Method to estimate Net Ecosystem Production of forest
|発表者||Tran Do Van （京都大学生存圏研究所・ミッション専攻研究員）|
英語スピーチ・Web配信（ZOOM Meeting ID: 932-778-813）
Net Ecosystem Production (NEP), carbon sequestration or carbon accumulation is a fundamental property of ecosystems. It was originally defined as the difference between the amount of organic carbon fixed by photosynthesis in an ecosystem and total ecosystem respiration. Based on this definition (Fig. 1), NEP represents the organic carbon available for storage within the system. In other ways, NEP is known as the rate of carbon accumulation in forest ecosystem.
Study was conducted in a tropical evergreen broadleaved forests of Copia Natural Reserve, Northwest Vietnam at 21°23′N and 103°38′E.
On the site, a plot of 30 m × 30 m was established in old-growth forest for NEP estimation. The NEP or rate of carbon accumulation in a forest ecosystem is estimated as NEP = ΔM + ΔCr + Lf + Fp − Rs, where ΔM is aboveground biomass increment, ΔCr is coarse root increment, Lf is aboveground litterfall, Fp is fine root production, and Rs is heterotrophic respiration (soil respiration). Fig. 1. Forest carbon cycle (estimating Net Ecosystem Production)
ΔM was estimated basing on measuring diameter at breast height (DBH) of all living stems at time ti and tj (tj > ti) (Fig. 2a), and applying allometry for AGB (aboveground biomass) in Eq. 1 ; AGB=ρ * exp[−1.499+2.148ln(DBH)+0.207(ln(DBH))2−0.0281(ln(DBH))3] (1) with ρ is wood specific gravity. ΔCr was estimated basing on alloemtry between CRB (coarse root biomass; root with φ > 2 mm) and AGB as CRB = 0.489AGB0.890  (Fig. 2a). Lf was estimated basing on litter trap technique, which was set up systematically under forest canopy (Fig. 2b). Fp was estimated basing on continuous inflow method using sequence soil core sampling and litter bag technique  (Fig. 2c). Rs was estimated basing on a closed chamber method  (Fig. 2d).
ΔM accounted for 57 % NPP, reducing to 30 % for Lf, 8.4 % for ΔCr, and Fp accounted for only 5 % (Table 1). Total carbon from NPP was 3.63 g m−2 day−2. Meanwhile, soil respiration was 1.8 g m−2 day−2 leading to high NEP of 1.83 g m−2 day−2, equaling to 6.6 Mg ha−1 year−1. This value was much higher than that in other forest around the world. The study forest was classified as young forest recovery from shifting cultivation, which had high ΔM while low soil respiration. Yung forests are more valuable in sequestration carbon compared to old-growth forest.
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