Growth effort and soil nitrogen availability drive age-related patterns in nitrogen resorption in a Larix Principis-rupprechtii plantation
Nitrogen (N) resorption as a key strategy for conserving N in forests, and is associated with stand development. However, our understanding of the age-related pattern of N resorption, and what drives the pattern in temperate forests, is limited. Here, we evaluated green- and senesced-leaf N concentrations and the nitrogen resorption efficiency (NRE) of Larix Principis-rupprechtii forest stands in different age classes (2, 11, 20, 45, and 100 years old) in a plantation in north China. We also examined the effects of the annual N requirement for tree growth and soil dissolved N concentration on N resorption.
We found a logarithmic increase in NRE and green leaf N concentration, and a logarithmic decrease in senesced-leaf N concentration along the stand-age chronosequence. Based on the study of 11-, 20-, and 45-year-old stands, the annual N requirement for tree growth positively affected N resorption, and the soil dissolved N concentration negatively affected NRE in a forest at the three stand ages. In addition, a 4-year exogenous N input experiment showed that the mean NREs in the 11- and 20-year-old stands decreased following a 50 kg N ha-1 yr-1 input. However, a similar response of NRE was not observed in the 45-year-old stand. These results not only imply that the trees could adjust their N resorption efficiency with stand development but also indicate that ignoring the age-related pattern of N resorption can lead to a bias in N cycle models when evaluating forest net primary production under increasing global N deposition.