N concentration of old leaves and twigs is more sensitive to stand density than that of young ones in Chinese fir plantations: a case study in subtropical China

Young leaves are conventionally used in the analysis to study the nutrient status of evergreen plants and their responses to environmental changes, but the role of old leaves remains poorly understood. We selected two stand types in 31-year-old Chinese fir (Cunninghamia lanceolata) plantations with similar soil conditions but different stand densities, to test the hypothesis that nitrogen (N) concentration of old leaves and twigs is more sensitive to stand density than that of young ones. Leaves and twigs were sampled and sorted into young (one-year-old) and old (two- and three-year-old) groups. Significant differences in N concentration and carbon: nitrogen ratio between the low-density stand and high-density stand were only found in the old leaves and twigs but not in the young ones. Although the N resorption efficiency did not vary significantly with stand density, the annual N resorption rates were increased in old leaves and relatively young twigs at high stand density. These results show the potential use of old tissues in the nutrient analysis in Chinese fir plantations. Testing the generality of these results could improve the use of foliar analysis as an indicator of nutrient status and environmental changes in evergreen tree species.