The Dynamics Of Leaf Area Index Of Typical Vegetation And Its Effect On Canopy Interception In Water-wind Erosion Crisscross Region

The automatic and accurate measurement of LAI is of great importance for the research of ecological processes.In the study,first,the digital hemispherical photography?DHP?and LAI-2200 were used to measure the canopy LAI of different growth stages of typical land in the northern Loess Plateau,including C.korshinskii,S.psammophila,alfalfa,S.bungeana,soybean and maize.And the accuracy of the optical fisheye lens is verified by comparing the two measurements.Second,photos were daily taken by fisheye webcam and processed by CAN-EYE software to obtain the dynamics of LAI in soybean,maize and C.korshinskii fields.Meanwhile,these measurements were compared with data obtained using the LAI-2200,?DHP?,and destructive sampling measurements?LI-3000A?.Finally,the fisheye webcam method is used to measure the influence mechanism of LAI on the canopy interception of shrubs.Taking C.korshinskii and S.psammophila fields as research objects,the rainfall redistribution patterns of the two fields in the natural growth state of 2017-2018 were measured to clarify the main controlling factors of the two shrub interceptions.And different LAI levels were set by artificially for the three S.psammophila fields,and the effect of LAI on canopy interception under the same meteorological conditions was compared.Main findings of this study are listed as follows:?1?The results showed that a linear correlation existed between the LAI measured by DHP and LAI-2200.The coefficient of determination?R²?was 0.85?P<0.05?and root mean square error?RMSE?was 0.256.The key parameters of professional software were affected by the solar radiation when taking pictures.When the downward webcam was used,the green index was the key parameter which increased with the increase of solar radiation.However,the brightness index decreased with the increase of solar radiation when the webcam was upward.Through the adjustment of the key parameters,the results of LAI of maize,soybean,and C.korshinskii were consistent with the LAI-2200 results,well reflecting LAI dynamics during the plant growth.The fisheye webcam could be used for monitoring the dynamic LAI of different vegetations.?2?In this study,an approach was proposed and evaluated to continuously monitor daily LAI in broadleaf crops?maize and soybean?and xerophytic shrub?C.korshinskii?using fisheye webcam.Fisheye webcam shows great potential for continuous LAI measurement in agricultural crop and shrub fields.As the upward-pointing fisheye webcams originally derived PAI_eff contains YAI is usually unreliable for maize during the senescent period?y=0.24x,RMSE=0.79?,remove senescent parts from fisheye photos was first reported in this study,which yielded a good agreement with LAI values from destructive method?y=0.75x,RMSE=0.37?.The downward webcam allows a more wider spatial representativeness and is suitable for low vegetation,such as soybean,alfalfa and S.bungeana.In contrast,the upward webcam is more suitable for tall vegetation,such as long-stalked crops?maize?,shrub and arbor,because of the small classification uncertainties of the upward photos.Potentially,the wood area index?WAI?can be estimated from the upward fisheye photos by selecting different classify pixels using CAN-EYE software.The fisheye webcam can achieve monitor canopy structural variables such as LAI,GF,and CI in an accurate,inexpensive,easy and fast manner.Further evaluation of this method should be made,in particular for dense canopies.?3?The interception increased with the increase of rainfall for C.korshinskii tree stand,and the steady interception rate was about 15%.Comparatively,the interception rate was relative low for S.psammophila tree stand and the steady interception rate was about 10%.For S.psammophila tree stand,when the LAI level is high?1.7?,the throuhfall is the least,the stemflow is in the middle,and the interception is the most;when the LAI is at the middle level?1.3?,the throuhfall is centered,the stemflow is the most,and the interception is the least;when the LAI is low?1.0?,the throuhfall is the most,the stemflow is in the least,and the interception is the centered.This indicates that the interception of S.psammophila is greatly affected by the stemflow.When the LAI level is low,the stemflow is the least,and it is inferred that the leaf has a positive effect on the stem flow,while compared with the high level of LAI,the stemflow is larger at the level of LAI,and the leaf seems to have a negative effect on the stemflow.The appropriate LAI could reduce the interception evaporation and increase the stemflow.