Measuring Canopy Phenology And Leaf Area Index In The Deciduous Broadleaved Forest At Maoershan Site

Near-surface remote sensing provides a novel approach to monitor canopy phenology and leaf area index（LAI）,it plays an important role for upscaling from ground observation to satellite remote sensing.Using the temperate broadleaved deciduous forest around the Maoershan flux tower in Northeast China as a case study,We quantified the effects of inclination angle（angle from horizon）and azimuth angle on vegetation indices（VI）from digital camera,hyper-spectrometer,and routine radiometer,then compared the differences in VI measured with three types of sensors on a flux tower.Finally,using the LAI based on litterfall collection method as a reference,we assessed the feasibility and advantage of tower-based VI for monitoring canopy LAI.The results indicated that:（1）All VIs from digital camera and spectrometer increased with inclination angle increasing in the spring expansion period,while they were roughly consistant between inclination angles in the autumn abscission period.The start of season（SOS）from four inclination angles advanced 1⁹ d for higher inclination angles;the end of season（EOS）varied within 5 d.The extension of growing season length（LOS）for higher inclination angle was mainly related to the contribution of understory to VI.In contrust,the inclination angles of radiometers did not affect the key phenophases.If we used a digital camera to tracking the canopy phenology at one site,we recommended that using 30⁴5°inclination angle were more appropriate,and we should separate layers to monitor understory and overstory.（2）Among azimuth angles,the LOSs for east and south sectors（the upper slope）was longer than those for the other directions（the lower slope）,and the interannual differences were distinct,indicating significant influences of complex terrian and composition of tree species on in-situ canopy phenology.Facing to the azimuth matched with footprint of CO₂ flux was more suitable.The integration of in-situ canopy phenology across sites should consider the effects of azimuth angles.（3）Because of the differences of spectral resolution and field of view among sensors,the differences of SOSs,EOSs,and LOSs from different sensors were 0¹0,0¹1,and 0¹2 d,respectively.Comparison of three methods,the routine radiometers could be widely used in tracking long-term continuous forest canopy phenology,but digital camera has the advantages of identifying and individuals and seperating different layers.（4）The normalized difference vegetation indice（NDVI_Bb）based routine radiameter was closely correlated linearly with the LAI derived from the litterfall collection method.The NDVI_Bb had the advantage in tracking the seasonal dynamics in LAI and extrapolating LAI to a broader scale.Given that most eddy flux towers havd equipped with energy balance measurements,a network of monitoring canopy LAI could be readily achieved if the reflectance of photosynthetically active radiation was measured synchronously.It has wide application prospects.