| CO2 fluxes of Phyllostachys edulis forest ecosystem from October in 2014 to September in 2015 in Anji, Zhejiang province were measured by the eddy covariance micrometeorological method and the eco-physiological method, to explore and definite the CO2 dynamic variations at daily, monthly, seasonal and annual scales, as well as the carbon budget of this ecosystem. Meanwhile, the results based on the eco-physiological method explained the change of CO2 fluxes based on the eddy covariance method, providing a physiological ecology basis for revealing CO2 exchange of the forest ecosystem. Besides, this paper analyzed the effects of environmental factors(such as light, temperature, humidity and else) on ecosystem CO2 fluxes, further to reveal the regulatory mechanism of carbon budget dynamics of forest ecosystem. What’s more, using the eco-physiological method and the eddy covariance method also could provide a certain theory and practice for evaluating carbon sources / sinks of different scales, even the entire forest ecosystem. In a word, attention to the carbon absorption of forest ecosystem under the background of global warming contributes to indicate a certain direction to ease the global greenhouse effect, and also has great significance for the sustainable and stable development of China’s economic and other aspects.We researched on the CO2 fluxes and environmental factors of Phyllostachys edulis forest ecosystem, the results showed that:(1) Annual average photosynthetic active radiation(PAR) was 527.61 μmol·m-2·s-1. The PAR generally showed a double peak curve in this year. The lowest average was in January, only 365.23 μmol·m-2·s-1, and the highest was in August, which was 672.18 μmol·m-2·s-1. Annual air temperature(Ta) showed an obvious single peak curve, and the annual average Ta was 14.98 ℃. In December, the average Ta was the lowest, only 4.22 ℃, and the highest was in August, which reached 25.62 ℃. Annual average relative humidity(RH) was about 76.30%, the lowest was in December, only 47.66%, and the highest was in July, reaching 94.91%. Affected by RH and Ta, the VPD regularity was poor. Annual average VPD was 0.44 kPa, the maximum value was 0.78 kPa in April, and the minimum value of 0.27 kPa was in July.(2) Average diurnal variation regularity of monthly net ecosystem exchange(NEE) during the whole year was strong, which showed a "U" or "V" type curve. The maximum value(negative) of NEE occurred at around 12:00. NEE were positive in the night and before dawn, but negative in the daytime. And NEE in summer was much higher than that in winter numerically. In addition, monthly mean NEE were negative, which suggested that this Phyllostachys edulis forest ecosystem was a carbon sink. The maximum value 0.163 mg·m-2·s-1 was in July, and the lowest value 0.039 mg·m-2·s-1 was in January. Monthly net photosynthetic rate(Pn) showed the change of "U" or "M"(because of light suppression), which was consistent with the increase or decrease of NEE. It told us that the response between photosynthesis and NEE was better.(3) From October in 2014 to September in 2015, the fitting relationships between monthly NEE and PAR values were better(R2>0.9) in the days of this Phyllostachys edulis forest ecosystem. Both the maximum net photosynthetic rate and dark respiration rate showed a decreased first and increased then decreased trend, the minimum values occurred in January, they were 0.543 and 0.067 mg·m-2·s-1, and the maximum values occurred in July, they were 1.889 and 0.296 mg·m-2·s-1, respectively. Fitting coefficients R2 between monthly Pn and PAR were higher than 0.96. Both the maximum net photosynthetic rate and dark respiration rate also showed a decreased first and increased then decreased variation trend as before. The maximum values occurred in July, they were 0.231 and 0.040 mg·m-2·s-1, respectively, and the minimum values were 0.075 mg·m-2·s-1(in January) and 0.007 mg·m-2·s-1(in December). The regularit ies of apparent quantum efficiency based on the ftting curves were poor, and the changes were small. In addition, monthly canopy maximum net photosynthetic rate and dark respiration rate based on eddy covariance and eco-physiological method showed a good positive correlation(r = 0.97, 0.93), and the former was higher than the latter, higher than 7.04% and 9.95%, respectively.(4) As for seasonal NEE, the highest was in summer(0.110 mg·m-2·s-1), and the lowest was in winter(0.060 mg·m-2·s-1). No matter daytime NEE or Pn and PAR response results in seasons showed that the maximum net photosynthetic rate and dark respiration rate showed a decreased after increased trend, and both showed the lowest in winter and the highest in summer. The apparent quantum efficiency fluctuated within 0.001-0.003 mg·μmol-1. Meanwhile, the Pearson correlation coefficient of seasonal maximum net photosynthetic rate based on two measurements was 0.91, and that of dark respiration rate was 0.95, which indicated that the measurement results of two methods had a high correlation.(5) The maximum net photosynthetic rate of eddy covariance method was 1.221 mg·m-2·s-1, ecosystem dark respiration rate was 0.263 mg·m-2·s-1. The maximum net photosynthetic rate of canopy based on eco-physiological method was 0.967 mg·m-2·s-1, dark respiration rate was 0.171 mg·m-2·s-1, and both were lower than the results of eddy covariance, decreased by 20.80% and 34.98%, respectively. Total net primary productivity of this ecosystem was 2581.33 g·m-2 during study year, and carbon absorption was about 0.704 kg·m-2, which indicated that this ecosystem was a high carbon sink. In addition, annual net pr imary productivity during the daytime were 4078.69 g·m-2 and 3277.50 g·m-2 based on the eddy covariance and eco-physiological method respectively, and the latter accounted for 80.36% of the former, with a higher proportion.(6) Monthly PAR and the net primary productivity during the day based on these two measurements showed a signif icant positive correlation(r = 0.95, P<0.01). With the increase of Ta, the maximum net photosynthetic rate and dark respiration rate of ecosystem showed an exponential increase, while the apparent quantum efficiency was decreased. At the same time, w ith the increase of VPD, the maximum net photosynthetic rate and dark respiration rate of ecosystem were showing a trend of decrease after increase at a turning point of 0.7 kPa, while the apparent quantum efficiency presented a decrease variation.The results above suggested that, during this period, carbon absorption capacity of Phyllostachys edulis forest ecosystem in Anji was better, which was a stronger carbon sink. The measurement results based on the two methods had better synchronization, and eddy covariance results were slightly higher than the physiological results. Moreover, eco-physiological results could explain some of the CO2 flux data changes. The photosynthesis of Phyllostachys edulis was critical process to affect the daytime NEE(carbon sink), which was also influenced by PAR、Ta and RH. |
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