| As one of the most important types of vegetation,The forest has the functions of water conservation,soil conservation,carbon fixation and oxygen release,climate regulation,biodiversity maintenance and recreation,and plays an important role in maintaining the balance of the global ecosystem.With the acceleration of global warming,the interaction between forest ecosystem and climate has become one of the hot issues in global change research.The temperate forest ecosystem in northeast China is an important ecological barrier in northeast China.However,the mechanism by which temperate forests influence regional climate by influencing the energy balance and the water cycle is not clear.Using multi-source remote sensing data and the most advanced WRF(Weather Research and Forecasting Model)regional climate model,this thesis quantitatively evaluated the effects of temperate forests on regional temperature and precipitation,and evaluated their biogeophysical mechanisms.The main conclusions are as follows:(1)Northeast China is an important forestry production base in China,with woodland accounting for about 40% of the total area.Arable land is the second-largest land-use type in the region,accounting for about 30% of the total area,and has a trend of continuous increase.From 1980 to 2015,except for cultivated land,urban and rural land,industrial and mining land,and residential land,the proportion of other land use types in Northeast China decreased.On the whole,the area change of various land use and cover types in northeast China is relatively stable.From 1982 to 2018,the annual mean temperature increased and precipitation remained stable in Northeast China,but the interannual variability was large.(2)Based on the regional climate model scenario simulation,it is found that the forest ecosystem has a warming and humidifying effect on the regional climate on an annual scale.The presence of forests makes the annual average temperature slightly increase by 0.04±0.02 ℃,and can effectively increase precipitation by 17.49±3.88 mm in northeast China.The regulation of the forest ecosystem on temperature and precipitation changes with seasons.The model simulations show that without forests,northeast China(including forested and farmland areas)would have colder winters and springs and warmer summers than observed,suggesting that temperate forests in northeast China act as an important buffer zone in the formation of regional climate patterns.The study also highlights the ability of temperate forests to regulate the regional water cycle in the Northeast.Due to high evapotranspiration,forests can transfer enough water to the atmosphere,and combined with regional moisture convergence,temperate forests in the northeast bring more rainfall to forest and agricultural ecosystems.Precipitation increases mainly in spring and summer.These seasonal rainfall accounts for 93.82% of the total increase.These results indicate that temperate forests have contributed significantly to the sustainable development of agriculture in northeast China.(3)The increase of forest vegetation coverage and evapotranspiration in Northeast China from 1982 to 2018 resulted in a decrease of 0.06±0.01 ℃ in local air temperature and an increase of 1.67±4.12 mm in precipitation.And climate changes and causes vary from season to season.In both autumn and winter,the increase of sensible heat flux and the absorption of more energy by soil reduced the temperature,evapotranspiration and precipitation.The decrease of spring air temperature is mainly due to warming effect caused by the decrease of surface albedo is completely offset by the cooling effect caused by the increase of evapotranspiration.In summer,a large increase in evapotranspiration and a decrease in net shortwave radiation lead to cooling and humidification.These results emphasize the important role of temperate forest vegetation changes in mitigating regional warming and drying over the past four decades. |
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