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.
Background: Prescribed burning is a common practice of site preparation before afforestation in subtropical forests. However, the effects of prescribed burning on carbon (C) dynamics of an ecosystem are poorly understood. Therefore, a Eucalyptus urophylla plantation (EU) and a naturally recovered shrubland (NS), each treated with prescribed burning and no burning were examined in subtropical China. Methods: Biomass of trees and shrubs in the 1st, 3nd, 4th, and 6th year after treatments were estimated by quadrat survey and allometric equations. Biomass of herbs and forest floors were estimated by harvest method. Plant biomass C storage was calculated by plant biomass multiplying by its C concentration. Soil organic C (SOC) storage in the 6th year after treatments was estimated by SOC concentration multiplying by soil bulk density and soil volumes. Results: Tree biomass C storage was significantly higher in the burned EU (BEU) than in the unburned EU (UEU) in the 1st year after treatments, yet the difference decreased over time. Conversely, tree biomass C storage was lower in the burned NS (BNS) than in the unburned NS (UNS), although the difference was not significant. However, in the 6th year after treatments, the total plant biomass C storage was 14.56% higher in the BEU than that in the UEU, and 59.93% higher in the BNS than that in the UNS, respectively, although the significant difference was only found between UNS and BNS. In addition, neither SOC storage at 0-20 cm nor ecosystem C storage in either the EU or NS was significantly affected by prescribed burning. Conclusions: Prescribed burning has little impact on overall C storage of forest ecosystems, we consider that prescribed burning may be an option for forest site preparation regarding plant biomass C accumulation.
