Research On Ecosystem Carbon-water Coupling Of Chinese Fir Plantations In Huitong,Hunan Province

Taking the Chinese Fir Plantations in Huitong,Hunan Province as the research object,this paper studied the temporal variations of its ecosystem respiration,gross ecosystem productivity(GEP),evapotranspiration(ET)and the ecosystem water-use efficiency(WUE)at different time scales and their correlation with environmental factors by analyzing the data from 2008 to 2017 measured by the open-circuit eddy correlation system and the meteorological factor observation system.And influence factors of GEP,ET and WUE were also discussed to provide a scientific theory for optimal water management of Chinese Fir Plantations.The results indicated that:1.The daily variation of RE was higher in the day and lower at night on the whole in January,April,July and October of 2017.In the meanwhile,the maximum values were at 15:30,14:30,14:30,0:30,the minimum at 0:30,6:00,19:30,18:00,and the daily average ranges of the month were 0.05,0.09,0.11,0.06 mg CO2/(Dm2 s1);The annual variation of RE from 2008 to 2017 showed a single peak.Meanwhile,the maximum value was in August(234.3 g C/(m2mon)),the minimum in July(172.2g C/(m2mon)),and annual average range was 145.5 g C/(m2mon);The inter-annual variation of RE from 2008 to 2017 was rising.The annual average value was 1544.8 g C/(m2a).Meanwhile,the maximum value was 1752.2 g C/(m2a)in 2015 and the minimum(1431.0 g C/(m2a))in 2011,and annual average range of this decade was 321:2 g C/(m2a);At the monthly scale,the RE had a strong quadratic polynomial relationship with Photosynthetically active radiation(PAR),soil water content(SWC),precipitation and vapor pressure deficit(VPD),and a very significant exponential function relationship with the soil temperature at a depth of 5 cm.2.GEP and ET in January,April,July and October of 2017 generally presented single-peak type.The maximum GEP values were in 12:30?11:00?9:30?11:30,and daily average ranges of the month were 0.53?0.80?0.87?0.83 mg CO2/(m2s):The maximum ET values were in 12:30?13:00?13:00?12:00,and daily average ranges of the month were 0.03?0.08?0.13?0.06 kg H20/(m2s):The annual variations of GEP and ET from 2008 to 2017 were in a single-peak type with highest in summer and lowest in winter.Their maximum values were in July or August,the minimum in January,and annual average range was 63.5 g C/(m2mon)?19.7 kg H2O/(m2non);Monthly total maximum value of GEP ranged from 185.5 C/(m2mon)to 249.0 g C/(m2mon).In the meanwhile,maximum value was in July of 2015(249.0 g C/(m2mon)),the minimum in July(185.5 g C/(m2mon)),and the annual average range of this decade was 63.5 g C/(m2mon);monthly total maximum value of ET ranged from 75.9 kg H2O/(m2mon)to 95.6 kg H2O/(m2mon).In the meanwhile,the maximum value was in August of 2012(95.6 kg H2O/(m2mon)),the minimum in July of 2008(75.9 kg H2O/(m2mon)),and the annual average range of this decade was 19.7 kg H2O/(m2mon);The GEP and ET from 2008 to 2017 exhibited a rising inter-annual variability.And ET was highest in 2016 and presented a value of 642.67 kg H2O/m2a),the lowest in 2011(528.79 kg H2O/m2a),and the annual average ET was 580.35±36.06 kg H2O/m2a;GET was highest in 2015(1952.17 g C/m2a),the lowest in 2011(1507.24 g C/m2a),and the annual average GEP was 1688.57±126.09 g C/m2a;At the monthly scale,GEP had an extremely significant quadratic polynomial relationship with PAR,VPD and precipitation;a significant exponential function relationship with air temperature.ET had an extremely significant quadratic polynomial relationship with PAR,VPD and precipitation;and a significant exponential function relationship with air temperature.3.The curve of the diurnal variation of WUE was a U shape in January,April,July and October of 2017.In the meanwhile,their minimum values were at 12:00,12:30,12:30,15:00,and daily average ranges were 8.9,10.3,6.5,7.9 g CO2/kg H2O;the mean annual variation of the ecosystem WUE from 2008 to 2017 showed that it varied greatly in winter and fluctuated little in summer.In the meanwhile,the maximum presented in December(4.7 g C/kg H2O),the minimum(2.6 g C/kg H2O)in August,and annual average range was 2.1 g C/kg H2O;The ecosystem WUE from 2008 to 2017 exhibited a rising inter-annual variability.In the meanwhile,the maximum value was in 2010(3.9 g C/kg H2O),the minimum value in 2016(3.5 g C/kg H2O)and the annual average WUE was 3.6±0.1 g C/kg H2O;At the daily scale,there existed a very significant linear relationship between the ecosystem WUE and PAR,VPD.The WUE decreased with PAR and VPD increasing.The correlation between the ecosystem WUE and air temperature,SWC was quadratic polynomial;At the monthly scale,the monthly mean WUE had an extremely significant linear relationship between PAR and monthly mean VPD;a strong exponential function relationship with precipitation;and a strong quadratic polynomial relationship with monthly mean air temperature.4.The correlation between GEP at the daily,monthly and annual scales and ET was quadratic polynomial from 2008 to 2017.GEP and ET were extremely correlated at the daily and monthly scales.GEP and ET were very correlated at the annual scale.5.From the relationship between non-effectively continuous precipitation days and drought grade,the author concluded that there appeared moderate drought in the summer of 2011 and extreme drought in 2013.ET increased and subsequently the ecosystem WUE decreased under moderate drought,while GEP didn't change;the ecosystem WUE decreased with GEP reducing under severe drought,while the ET was not influenced;both GEP and ET decreased under extreme drought,while the ecosystem WUE nether increased nor decreased in 2013.