Effects Of Climate Change And Natural Disturbances On Carbon Budget Of Forest Ecosystem In The Three Gorges Reservoir Area

Forest is the domination of the terrestrial ecosystem.And the carbon budget of forest ecosystems is highly affected by climate change and natural disturbances.Climate change not only directly affects forest growth through changes in meteorological factors such as temperature and precipitation,but also indirectly affects the carbon budget of forest ecosystems by changing the area and intensity of natural disturbances.However,little is known about that how climate change and natural disturbances affect the carbon storage and carbon budget of forest ecosystems at the regional scale.Furthermore,whether climate change and natural disturbances will change the carbon source/sink status of forest,and whether the interaction effect of them will aggravate or mitigate the loss of forest ecosystem carbon budget are still unclear.This study took the arbor forest ecosystem in the Three Gorges Reservoir Area(TGRA)as the research object,and used the following data sets as the main data source,which consist of measured meteorological dataset from 1973 to 2018,regional climate model(Reg CM 4.0)projection from 2006 to 2050,forest resource planning,designing and surveying data in 2009,and annual statistics of nature disturbances(fire,insect and disease)from 1998 to 2018.Based on the assumption that the forest area in the TGRA would be unchanged in the future,without taking the land use change,natural forest regeneration and human management into consideration,this paper studied the following contents.(1)Applied Mann-Kendall trend and abrupt analysis to describe the climate change characteristics of the TGRA in the past 46 years,and revised the future regional climate model projection accordingly to obtain three future climate scenarios.(2)Utilized stepwise regression to fit the linear relationship between climate elements and the disturbed area to predict the areas of various disturbances would occur in the future.(3)Incorporated an ecological process model(3-PG)with a carbon budget model(CBM-CFS3)to evaluate the spatio-temporal dynamics of both carbon storage and carbon budget of forest ecosystem in the TGRA under various climate-disturbance scenarios from2009 to 2050.(4)Compared the results of different scenarios to estimate the impact of climate change on forest volume,and quantified the specific impacts of both climate change and natural disturbances(fire and pests)on forest ecosystem carbon storage and productivity,and then assessed the interaction of climate change and the natural disturbances on forest carbon budget.Aiming at identifying the risks of forest ecosystems in the TGRA change from carbon source to carbon sink in the future,this study revealed the response patterns of the forest ecosystem to the natural drivers.It would provide a scientific basis for the maintenance of regional ecological security and sustainable social development.The annual maximum temperature,annual average temperature,annual minimum temperature,and annual precipitation in the TGRA were 22.1?,17.7?,14.7?,and1120.8 mm,respectively,during the period of 1973 to 2018.Assumed average values of climate elements in this historical period as the baseline climate scenario(BS),and set the revised regional climate model results as the climate change scenarios(RCP4.5 and RCP8.5)in the future.Both annual average temperature and annual precipitation under climate change scenarios(RCP4.5 and RCP8.5)in the TGRA are higher than that of BS scenario by 0.7-0.8?and 1.3-24.6 mm,respectively,which is in line with the climate change trend on the nation scale.Both volume and carbon stock of the forest ecosystem in the TGRA showed a trend of rapid increase at first and then become steadly increasing from 2009 to 2050.Meanwhile the net primary production(NPP),net ecosystem production(NEP)and net biome production(NBP)of forest ecosystem showed a gradual decreasing trend.If no disturbance occurs,NBP equals NEP.During the entire simulation period,the average values of forest volume,ecosystem carbon storage,NPP,NEP and NBP under the baseline climate with non-disturbance scenario would be 3.18×10⁸ m³(or 124.30 m³·ha^-1),286.22 Tg C(or 111.81 Mg C·ha^-1),7.30Tg C·a^-1(or 2.85 Mg C·ha^-1·a^-1),2.13 Tg C·a^-1(or 0.83 Mg C·ha^-1·a^-1)and 2.13 Tg C·a^-1(or0.83 Mg C·ha^-1·a^-1),respectively.Compared with these results,climate change will increase the ranges of average values of forest volume,ecosystem carbon storage,NPP,NEP,and NBP in the TGRA by 4.29%-4.80%,2.49%-2.77%,3.42%-3.82%,6.87%-7.67%and 6.87%-7.67%,respectively,during the simulation period.The natural disturbance types include fire,insects and diseases,for the insects and diseases are the dominant disturbances in the TGRA.Under the baseline climate with non-disturbance scenario,the annual area of natural disturbances is1.22×10⁵ ha,which accounting for 4.76%of total area of the forest ecosystem in the TGRA.Compared with the corresponding results under baseline climate with non-disturbance scenario,natural disturbance would reduce the average values of carbon storage,NPP,NEP and NBP of the forest ecosystem in the TGRA by 6.47%,13.00%,38.47%and 39.87%,respectively.The loss of ecosystem production brought about by insects and diseases accounted for99.39%-99.59%of total loss,and the loss caused by fire accounted for only 0.41%-0.61%of the total loss.Climate change would exacerbate the occurrence of natural disturbances in the future,increasing the cumulative disturbance area by 7.78%-14.44%,which would lead to increased annual average losses of forest ecosystem carbon stocks,NPP,NEP,NBP of0.71%-1.07%,1.10%-1.67%,3.02%-4.68%and 3.12%-4.98%,respectively.The high-value areas of both carbon storage and production of forest ecosystem in the TGRA are concentrated in the eastern,northern and central-southern parts where elevations are high,presenting a distribution pattern of"high values in the eastern and northern parts,low values in the western and southern parts".Forest types in these high-value areas are dominated by deciduous broad-leaved forests and evergreen broad-leaved forests.Climate change has the strongest promoting effect on the growth of evergreen broad-leaved forests among 8 forest types,increasing the NEP of per hectare by 14.88%-16.04%.Assuming that the existing forest area remains unchanged,natural disturbances have transformed the forest ecosystems which located in the central and western parts of TGRA to carbon sources in the late period of simulation.Moreover,climate change would further aggravate the carbon loss of forest ecosystems in these regions.While the forests on the northeast side and the south-central edge of the TGRA at higher altitudes are less affected by natural disturbances,and would act as carbon sinks under the influence of nature drivers and would show a good adaptability to the future climate changeResults demonstrated that the method of coupling 3-PG with CBM-CFS3 not only takes advantage of 3-PG to simulate the impact of climate change on forest growth,but also combines the CBM-CFS3 model to assess the influence of natural disturbances to forest ecosystems comprehensively.Based on the forest resource survey data,this method can accurately estimate the impacts of climate change and natural disturbances on both carbon stock and budget of forest ecosystem in the TGRA.The stimulation effect is ideal,which revealed this method is suitable for predicting the dynamic of regional forest carbon budget in the long-term time series and appropriate for evaluating the impact of natural drivers.With a trend of continuous warming and precipitation slightly increased,climate change would increase both forest volume and ecosystem carbon stock of the research area in the future.Without natural disturbances,the forest ecosystem in the TGRA would serve as a carbon sink,and climate change would enhance its carbon sequestration capacity.Dominated by insects and diseases,the natural disturbances would reduce the carbon stock and production of the forest ecosystem.Natural disturbances would change the forests in the central and western regions of the TGRA into carbon source in the late simulation period with the assuption that every forest type area remains unchanged,without considering land use changes and natural forest regeneration.The losses of carbon storage and carbon budget caused by natural disturbances are greater than the promotion of climate change on forest growth.Climate change would exacerbate the loss of forest carbon budget caused by natural disturbances.Forest management and pest control should be strengthen in the TGRA in the future,with the practices of tending and afforestation to adjust the current age structure.Depending on the rule of matching tree species with site,evergreen broad-leaved species should enjoy high priority in afforestation,in order to enhance the regional forest's adaptability to climate change.Comprehensive management measures should be practised on the forests in the central and western parts of the TGRA,in order to enhance the carbon sequestration capacity of the forest ecosystem,decrease the losses caused by disturbances,protect the forest from converting to a carbon source,promote the long-term sustainable development of regional forest resources and ensure the ecological security of the Yangtze River Basin.