| Usually,leaf area index(LAI)is defifined as one half the total leaf surface area per unit ground surface area.In agriculture,LAI can help to understand the growth of crops,guide field fertilization and efficient management.Ecologically,LAI is a key parameter for the study of ecosystem processes,logistics and energy flow between systems.At present,the methods of measuring LAI are mainly divided into direct measurement and indirect measurement.Direct measurement mainly includes leaf collection process and leaf area measurement technology,which is a very high accuracy method and can judge the reliability of indirect measurement.Because of its time-consuming and labor-intensive characteristics and other operational limitations,it is not commonly used in practice.Indirect measurement is to estimate LAI with reference to another variable,which mainly includes inclined point quadrat,allometric techniques,radiometric method and gap fraction inversion.It has the advantages of faster speed,convenient use and easy automation.Measuring LAI based on hemispherical canopy photography belongs to the gap fraction inversion,which is a technology to study the canopy structure through the images collected by camera equipped with fisheye lens.In this paper,hemispherical canopy photography system,LAI Data Acquisition and Transmission Instrument,was used to collecte more than 40 groups of canopy images form four sample plots with tall trees.Then a series of digital image processing methods were used to obtain the canopy gap fraction,and finally the gap fraction was brought into the LAI inversion model to obtain the LAI.After comparing the inversion results with LAI-2200 C,it is concluded that the method of measuring leaf area index based on hemispheric canopy photography is feasible and scientific.The details are as follows:(1)A threshold segmentation method suitable for hemispheric canopy image was developed.The key to the accuracy of gap rate extraction based on hemispherical canopy photography lies in the performance of threshold segmentation algorithm.The effectiveness of the algorithm is verified by visual inspection and quantitative analysis.(2)The inversion method of leaf area index was studied.This paper focuses on the radiometric method and gap inversion method.It is found that the extinction coefficient of radiometric method is not a constant like the theoretical value,which leads to the difficulty of practical application.For gap fraction inversion,only canopy gap fraction is needed when estimating LAI.Therefore,it is determined to use the gap fraction inversion method represented by hemispheric canopy photography to calculate leaf area index.(3)To verify the scientificity and feasibility of hemispheric canopy photography.The canopy images were collected and measured simultaneously with LAI-2200 C.The correlation coefficient is more than 0.89,and the corrected results are in good agreement with LAI-2200 C,which proves that hemispheric canopy photography is an effective method to measure LAI. |
