Spatio-temporal Dynamics Of Carbon And Water Fluxes In The Terrestrial Ecosystems Of The Yangtze River Delta Region And Their Relations To Driving Variables

The establishment of tree plantations is a potential approach to reducing atmospheric carbon dioxide concentrations to mitigate climate change.However,carbon storage gains correspond to an amount of water loss.Carbon and water cycles are closely related in the terrestrial ecosystem because the exchange of carbon dioxide and water vapor between biosphere and atmosphere are both controlled by stomata.Water use efficiency(WUE),the ratio of gross primary productivity(GPP)to evapotranspiration(ET),can be used to quantify this coupling relationship.A better understanding of WUE will provide an alternative approach for carbon and water budget assessment.Meanwhile,understanding the characteristics of GPP,ET,WUE and their relationship with related environmental factors can greatly enhance our knowledgeoftheir control processes as well as the ability to predict how climate change may affect the carbon and water budgets.For the research of carbon and water cycles in the terrestrial ecosystem,the ground-based flux observation and remote sensing observation are complementary techniques.On one hand,the development of global remote sensing observation system contributes to quantify these spatial characteristics at the regional or global scale.On the other hand,the long-term ground-based flux observation can be used to illustrate the control mechanism of carbon and water cycles.The combination of these two techniques will promote the recognition and understanding to the process of carbon and water cycles.With fractionation effect,water vapor stable isotope composition provides important information for revealing water vapor transport process and could be integrated with water vapor flux data for a further understanding of water exchange characteristics and dissipation mechanism.Using MODIS terrestrial stardard products,the vegetation productivity model(VPM),the revised remote sensing-Penman-Monteith(RRS-PM)model,eddy covariance flux observation and water vapor stable isotope data,the spatial and temporal dynamics of carbon and water fluxes in the terrestrial ecosystems of the Yangtze River Delta(YRD)region and therir relations to driving factors were analysed.The major conclusions were as follows.(1)Temporal and spatial variations of GPPThe seasonal pattern of GPP showed a bimodal distribution in the coniferous forest,broad-leaved forest,grassland and cropland,with their peak values occurring in May and August.By contrasn,the seasonal pattern of GPP in moso forest had a unimodal pattern,with its maximum in July.During the study period,GPP in most biomes appeared a downward trend,the regional annual average GPP also decreased.In general,high value of GPP appeared in the southern YRD and relative low value appeared in the northern YRD.This was likely to be due to the different land-cover types.In the south,there was more forest,while in the north the land-cover type was almost entirely cropland(2)Temporal and spatial variations of ETThe seasonal variations of ET in all biomes showed a unimodal pattern,with maximum in July.During the study period,ET appeared a rising trend in all forest ecosystems and a downward trend in the grassland and cropland ecosystems.The regional annual average ET had a rising trend from 2001 to 2014.The spatial distribution of ET was similar to that of GPP,with a higher value occurring in the south.(3)Temporal and spatial variations of WUEThe seasonal pattern of WUE showed a bimodal distribution in the coniferous forest,broad-leaved forest,grassland and cropland,with their peak values occurring in April and October.However,the seasonal variation of WUE in moso bamboo forest was more complicated and did not show any pattern.During the study period,WUE in all forest biomes appeared a downward trend,the regional annual average WUE also decreased.In general,high value of WUE appeared in the southern and northeastern YRD and relative low value appeared in the northwestern and middle part of YRD.(4)Response of GPP,ET and WUE to control factorsIn the terrestrial ecosystems of YRD,the relationships of GPP with atmospheric temperature(Ta)were fitted with exponential growth equations,while GPP increased linearly with increasing annual mean precipitation and leaf area index(LAI).In moso bamboo forest,the relationship of GPP with net radiation(Rn)was fitted with exponential growth equation while the relationship between GPP and vapor pressure deficit(VPD)was fitted with a quadratic polynomial equation.In the coniferous forest,broad-leaved forest,grassland and cropland,the relationships of ET with Ta were fitted with exponential growth equations,while ET increased linearly with increasing annual mean precipitation.The relationship between ET and LAI was relatively complex.ET increased linearly when LAI was less than 2,the growth rate declined when LAI was greater than 2.In moso bamboo forest,the relationships of ET with Ta,Rn and VPD were all fitted with linear equations.WUE decreased as vapour pressure deficit(VPD)increased in all of the biomes.(5)The water vapor stable isotope characteristics in the process of water circulationIn broadleaved deciduous mixed forest,deuterium excess presented a negative linear relationship with relative humidity,and deuterium excess was positively linear correlated with ET.The results showed that the ET was the main driving force of deuterium excess in the afternoon.