Effects of Crop Leaf Angle on LAI-Sensitive Narrow-Band Vegetation Indices Derived from Imaging Spectroscopy

Xiaochen Zou,Matti Mõttus,Petri Pellikka,Pirjo Mäkelä,Iikka Haikarainen,Iina Haikarainen

doi:10.3390/app8091435

Abstract

Leaf area index (LAI) is an important biophysical variable for understanding the radiation use efficiency of field crops and their potential yield. On a large scale, LAI can be estimated with the help of imaging spectroscopy. However, recent studies have revealed that the leaf angle greatly affects the spectral reflectance of the canopy and hence imaging spectroscopy data. To investigate the effects of the leaf angle on LAI-sensitive narrowband vegetation indices, we used both empirical measurements from field crops and model-simulated data generated by the PROSAIL canopy reflectance model. We found the relationship between vegetation indices and LAI to be notably affected, especially when the leaf mean tilt angle (MTA) exceeded 70 degrees. Of the indices used in the study, the modified soil-adjusted vegetation index (MSAVI) was most strongly affected by leaf angles, while the blue normalized difference vegetation index (BNDVI), the green normalized difference vegetation index (GNDVI), the modified simple ratio using the wavelength of 705 nm (MSR705), the normalized difference vegetation index (NDVI), and the soil-adjusted vegetation index (SAVI) were only affected for sparse canopies (LAI < 3) and MTA exceeding 60°. Generally, the effect of MTA on the vegetation indices increased as a function of decreasing LAI. The leaf chlorophyll content did not affect the relationship between BNDVI, MSAVI, NDVI, and LAI, while the green atmospherically resistant index (GARI), GNDVI, and MSR705 were the most strongly affected indices. While the relationship between SR and LAI was somewhat affected by both MTA and the leaf chlorophyll content, the simple ratio (SR) displayed only slight saturation with LAI, regardless of MTA and the chlorophyll content. The best index found in the study for LAI estimation was BNDVI, although it performed robustly only for LAI > 3 and showed considerable nonlinearity. Thus, none of the studied indices were well suited for across-species LAI estimation: information on the leaf angle would be required for remote LAI measurement, especially at low LAI values. Nevertheless, narrowband indices can be used to monitor the LAI of crops with a constant leaf angle distribution.

Highlights

Leaf area index (LAI) is an important biophysical variable that indicates the radiation absorption and photosynthetic capacity of a crop canopy [1,2]
The value of τ k lies between −1 and 1, with τ k = −1 indicating a perfect negative between species, we could analyze sensitivity of the Vegetation indices (VIs)–LAI relationship to mean tilt angle (MTA) in the fieldcorrelation between the not paired data, τthe k = 0 the lack of a relationship and τ k = 1 a perfect positive measured dataset
The accuracy of the indices in estimating LAI was restricted by the variation in MTA and

Summary

Introduction

Leaf area index (LAI) is an important biophysical variable that indicates the radiation absorption and photosynthetic capacity of a crop canopy [1,2]. LAI is defined as one half of the total green leaf area per unit of horizontal ground area [3]. It is a unitless measure, units of m2 /m2 are often quoted. The determination of LAI, or its temporal course, allows an understanding of ongoing biophysical processes and the prediction of plant growth and, crop productivity. In situ measurement of LAI is time consuming and cannot be operationally applied to large areas

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