Impact Of Harvesting On Carbon Source/sink Of Alnus Sibirica Swamps In Daxin'anling Discontinuous Permafrost Region Of China

Degradation of permafrost caused by climate warming will increase greenhouse gas emissions from wetlands above it,yet it is still unclear that how harvest disturbance affect greenhouse gas emissions from wetlands on frozen soil.Annual carbon emission fluxes(ACE)(CO2 and CH4),the net primary productivity(ACE)and annual net carbon sequestration of vegetation(VNCS)of four different harvest treatments(control-D,low intensity selective cutting 15%-QZ,high intensity selective cutting 45%-Zz and clear cutting-J)of Alnus sibirica var,hirsuta swamp were measured synchronously by using the static chamber gas chromatography method and the relative growth equation,as well as related environmental factors(soil temperature,water level,thawing depth,soil carbon and nitrogen content,etc.),at Nanwenghe in Daxin'anling discontinuous permafrost region of China,in order to reveal the influence of harvesting disturbance on the carbon source/sink of the cold temperate forested wetlands according to the balance of the net carbon budget of the ecosystem.(1)Both selective(low intensity selective cutting and high intensity selective cutting)and clear cutting increased the non-growth seasonal water level,freezing depth and temperature(P>0.05),high intensity selective cutting and clear cutting significantly decreased the growing season average water level and snow thickness(P<0.05);Low intensity selective cutting,high intensity selective cutting and clear cutting all significantly increased the soil temperature of individual soil layers(P<0.05),and significantly reduced the organic carbon content,soil C/N and soil moisture content of individual soil layers.(P<0.05).(2)The annual average flux of CH4 was between 0.008~0.019 mg·m-2h-1,high intensity selective cutting and clear cutting significantly reduced the average annual flux of CH4 in soil by 52.6%-57.9%(P<0.05),while low intensity selective cutting,is similar to control(-10.5%,P>0.05);The seasonal dynamic trends took on two types(D,Qz bimodal-low emission;Zz and J bimodal-low absorption);Low intensity selective cutting and high intensity selective cutting have changed the seasonal distribution patterns form summer>spring ?autumn>winter(D and J)to summer>autumn ? winter?winter(Qz)and summer>spring>autumn>winter(Zz).(3)The annual average flux of CO2 was between 103.69~133.65 mg m-2·h-1 low intensity selective cutting,high intensity selective cutting and clear cutting were made the mean annual fluxes of CO2 significantly decreased by 19.9%,14.4%and 22.4%(P<0.05);The seasonal dynamic trends also existed two types(D,Qz unimodal type-peak at late summer;Zz and J unimodal type-peak advance to midsummer);Selective cutting(low intensity and high intensity)have changed the seasonal distribution patterns form summer-autumn>spring>winter(D)to autumn>summer>spring>winter(Qz)and summer>autumn>spring>winter(Zz and J).(4)The soil CH4 flux was controlled by temperature,water level,freezing depth and snow thickness.After harvesting disturbances,selective cutting(low intensity and high intensity)increases the correlation between CH4 flux and deep soil temperature,high intensity selective cutting and clear cutting were converted snow thickness from positive correlation to a negative correlation and water level was converted from positive correlation to irrelevant;Soil CO2 flux was controlled by temperature,freezing depth,soil water content,soil organic carbon content and C/N ratio,synthetically.After harvesting disturbances,both selective and clear cutting increased the correlation between CO2 flux and deep soil temperature,and reduced its correlation with soil C/N,but the correlation with organic carbon content increased or decreased(high intensity selective cutting and clear cutting increased and low intensity selective cutting reduced);(5)The net primary productivity(NPP)and annual net carbon sequestration of vegetation(VNCS)between 5.07-8.83 t hm-2·a-1 and 2.10-3.83 tC·hm-2-a-1,shown a declining trend with increasing harvesting intensity,NPP and VNCS of low intensity selective cutting were similar to those of control(9.8%and 3.0%,P>0.05),high intensity selective cutting and clear cutting made NPP and VNCS significantly decreased by13.7%-36.9%and 14.2%-43.5%compared to control(P<0.05)respectively,and those of clear cutting were significantly lower than that of high intensity selective cutting by 26.9%and 34.2%(P<0.05);(6)The net ecosystem carbon budget was between-0.42~1.30 tC·hm=2·a=1,control,low intensity selective cutting,and high intensity selective cutting were all carbon sinks(0.50~1.3 tC·hm-2·a-1),which the sink strength of low intensity selective cutting was significantly higher than that of control and high intensity selective cutting by 1.6 and 1.2 times(P<0.05),but clear cutting had been converted it into carbon source(-0.42 tC·hm-2·a-1,p<0.05).Therefore,after eight years of harvesting,Alnus sibirica var.hirsuta swamp has restored the carbon sink function at low intensity selective cutting and high intensity selective cutting site,while it still maintained the carbon source at clear cutting site in the cold temperate Daxin'anling discontinuous permafrost region,so selective cutting should be adopted in the management of wetland carbon sinks and to avoid clear cutting.