The Water Use Efficiency And Its Driving Factors In Larch Forest

While working as an important index for revealing the interrelationship between forest productivity and forest water consumption,water use efficiency is also a key factor for carbon-water coupling research.Taking larch(Larix gmelinii Rupr.) forests as the research object,this paper measured the carbon flux and water vapor flux through eddy covariance technique to calculate the water use efficiency.Environmental factors were monitored through the meteorological observation tower and the least square method and the energy balance ratio method were adopted to calculate the energy closure.Dynamic characteristics and driving factors of the Larch forest water use efficiency were analyzed by utilizing the Pearson correlation analysis method and stepwise regression analysis,so as to provide theoretical support for the research of carbon-water coupling process.The results show that:(1)The energy closure was 0.6.linear regression analysis of the energy data was implemented through the least square method;the average intercept during the growing season was-12.95,the average slop was 0.61 and the determination coefficient(R2) was 0.76.It Prove that the data quality of the site is reliable.(2)The total sensible heat flux was 328.21kW·m-2,and the total latent heat flux was 454.75kW·m-2 in the growing season.The total amount of canopy heat reserves and soil heat flux in growing season is small,and the energy distribution in growing season of Larch forest is mainly used for latent heat flux.Among the ratios of each component to net radiation,total latent heat flux growth season is about 35% of total net radiation,which is higher than other components.Energy is used for tree life activities throughout the growing season.(3)The daily variation of Larch forest water use efficiency takes a single-peaked inverted U-type distribution and the starting and stopping time of water use efficiency in different months during the growing season 1s relatively consistent.Obvious fluctuations could be observed around 5 o'clock every day;then a rise appears around 6 o'clock,reaches to the peak from 10 to 12 o'clock with small fluctuations existing in certain ranges,and gradually declines to 18 o'clock.The water use efficiency from May to September is divided into-O.041mg·g-1,0.028mg·g-1,0.017mg·g-1,-0.034mg·g-1 and-0.062mg·g-1.(4)The driving factors of water use efficiency of Larch forest were studied.It was found that the response of water use efficiency to diurnal variation of total radiation was the strongest,and the response of water use efficiency to mouthly variation of air humidity at 60 m was more obvious.This indicated that the response of water use efficiency to soil factors was weak in Larch forest.Through stepwise regression analysis,the response equations of water use efficiency to diurnal and temporal changes of environmental factors are obtained as follows:yWUE=0.001xSR-1.692E-6x2SR+2.5 18E-5x3Ta42m-4.738E-6x3TS-3.428E-5x3GS-0.077,yWUE=-4.636E-5x3GS+0.001xSR-1.469E-7x3RH42m-0.042.