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Carbon and nitrogen dynamics in a Pinus densiflora forest with low and high stand densities

Understanding carbon (C) and nitrogen (N) dynamics and their dependence on the stand density of an even-aged, mature forest provides knowledge that is important for forest management. This study investigated the differences in ecosystem total C and N storage and flux between a low-density stand (LD) and a high-density stand (HD) and examined the effects of stand density on aboveground net primary productivity (ANPP), total belowground C allocation (TBCA ) and net ecosystem production (NEP) in a naturally regenerated, 65- to 75-year-old Pinus densiflora S. et Z. forest.
LD (450 trees ha−1) and HD (842 trees ha−1) were established in an even-aged, mature P. densiflora forest in September 2006. The forest had been naturally regenerated following harvesting, and the stand density was naturally maintained without any artificial management such as thinning. The diameter at breast height (DBH ≥ 5.0 cm) of all live stems within the stands was measured yearly from 2007 to 2011. To compare C and N storage and fluxes in LD and HD, C and N pools in aboveground and belowground biomass, the forest floor, coarse woody debris (CWD) and soil; soil CO2 efflux (RS); autotrophic respiration (RA); litter production; and soil N availability were measured. Further, ANPP, TBCA and NEP were estimated from plot-based measurement data.
Important Findings
Ecosystem C (Mg C ha−1) and N (Mg N ha−1) storage was, respectively, 173.0 ± 7.3 (mean ± SE) and 4.69 ± 0.30 for LD and 162 ± 11.8 and 4.08 ± 0.18 for HD. There were no significant differences in C and N storage in the ecosystem components, except for soils, between the two stands. In contrast, there were significant differences in aboveground ANPP and TBCA between the two stands (P < 0.05). Litterfall, biomass increment and RS were major C flux components with values of, respectively, 3.89, 3.74 and 9.07 Mg C ha−1 year−1 in LD and 3.15, 2.94 and 7.06 Mg C ha−1 year−1 in HD. Biometric-based NEP (Mg C ha−1 year−1) was 4.18 in LD and 5.50 in HD. Although the even-aged, mature P. densiflora forest had similar C and N allocation patterns, it showed different C and N dynamics depending on stand density. The results of the current study will be useful for elucidating the effects of stand density on C and N storage and fluxes, which are important issues in managing natural mature forest ecosystems. 

Keywords: aboveground net primary productivity, net ecosystem production, Japanese red pine, stand density, total belowground carbon allocation
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