Dynamics Of Carbon Fluxes And Stocks Of Forests And Underlying Driving Factors In Xinjiang,China

The Xinjiang Uygur Autonomous Region,China is in a typical arid area,characterized by fragile ecological environment,large area and complicated topography,leading to significant differences in environmental factors(e.g.climate,soil)among different ecological zones.There are farming and irrigation in the oasis.Consequently,types of forests are diverse here.The forests in this region are important components of boreal forest belt of the five countries in Central Asian and even the Eurasia.Although the forest areas in Xinjiang are smaller compared to the forest areas in the east part of China,they play a key role in maintaining the balance of ecological systems for the arid area.Forest ecosystems in Xinjiang are sensitive to climate change.Thus,the study on forest carbon fluxes and stocks here is of scientific significance for assessing the response of arid zone ecosystems to climate change.On the basis of multi-source data,including forest inventory,remote sensing data,meteorological data,soil data and leaf area index,this study employed geo-spatial statistical analysis and process-based ecological modelling methods to investigate the dynamics of carbon fluxes and stocks of forests and underlying driving factors.Biomass and productivity(NPP,Net Primary productivity)of forests in Xinjiang were estimated.The variations of NPP with forest age were analyzed.The temporal and spatial variations of forest carbon fluxes and carbon stocks in Xinjiang during 1901-2012 were simulated using the Integrated Terrestrial Ecosystem Carbon(InTEC)model.The sensitivity of the model results and the contributions of different driving factors to forest carbon fluxes and carbon stocks accumulation in Xinjiang were quantitatively evaluated.The main results of this study were as follows:(1)Spatial patterns of forest biomass and NPP in XinjiangEstimated using Forest Management Inventory data,the average values of forest biomass and NPP in 2011 were 123 t ha-1 and 458 g C m-2 yr-1 in Xinjiang,respectively.The total NPP in the ecological zone I(Altai-mountain forest and grassland ecological zone)and ecological zone III(Tianshan mountain forest and grassland ecological zone)accounted for 70%of the total forest NPP.The average NPP of deciduous needleleaf forests(564 g C m-2 yr-1)was the largest among four forest types(evergreen needleleaf forest,deciduous needleleaf forest,broadleaved deciduous forest and mixed broadleaf-needleleaf forest).The evergreen needleleaf forests made the largest contribution(39%)to the total forest NPP in Xinjiang.The average NPP of natural forests was 15 g C m-2 yr-1 smaller than that of planted forests.However,the total area of natural forests was much larger than that of planted forests,resulting in their total NPP nearly 3 times higher than the total NPP of planted forests.As to broadleaf forests,the average NPP values of poplar,birch,and elm were above 600 g C m-2 yr-1,but the average NPP of Euphrates poplar was only 248 g C m-2 yr-1.As to coniferous forests,the average NPP values of spruce and larch were 481 g C m-2 yr-1 and 558 g C m-2 yr-1 respectively.These two tree species contributed 39%and 23%to the total NPP of forests in Xinjiang,respectively.Both the total and average of forest NPP increase with the increase of elevation and slope.The average of NPP was higher in shady slopes(north,northwest,northeast,and east)with larger forest areas,while the average of NPP was lower in sunny slopes(southeast,southern,and southwest)with smaller forest areas.(2)Spatial patterns of forest age and its effects on NPP in XinjiangMature forests account for the largest areas(26%)in Xinjiang.The near-mature,mature and over-mature stand forests were mainly concentrated in the mountains areas;while the middle-aged forests were distributed in the northwest areas of the Tarim;and the young stands were mainly distributed in the southern margin of the Tarim Basin,the south slope of Tianshan Mountains basin,and Tacheng basin area.The forest age frequency distribution of natural and planted forest was bimodal and unimodal,respectively.Nevertheless the forest age frequency distribution of all the species was normal,but different species had different peak ages.Forest age generally increases firstly and then decreases with increasing elevation.The change of forest NPP with age basically showed the downward-opening parabola distribution.The age at which NPP maximized changed significantly with forest types and species.The NPP of the evergreen needleleaf,deciduous needleleaf,broadleaved deciduoust,and mixed broadleaf-needleleaf forests reached the peaks at ages of 141,86,32 and 78 years,respectively.The corresponding ages with peak NPP were 90,102,42 and 15 years for the larch,spruce,Euphrates poplar and poplar,which were the dominated species in Xinjiang.The refined relations of forest NPP with ages were used as inputs into the InTEC model.(3)Temporal and spatial patterns of forest carbon fluxes and carbon stocks in XinjiangThe change characteristics of the average forest NPP,net ecosystem productivity(NEP)and net biome productivity(NBP)simulated by the InTEC were basically consistent in Xinjiang during the period of 1901-2012.The average values of NPP,NEP and NBP were 337.0 g C m-2 yr-1,43.8 g C m2 yr-1 and 34.8 g C m-2 yr-1,respectively.From 1960 to 2012 in Xinjiang,the forest biomass carbon stocks,soil carbon stocks and ecosystem carbon stocks(including biomass and soil carbon stocks)increased significantly.The forest ecosystems acted as a carbon sink of 1.5 Tg C yr-1 averaged over the period of 1960-2012.In 1901-2012,the increase rate of forest NPP in the west of Xinjiang was larger than that in the east.Meanwhile,the increase rate of forest NPP in the mountainous area was larger than that in the plain area.The forest NEP in the northern Xinjiang decreased,while it increased in the southern Xinjiang.In 2012,the carbon stock density of biomass,soil and the total ecosystem in mountainous areas showed larger values than those in plains.During 1960-2012,except for Junggar basin-western mountain and desert grassland ecological zone,forest ecosystem carbon stock density generally increased.During 1901-2012,the average NPP of Altai-mountain forest and grassland ecological zone was the highest,among 5 ecological zones.Forests in Tianshan mountain forest and grassland ecological zone had the highest average annual total NBP and average ecosystem carbon stocks.The deciduous needleleaf forest had the largest mean NPP(483.9 g c m-2 yr-1),while the evergreen needleleaf forest had the largest average annual total NBP(0.45 Tg c yr-1)and average carbon stocks(202.8 Tg C),among 4 forest types.Both the forest carbon fluxes and average ecological system carbon stocks were larger in natural forests than planted forests.The forest average annual total NBP and average ecological system carbon stocks of spruce and larch were larger than those of other species.(4)Sensitivity of simulated carbon fluxes and average annual variation of carbon stocks to model inputs and parametersThe sensitivity of average carbon fluxes and average annual variation of carbon stocks simulated using the InTEC model by specific perturbation values of input data and model parameters was assessed during 1960-2012.With temperature increasing or decreasing by 1?,the average NBP would decrease by 1.1%or increase by 1.6%.With precipitation increasing or decreasing by 25%,the forest average NBP would increase by 0.6%,or decrease by 1.1%.The sensitivity of the simulated forest NBP was low to solar radiation and vapor pressure.The forest average NBP would increase by 1.1%or decrease by 1.4%corresponding to the nitrogen deposition increasing or decreasing by 25%.The impacts of forest age and NPP in the reference year(NPPref)were significant on model results.With the NPPref increasing or decreasing by 25%,the average carbon fluxes and the average annual variation of carbon stocks would increase or decrease by about 20%.The forest average NBP would increase by 5.6%or decrease by 6.3%,when forest age increasing or decreasing by 5 years.When the sensitivity coefficient of ecosystem respiration to temperature(Qio)increase or decrease by 0.5,the forest average NBP would decrease by 7.8%or increase by 16.2%.(5)Contributions of different factors to the forest carbon fluxes and stocks accumulation in XinjiangDuring 1901-2012,the disturbance factor(forest age)had positive effects on forest NPP and NBP in Xinjiang,while climatic factor had negative effects on forest NBP.The climatic factor,nitrogen deposition and CO2 concentration contributed-20%,2%and 19%to the accumulation of ecological system carbon stocks,respectively.However,different factors had spatially different impacts on forest carbon fluxes and stocks.In Altai-mountain forest and grassland ecological zone,the disturbance factor had the largest contribution to forest NBP,followed by CO2 concentration,and nitrogen deposition.The climate factors had negative impacts on NBP.The disturbance and non-disturbance factors contributed 85%and 15%to the accumulation of ecological system carbon stocks.However,in Junggar basin-western mountain and desert grassland ecological zone,the corresponding contributions of disturbance and non-disturbance factors were-64%and 36%to the accumulation of ecological system carbon stocks.In Tianshan mountain forest and grassland ecological zone,the forest NBP was decreased by climate factors,while it was increased by disturbance factors.The disturbance and non-disturbance factors contributed 82%and 18%to the accumulation of ecosystem carbon stocks,respectively.In Tarim basin-desert ecological zone,the disturbance factor and climatic factor caused reduction in the forest NBP.The contributions of disturbance and non-disturbance factors were-63%and 37%to the ecosystem carbon stocks accumulation.In Pamirs-Kunlun mountain-Altun alpine desert grassland ecological zone,the disturbance and non-disturbance factors made 21%and 79%contributions to the accumulation of ecosystem carbon stocks,respectively.