Analysis Of The Dynamics And Driving Forces Of Carbon Budget In Two Typical Bamboo(Phyllostachys Edulis And Phyllostachys Praecox) Forest Ecosystems

Forests in the East Asian monsoon region play an important role in the global carbon cycle.As an important component of subtropical forests in China,bamboo forests have a strong carbon sequestration capacity,and play an increasingly major role in mitigating global warming.The carbon budget of bamboo forests is easily affected by many factors.Lack of long-term continuity observation of bamboo forests,little is known about the dynamics and reason of its carbon budget.Especially how to determine the drivers in the variation of carbon fluxes and the casue of its variation on different time scales.Combined with the eddy covariance technique and field experiments,the variation of carbon fluxes(net ecosystem exchange,NEE,ecosystem respiration,RE,gross ecosystem exchange,GEE),carbon use efficiency(CUE)and its drivers were analyzed in Moso bamboo forest and Lei bamboo forest during 2011-2016.Using the data of soil respiration(Rs),the best model was selected to estimate its contribution to ecosystem fluxes of both bamboo forests.Finally,the variation of carbon fluxes in both forests responded to the high temperature and heat waves weather and its causes were explored.The main results were as follows:(1)Random forest model was performed to screen the driving factors of carbon fluxes in both of the ecosystems at different time scales.On mean diurnal scale,the significantly importance variables of NEE,RE,GEP of Moso bamboo forest was vapor pressure deficit(VPD)>soil temperature(Ts)>air temperature(Ta)>photosynthetically active radiation(PAR)>soil water content(SWC);Ts>Ta;VPD>Ts>Ta,respectively.For NEE,RE,GEP of Lei bamboo forest was PAR;soil temperature at 5 cm depth(Ts5),soil temperature at 100 cm depth(Ts100);PAR,respectively.On monthly scale,the significantly importance variables of NEE,RE,GEP in Moso bamboo forest was Ts>Ta>leaf area index(LAI)>PAR;Ts5>air temperature>evapotranspiration;Ta>Ts5>LAI.It was PAR,canopy temperature,VPD for NEE,RE,GEP in Lei bamboo forest.(2)Structural equation model(SEM)was used to explain the variation of carbon fluxes in the two bamboo forests.After the selection of variables,SEM of the carbon flux of the Moso bamboo forest established could explain the 54.9%,84.5%,72.2%of its variation in NEE,RE and GEE.the SEM for Lei bamboo forest could explain the variation of NEE,RE and GEE was 36.6%,83.1%and 85.4%,respectively.(3)Combined with data of flux and LAI,the variation of CUE and its cause of the two bamboo forests were analyzed.CUE of Moso bamboo forest(-0.5～0.6)was higher than Lei bamboo forest(-0.4～0.3).The ranking of the significant related factors of CUE in the Moso bamboo forest was Ts>Ta>LAI>PAR>VPD>rainfall.SEM based on these drivers can explain 31.6%of its CUE variation.For Lei bamboo forest,it was canopy temperature>Ts5,LAI>PAR,VPD.The SEM could explain 23.5%of its CUE variation.(4)Analyses were conducted on the monthly changes of Rs and its influencing factors of the both forests.The variation pattern of Rs in the forests both exhibited a single peak curve,being the maximum in June and August,respectively.Model 10 was the optimal model for both ecosystems.Soil respiration correlated insignificantly(P>0.05)with soil water content in the bamboo forests.However,soil respiration correlated positively with soil temperature(P<0.01)in the two forests.Soil temperature in 3 years could explain 94%(2013),77%(2016)and 63%(2017)variation of Rs in Moso bamboo forest,respectively,and soil temperature could explain 87%(2013),63%(2016)and 53%(2017)variation of Rs in Lei bamboo forest,respectively.(5)Based on the optimal model of Rs,the contribution of Rs to ecosystem carbon fluxes(RE and GEP)in the two forests was estimated.The annual average contribution of Rs to RE and GEP was 47.1%and 27.4%and the accumulation of soil respiration accounted for RE,GEP 51.05%and 32.74%in Moso bamboo forest.And annual average contribution of Rs to RE and GEP was 63.1%and 58.5%,respectively in Lei bamboo forest.The accumulation of Rs in Lei bamboo forest was 66.59%of RE and 60.92%of GEP.(6)Under the condition of high temperature and heat waves weather,the variation of carbon fluxes and the causes of the two bamboo forests were studied.NEP of Moso bamboo forest in July and August of 2013 decreased by 59.9%and 80%,respectively,compared with the same period of 2011.The main reason of this included:GEP was greatly limited by lower soil water content and higher vapor pressure deficit,RE was increased by higher VPD to some extent,and both of them resulted in the reduction of NEP.But for Lei bamboo forest,the influence of extreme weather on its carbon fluxes was relatively not significant,indicating that soil drought in a short time(due to irrigation timely)cannot make its carbon fluxes decreased.