Spatio-temporal Variations And Driving Mechanism Of Vegetation NPP And WUE In The Poyang Lake Basin

Terrestrial ecosystem and its dynamic processes are affected by climate change and human activities,and tis complexity and variabilityare very prominent.In the context of global change,the basic pattern of the carbon-water cycle in the terrestrial ecosystem has changed.In-depth study of the relative effects of climate change and human activities on net primary productivity(NPP)and water use efficiency(WUE)dynamics of the ecosystem is of great significance to clarify the response mechanism of global climate change,accurately assess and predict regional carbon budget,optimize regional water and soil resource management,and promote the healthy development of the ecological environment.Takes the typical subtropical mountain basin—Poyang Lake basin as the study area,this paper systematically analyzed meteorological and hydrological variates and and land use changes of the basin based on the collection of meteorological data,normalized vegetation index(NDVI)and MODIS evapotranspiration(ET)products and multi-period land use data during the period of 2000—2017.On this basis,the NPP of the Poyang Lake basin duing the past18 years was estimated by application of the CASA model,and the WUE was then calculated.According to the NPP and WUE dataset,this paper revealed the characteristics of the spatiotemporal variations of NPP and WUE of the terrestrial ecosystem of the Poyang Lake Basin,and deeply analyzed the relative effects of climate factors and human activities on the inter-annual variation of NPP and WUE.The main conclusions obtained are as follows:(1)During the study period,the annual averages of temperature,precipitation and solar radiation in the Poyang Lake Basin were 18.56?,1638.12 mm,and 4900MJ/m²,respectively.In terms of inter-annual changes,both temperature and precipitation showed an increasing trend,while solar radiation showed a decreasing trend.Only the change trend of temperature passed the significance test(P<0.05).Spatially,the change trend of annual average temperature decreases gradually from south to north.Change trend of precipitation and solar radiation shows the similar is similar spatial characteristic,which decreases from east to west.(2)The annual average ET of the basin during 2000—2017 was 829.54 mm.The difference of annual average ET among the sub-basins was small.During the past 18years,inter-annual variation of ET of the Poyang Lake Basin showed a significant increasing trend(P<0.01).The change trend of ET decreases from the surrounding mountains to the center lake region.The area with significant increasing trend of ET accounts for 32.7%of the whole basin,which mainly distributed in the southern part of the basin.However,the area with significant decreasing trendonly accounts for about2.3%of the total basin,and scattered in the north of the Poyang Lake Basin.The highest ET of different land use types was forested land(869.41 mm),and the lowest was paddy field(741.48 mm).ET of all the land use types showed a significant increasing trend(P<0.01)during the study period.(3)During the study period,the conversion area of land use in Poyang Lake Basin accounted for 1.9%of the whole basin.In general,the area of forest land increased,and the area of arable land and grassland decreased.The most active transformation was among the secondary classifications of forest land.The area changes of the land use types in the six sub-basins in different periods were relatively complicated.Since the Ganjiang sub-basin occupies the largest area of the study area,the distribution of each land type in the Ganjiang sub-basin was most extensive.Therefore,the conversion between land use types was also the most obvious.At this stage,land use changes was greatly influenced by local policies.Due to the escalating impact of human activities,the structure of land use in the basin is unstable and adjusted rapidly.(4)The annual average value of NPP in the Poyang Lake Basin is 741.10 g C·m^-2,and spatially decreases from the surrounding to the lake area.Among different sub-basins,NPP shows the highest in the Raohe sub-basin and the lowest in the lake region.In terms of inter-annual changes,NPP of the whole basin as well as all the sub-basins show an increasing trend.Among different land use types,forest land has the largest NPP,which is as high as 1000 g C·m^-2,and shrub has the lowest NPP,which is about 448.43 g C·m^-2.Correlation analysis indicates that temperature,among the main climate factors,has the prominent impact on annual NPP of the basin,followed by precipitation and solar radiation.The attribution analysis further demonstated that the changes of NPP in the Poyang Lake basin were mainly affected by human activities during the study period.However,the impact of both driving factors on NPP varied in different periods.During the period 2000—2009,NPP changes of the basin was mainly affected by human activities,while the change of NPP was dominated by climate change during the period 2010—2017.(5)The average annual WUE of the Poyang Lake Basin was 0.899 g C/kg H₂O during the study period.The spatial distribution of WUE is similar to that of NPP.In the past 18 years,WUE of the Poyang Lake basin showed a slight decreasing trend.The proportions of the areas with increasing and decreasing WUE in the basionwere about41.4%and 58.6%,respectively.WUE of different land use types is quite different.Among them the major land use types,woodland has the highest WUE(1.247g C/kg H₂O),and shrub land has the lowest WUE(0.543 g C/kg H₂O).Except for slight increasing trend of other woodlands and grasslands,WUE of all the other land use typesshowed a decreasing trend.During the study period,precipitation was the main meteorological factor affecting the WUE changes in the Poyang Lake basin,followed by solar radiation and temperature.The change of WUE in the Poyang Lake Basin was mainly affected by the negative effects of human activities,while the impact of climate change was relatively weaker.