Characteristics And Influencing Factors Of Surface Energy Balance In An Artemisia Ordosica Shrub Ecosystem In The Mu Us Desert

The surface energy balance of arid and semi-arid ecosystems could be fed back to local and regional climates and be extremely important in the carbon-water cycle.Typical sandy shrub ecosystems were widely distributed in arid and semi-arid areas in northern China.The energy exchange processes between typical sandy shrub ecosystems and the atmosphere was extremely susceptible to climate change and was crucial to help predict the processes and functions of regional arid and semi-arid ecosystems.However,there were few studies on energy balance in arid and semi-arid areas,and there was a lack of systematic research to quantify how energy balance responded to climate change.In order to determine the main factors affecting the energy balance in the semi-arid area and help predict how the ecosystem in the semi-arid area will respond to climate change,this study took the typical shrub ecosystem in the Mu Us Sandy Land as the research object,combined with eddy covariance method,radiation observation,soil heat flux observations and micro-meteorological factors observations,evaluated the energy balance closure of the shrub ecosystem from 2012 to 2018,analyzed the seasonal and inter-annual dynamics of energy components,and discussed the biological and abiotic factors affecting energy partitioning on different time scales.The main conclusions are as following:(1)The energy balance of the shrub ecosystem in the study area was not closed.The energy balance closure conformed to the reports of the scope of the FLUXNET and China FLUX.On the half-hour scale,the slope of the OLS(Ordinary Least Squares)linear regression was between 0.59 and 0.64,with an average value of 0.61,and the average EBR(Energy Balance Ratio)of the 7-year was 0.82;the overall linear regression line for 7 years had a slope of 0.64 and an EBR of 0.82.On a daily scale,the slope of the OLS linear regression line was between 0.69 and 0.78,with an average value of 0.72,and the 7-year EBR average was 0.77;the 7-year overall energy balance closed OLS linear regression line had a slope of 0.72and EBR of 0.78.(2)Net radiation(R_n),sensible heat flux(H),latent heat flux(LE),and soil heat flux(G)had similar seasonal changes over the seven years.The sensible heat flux(H)was the main energy component that consumed net radiation(R_n)on both the annual and seasonal scales.Due to water restriction,LE did not exceed H even during periods of plant growing most vigorously and rain falling most heavily.G played an important role in energy balance on daily and seasonal scale,but it was negligible on an interannual scale.The main characteristics of the energy distribution of the shrub ecosystem in the seven years were:high Bowen ratio(?=H/LE;of mean value 6.60),high H/R_n(of mean value 0.53)and low LE/R_n(of mean value 0.14).Compared with other ecosystems in arid and semi-arid areas,the drought of the shrub ecosystem in our study area were also significant.(3)There were significant differences between biological and abiotic factors on energy distribution at different time scales.Surface conductance(g_s)and normalized difference vegetation index(NDVI)were negatively correlated with energy distribution(?)on the half-hour,daily,and monthly scales,and g_ssignificantly affected?in both seasons and years.On half-hour and daily scales,?was positively correlated with saturated water vapor pressure difference(VPD);on the monthly scale,?was negatively correlated with VPD.In growing seasons,VPD had no significant effect on?.At various time scales(half an hour,daily,and monthly),?decreased with increasing VWC.(4)The analysis of the factors influencing the interannual variation of?using the structural equation model showed that g_s had a significant direct effect on?,with a direct path coefficient of-0.86.The regulation of?by NDVI and VWC were mainly indirect effect through g_s,and the indirect path coefficients were-0.53 and-0.18,respectively.The impact of VPD on the interannual variability of?were directly effect on?on the one hand,and indirectly effect on?through g_s on the other hand,the total path coefficient was-0.30.The main effects of g_s and NDVI on?indicated that vegetation played an important role in regulating energy partitioning.Given that vegetation could improve the local climate by affecting energy balance,measures such as vegetation restoration in semi-arid areas could play an important role in improving the local climate and reversing desertification.