Characteristics Of Daytime Net Ecosystem Carbon Exchange In A Deciduous Broad-leaved Forest Of Maoershan

Using the temperate deciduous broad-leaved forest at the Maoershan site on an average slope of 9° and a azimuth of 296° as a case study,we investigated the diurnal patterns and interannual variations in net ecosystem exchange(NEE)in the daytime and its driving factors in the growing season(May to September)during 2007-2018,assessed the differences in estimating the light response parameters and the explanations of NEE by other environmental factors between the photosynthetically active radiation(PAR)measured by horizontal and slope-parallel radiometers.The results showed:(1)The diurnal change of NEE for each month of the growing season presented a morning-afternoon asymmetrically unimodal curve:the NEE became negative(net carbon absorption)about 2.5 h after sunrise,reached the peak between 10:00-13:30,then approached zero again 2 hours before sunset.The daily peak value of net uptake maximized in July,and minimized in May.On the annual scale,the daily peak value of net uptake from May to September and June gradually increased.The correlation between the residual of the rectangular hyperbola light response curve of NEE(?NEE)and air temperature(Ta),vapor pressure deficit(VPD),and broadband normalized difference vegetation index(NDVI)in the entire growing season(May-September)were in the order of r(?NEE vs VPD)>r(?NEE vs Ta)>r(?NEE vs NDVI);?NEE has a very significant positive correlation with Ta and VPD while generally showed a negative correlation with NDVI.Diurnal,seasonal,and interannual changes in NEE are jointly controlled by PAR,air temperature(Ta),vapor pressure deficit(VPD),and normalized difference vegetation index(NDVI),the diurnal changes were mainly driven by PAR,whereas the seasonal and interannual changes were driven more by Ta and VPD.(2)From 2007 to 2018,the photo synthetic quantum yield(?),maximum photosynthetic efficiency(Amax),and the daytime respiration rate(Rd)in May and September fitted through the rectangular hyperbola function were smaller than those in other months,Amax and Rd maximums occurred in July or August,and the photosynthetic parameters vary between months,which was a result of the combined effect of environmental and biological factors;The Amax in May-September,June,and July,and the Rd in May-September and August have increasing inter-annual fluctuation trends,this interannual changes in photo synthetic parameters were mainly driven by Ta and VPD.Taken together,the photosynthetic parameters of each year and month during the study period(12 years in total)were mainly driven by NDVI.(3)During the growing season of 2016,the time-lag and difference in the PAR measured by the horizontal versus slope-parallel radiometers led to that the PAR values measured by the horizontal radiometer increased the a and Rd,and decreased the Amax;All the Rd,and Amax in the afternoon overall greater than that in the morning.Weather conditions also affected the light response parameters:on cloudy days,the Amax was higher than that in sunny days,the a and Rd were lower versus those in sunny days for most conditions.However,the monthly Amax and Rd were generally higher for horizontally measured PAR than for slope-parallelly measured PAR,especially for the Amax in the cloudy afternoon.Besides,the radiometer-orientation affected the explanation of daytime NEE by Ta and VPD:the correlation of the NEE residual based on the slope-parallel radiometer with Ta and VPD(r ranges:0.013 to 0.197,0.098 to 0.210)was tighter than that based on the horizontal radiometer(r ranges:0.082 to 0.219,0.162 to 0.237)when the negative correlations with Ta for September was excluded.However,there was still substantial errors in rotating the horizontal measured radiation to the inclined plane,which limited to analyze the historical data with horizontally measured radiation.This study indicated that the Maoershan deciduous broad-leaved forest had a strong carbon absorption capacity,and its maximum carbon assimilation capacity(Amax)is at a high end of the temperate forests,and its carbon assimilation capacity has been continuously strengthened from 2007 to 2018.The trend of correlation between NEE and environmental factors,especially Ta and VPD,was also increasing.The measurements of PAR on the inclined terrains could cause errors in the environmental interpretations of NEE,which should be considered for the radiometric measurement of mountain vegetations and the reasonable interpretation of carbon fluxes in terrestrial ecosystems.