| As one of the most important terrestrial ecosystems, forest ecosystem had a decisive function in balancing the global carbon budget and mitigating negative impact of global climate change. Subalpine forest region of western Sichuan, which was located in the Southeast edge of the Tibetan Plateau, played an irreplaceable role in maintaining the ecological balance and regulating regional water-cycle and carbon-cycle in upper Yangtze River. Therefore, the study on the aboveground biomass of regional main forest types would help to enhance the understanding of matter cycle and energy flowing of forest ecosystem. The former researches of forest biomass in this region was mainly focused on the artificial forest and old growth forest, but lack of corresponding datas on various kinds of natural forests and comparative analysis among different recovery approaches.On basis of typical community plot investigation and dendrochronology, stand ecological characteristics and biomass pattern changes for main four forest types (Natural birch forest (NBF), Natural coniferous-broadleaved mixed forest(NMF), Spruce artificial forest(SAF) and Old growth forest(OF)) were studied, so as to provide guidance for evaluation on carbon budget and regional ecosystem management. The major results were summarized as follows:(1) Effect of age and elevation on the aboveground biomass in subalpine forests, western SichuanAge effect: In the process of natural recovery succession, mean aboveground biomass (MAB) of tree layer and individual for Natural birch forest(NBF) increased significantly (P < 0.01)with the stand age increased; mean aboveground NPP(MANPP) reached its peak value (7.88 t·hm-2·a-1) at 30 a and then decreased later. Biomass of branch and leaf took up a greater proportion (55%) at 20 a in Natural birch forest, while stem account for 55% of total tree layer biomass at 30 a, proportion of stem decreased after 30 a. Both stand stock of tree layer and individual volume went up with the increasing stand age, and so it is for mean annual increment and current annual increment, which reached their maximum at 50 a.Elevation effect: Mean aboveground biomass (MAB) of Natural coniferous-broadleaved forest (NMF) in Western Sichuan subalpine region declined with increasing elevation, and those of coniferous species and broadleaved species showed a similar falling trend. Regression analysis showed that the relationship between biomass and altitudinal gradient for community and broadleaved species reached significant level (P<0.01). Mean aboveground NPP (MANPP) for tree layer and individual tree decreased with increasing elevation (tree layer reached the significant level). In terms of biomass distribution among different organs, the proportion for stem and branch was dropped, compared with the obvious upward trend in leaf. Mean aboveground biomass and NPP of individual tree for broadleaved species significantly went down (P <0.01) with the increasing altitudinal gradient, however, the corresponding values for coniferous species was went up with the increasing elevation without reaching the significant level (P > 0.05). Coniferous species had a higher MAB and MANPP than those of broadleaved species within the same gradient when the elevation exceeded 3300 m. The response of broadleaved species to altitudinal gradient was more sensitive than that of coniferous species.(2) Comparative analysis among different forest types The analysis of dendrochronology for four forest types showed that: Diameter classstructure for NBF and NMF at 40 years old displayed an inverted J shape, while that for SAF showed a normal distribution shape. Distribution features of age class structure for NBF was confirming with normal distribution, however, most individuals for NMF and SAF were distributed in few special age classes. NMF had been keeping its highest MAB and stand volume in the whole recovery process, and NBF had a comparatively lower MAB, compared with that for SAF after 30 years. Before 20 years , the stand volume of NBF was higher than that of SAF; after 20 years, in contrast, SAF had an obviously higher volume value. MANPP for all three forest types went up with the increasing stand age and displayed S shape fluctuation. Compared with Afforestation (SAF), the MANPP of Natural recovery types (NBF and NMF) had a higher fluctuation after 20 years. The size order of MANPP for three forest types before 30 years is NMF > NBF > SAF; after 30 years, the order changed to NMF > SAF > NBF.Old growth forest showed the widest span in the diameter class and age class, which distributed from 5—10 cm to > 75cm and 20—40 a to 300 a, respectively, and most of the individuals were distributed in 100 years age class. Mean aboveground biomass and stand volume increased with a J shape with the increasing stand age, and the MANPP showed a S shape increasing curve.(3) Relationship between forest aboveground NPP and environmental factors.Correlation analysis showed that: In subalpine region of Western Sichuan, temperature, especially the monthly mean minimum temperature, was the main limiting factor in the process of stand development, and showed strongly positive correlation. But the correlation between the precipitation and MANPP was not significant.
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