LAUSR

Net ecosystem CO2 exchange (NEE) over a 5-year-old rubber plantation was measured, using Eddy Covariance technique in 2014 to (1) quantify the seasonal distribution of NEE and its partitioning into daily gross primary production (GPP) and ecosystem respiration (Rd) and (2) examine how key environmental factors influence those carbon fluxes. We found that the daily amplitude of carbon fluxes varied substantially within the growing season, with the largest daily changes occurring in July. The daily maximum NEE generally occurred before noon, while maximum Rd occurred around sunset. The NEE peak of each growth stage was different. The seasonal variations of NEE, GPP and Rd, all of which were closely related to the rubber phenology, reached the peak value in July. Annual NEE, Reco and GPP were 715.21, 591.98, and 1298.93 g C m–2 y–1, respectively. The 5-year-old rubber plantation behaved as a carbon sink from the end of refoliation stage to the beginning of defoliation period, while it acted as a carbon source within 1–2 weeks. The response of daytime NEE to light (PAR, photosynthetic active radiation) showed that 37–69% of the variation in NEE was explained by the change in net solar radiation. The effect of PAR was dependent on growth stage of rubber plantation. The 3-light response parameters, maximum photosynthetic capacity (Pmax), ecosystem apparent quantum yield (α) and ecosystem respiration (Rd), varied with the growth stage, i.e., initial stage, refoliation, fully expanded leaf and defoliation. Range of Pmax, α, and Rd were −9.44 to −46.61 μmol CO2 m –2 s–1, −0.0041 to −0.0428 μmol CO2 μmol −1 photon, and 1.06–4.91 μmol CO2 m –2 s–1, respectively. The highest values of Pmax occurred in defoliation stage. Both of α and Rd were highest in fully expanded leaf stage. The impact of air temperature (Ta) and vapour pressure deficit (VPD) on NEE light response was studied. The magnitude of Pmax , α, and Rd decreased with increasing of Ta. Pmax increased with the increase in VPD, the maximum value of Pmax occurred at low level of VPD (VPD¶ 1 kPa). Values of α and Rd at medium level of VPD (1< VPD< 2) were the highest. Multiple linear regression and correlation methods were used to assess the relationship between NEE and environmental factors. The environmental factors controlling NEE were different depending on growth stage.