| Understandings of the impact of environmental factors on ecosystem processes at different time scales remained limited.Climate models predicted that the frequency of extreme weather events in the northern temperate forests has and will continue to be intensified.Understanding energy partitioning and water vapor exchange,and their relationship with environmental factors in temperate forest ecosystems is very important for the development of adaptive management strategies for regional natural forests.In this study,the eddy-covariance method(EC)was used to continuously monitor the water and heat flux in situ and their environmental factors in a typical natural deciduous broad-leaved mixed forest ecosystem in Songshan,Beijing in 2019.The wavelet analysis was used to analyze the regulation mechanism by biotic and abiotic factors at different time scales.The main results are as follows:(1)The annual average values of air temperature(T_ain?),relative humidity of air(RH in%),vapor pressure deficit(VPD in k Pa),photosynthetically active radiation(PAR in mol/(m~2·day)),and bulk surface conductance(g_s)in 2019 were 7.00?,48.02%,0.64 k Pa,24.79 mol/(m~2·day),and 2.50mm/s,respectively.The annual average values of volumetric water content(VWC in m~3/m~3)at 10-cm depth and normalized difference vegetation index(NDVI)during the growing season were 22.79%and0.62,respectively.The rainfall was unevenly distributed over the season,with 78.4%being from June to September,and with the total annual rainfall of 503.3 mm.(2)The net radiation(R_n),soil heat flux(G),corrected latent heat flux(LE_c),and corrected sensible heat flux(H_c)showed similar diurnal dynamics,with an"inverted U-shape".At a daily scale,the LE_cwas larger than the H_cat noon from June to September.But at a seasonal scale,H_cwas greater than LE_cin the spring,and then H_cdecreased,while LE_cgradually increased,surpassing H_cin the middle growth period.At the end of growing season,LE_csurpassed H_c.The average annual Bowen ratio(?)was 1.53,probably due to the relatively short growing season.G played an important role in ecosystem energy balance at a short time scale(daily scale),but the proportion of soil heat flux in net radiation at a long-term scale(annual scale)was negligible.(3)The evapotranspiration(ET)had obvious seasonal dynamics.The maximum daily evapotranspiration was 5.01 mm/day,and the cumulative annual evapotranspiration was 476.23 mm,the cumulative annual rainfall is 503.3mm.(4)The main influencing factors of the water and heat fluxes varied with time scales,being mostly controlled by VPD at the daily scale,and by PAR at the seasonal scale.At the seasonal scale,PAR was likely to have an indirect impact on ET by affecting VPD and a direct impact on?.In summary,We found that the main factors affecting the water heat flux of natural deciduous broad-leaved forest ecosystem in Songshan are different at different time scales.In addition,we also found that the radiation may indirectly affect the water heat flux of the ecosystem through the leaf water potential.The results can provide scientific basis for evaluating ecosystem services of temperate deciduous broad-leaved forest in climate regulation and water conservation. |
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