Biophysical Regulation Mechanisms Of Gross Primary Productivity And Light And Water Use Efficiency For Temperate Forest Ecosystems

Global climate change and frequent extreme climate events significantly affect terrestrial ecosystem functions and resource use efficiency.Temperate forests play a crucial role in regulating global carbon and water cycles,mitigating global climate change and biomass energy production.However,the regulatory mechanisms of multiple interacting biophysical factors on productivity and resource use efficiencies(i.e.,water use efficiency and light use efficiency)in temperate forests are still unclear.In this study,we first conducted the long-term localized observations of carbon,water,and energy fluxes,and environmental elements from 2014 to 2017 over a poplar(Populus×euramericana)plantation in northern China using the eddy covariance technique and microclimate observations to investigate the impact mechanisms of various biophysical factors on ecosystem gross primary productivity(GPP)and light and water use efficiency.In addition,this study compared the regulatory mechanisms of biophysical factors for nine temperate ecosystems using the global flux network(FLUXNET)dataset during extreme heat conditions;62 temperate forest sites were selected to explore the long-term dynamics and regulatory mechanisms of light and water use efficiency in temperate forest ecosystems.The results are as follows:(1)Diffuse radiation is the primary driver for regulating the photosynthesis of the poplar plantation in northern China.The GPP of the poplar plantation peaked when the clearness index was between 0.45 and 0.65,at which point diffuse radiation reached its maximum.Besides,the decrease in cloudiness increased the light use efficiency of the poplar plantation.(2)Canopy conductance(Gc)is the most crucial biophysical factor regulating water consumption and energy partitioning of the poplar plantation in northern China.The reduction of diffuse radiation increased the proportion of latent heat.The rate of stomatal sensitivity and reference canopy conductance was 0.37 mol m-2 s-1 ln(kPa)-1 and 0.49 mol m-2 s-1 ln(kPa)-1 during the clear sky and cloudy sky conditions,respectively,which indicated that this poplar plantation lived with an anisohydric behavior.Also,this poplar plantation with a loose water use strategy under clear skies would consume more water compared with cloudy skies.(3)Gc was the dominant factor regulating water use efficiency(WUE)of the poplar plantation in spring and autumn.The fraction of diffuse radiation(fDPAR)was the primary controlling variable in summer;fDPAR enhanced WUE by depressing evapotranspiration(ET)and jointly promoting GPP.This plantation could utilize diffuse radiation efficiently by regulating stomatal function when fDPAR was lower than 0.6.At the canopy scale,the stomatal optimization theory did not work at vapor pressure deficit(VPD)>3 kPa.(4)During extreme heat conditions,the variation of soil water was the primary variable responsible for the changes in GPP during heatwaves across all nine temperate ecosystems.Compared with other ecosystems,forest ecosystems were more resistant and resilient to extreme heat events.In addition,the diurnal patterns of GPP and ET were asymmetric during periods of extremely high temperature and ecosystem functions were more sensitive to extreme heat in the afternoon.(5)In the long run,light and water use efficiency of evergreen forests gradually declined after maturity;while they continuously increased with age for mature deciduous forests based on the data of this study.Canopy photo synthetic capacity was the main cause of long-term dynamics in light and water use efficiency across temperate forest ecosystems.