucial role of the distribution of N within different litter C fractions, may improve the mechanistic understanding of ecosystem functioning. (C) 2013 Elsevier B.V. All rights reserved.