Sensitivity of common vegetation indices to the canopy structure of field crops

Xiaochen Zou, Matti Mõttus

    Research output: Contribution to journalArticleScientificpeer-review

    7 Citations (Scopus)

    Abstract

    Leaf inclination angle distribution is an important canopy structure characteristic which directly impacts the fraction of the intercepted solar radiation. Together with the leaf area index (LAI) it determines the structure and fractional cover of a homogeneous crop canopy. Unfortunately, this key canopy parameter has usually been ignored when applying common multispectral vegetation indices to the mapping of LAI, although its impact is known from model simulations. Therefore, we measured leaf angles and determined their distribution (quantified using the leaf mean tilt angle, MTA) for six crop species with different structures growing on 162 plots with a broad range of LAI (1.1-5.0) and leaf chlorophyll content (26-94 μg cm-2). Next, we calculated six vegetation indices widely used for LAI monitoring-the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), the two band enhanced vegetation index (EVI2), the modified triangular vegetation index (MTVI2), the optimized soil adjusted vegetation index (OSAVI) and the wide dynamic range vegetation index (WDRVI)-from airborne imaging spectroscopy data. We calculated the Spearman's correlation coefficient Rs, a non-parametric statistic chosen because of the non-normal distribution of canopy parameters. All studied indices depended on the LAI (0.50 ≤ Rs ≤ 0.71), but the dependence on the MTA was of similar magnitude (-0.83 ≤ Rs ≤ -0.53) with EVI, EVI2, OSAVI and MTVI2 depending more strongly on MTA than on LAI. All studied indices were good proxies (0.78 ≤ Rs ≤ 0.88) for vegetation fractional cover (Fcover) which, for homogeneous crop canopies, is a nonlinear function of LAI and MTA. EVI2 and MTVI2 were the most strongly correlated with Fcover, although the difference to the other studied indices was small. This first study involving a large range of crop structures confirms the results from canopy reflectance simulations and emphasizes the necessity of leaf angle information for the successful mapping of LAI with Earth observation data.

    Original languageEnglish
    Article number994
    JournalRemote Sensing
    Volume9
    Issue number10
    DOIs
    Publication statusPublished - 1 Oct 2017
    MoE publication typeA1 Journal article-refereed

    Fingerprint

    vegetation index
    leaf area index
    canopy
    crop
    tilt
    canopy reflectance
    NDVI
    simulation
    solar radiation
    chlorophyll
    soil
    spectroscopy
    vegetation
    monitoring

    Keywords

    • Canopy structure
    • EVI
    • EVI2
    • Imaging spectroscopy
    • Leaf angle distribution
    • Mean tilt angle
    • MTVI2
    • NDVI
    • OSAVI
    • PROSAIL
    • WDRVI

    Cite this

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    title = "Sensitivity of common vegetation indices to the canopy structure of field crops",
    abstract = "Leaf inclination angle distribution is an important canopy structure characteristic which directly impacts the fraction of the intercepted solar radiation. Together with the leaf area index (LAI) it determines the structure and fractional cover of a homogeneous crop canopy. Unfortunately, this key canopy parameter has usually been ignored when applying common multispectral vegetation indices to the mapping of LAI, although its impact is known from model simulations. Therefore, we measured leaf angles and determined their distribution (quantified using the leaf mean tilt angle, MTA) for six crop species with different structures growing on 162 plots with a broad range of LAI (1.1-5.0) and leaf chlorophyll content (26-94 μg cm-2). Next, we calculated six vegetation indices widely used for LAI monitoring-the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), the two band enhanced vegetation index (EVI2), the modified triangular vegetation index (MTVI2), the optimized soil adjusted vegetation index (OSAVI) and the wide dynamic range vegetation index (WDRVI)-from airborne imaging spectroscopy data. We calculated the Spearman's correlation coefficient Rs, a non-parametric statistic chosen because of the non-normal distribution of canopy parameters. All studied indices depended on the LAI (0.50 ≤ Rs ≤ 0.71), but the dependence on the MTA was of similar magnitude (-0.83 ≤ Rs ≤ -0.53) with EVI, EVI2, OSAVI and MTVI2 depending more strongly on MTA than on LAI. All studied indices were good proxies (0.78 ≤ Rs ≤ 0.88) for vegetation fractional cover (Fcover) which, for homogeneous crop canopies, is a nonlinear function of LAI and MTA. EVI2 and MTVI2 were the most strongly correlated with Fcover, although the difference to the other studied indices was small. This first study involving a large range of crop structures confirms the results from canopy reflectance simulations and emphasizes the necessity of leaf angle information for the successful mapping of LAI with Earth observation data.",
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    author = "Xiaochen Zou and Matti M{\~o}ttus",
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    Sensitivity of common vegetation indices to the canopy structure of field crops. / Zou, Xiaochen; Mõttus, Matti.

    In: Remote Sensing, Vol. 9, No. 10, 994, 01.10.2017.

    Research output: Contribution to journalArticleScientificpeer-review

    TY - JOUR

    T1 - Sensitivity of common vegetation indices to the canopy structure of field crops

    AU - Zou, Xiaochen

    AU - Mõttus, Matti

    N1 - Project code: 113325

    PY - 2017/10/1

    Y1 - 2017/10/1

    N2 - Leaf inclination angle distribution is an important canopy structure characteristic which directly impacts the fraction of the intercepted solar radiation. Together with the leaf area index (LAI) it determines the structure and fractional cover of a homogeneous crop canopy. Unfortunately, this key canopy parameter has usually been ignored when applying common multispectral vegetation indices to the mapping of LAI, although its impact is known from model simulations. Therefore, we measured leaf angles and determined their distribution (quantified using the leaf mean tilt angle, MTA) for six crop species with different structures growing on 162 plots with a broad range of LAI (1.1-5.0) and leaf chlorophyll content (26-94 μg cm-2). Next, we calculated six vegetation indices widely used for LAI monitoring-the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), the two band enhanced vegetation index (EVI2), the modified triangular vegetation index (MTVI2), the optimized soil adjusted vegetation index (OSAVI) and the wide dynamic range vegetation index (WDRVI)-from airborne imaging spectroscopy data. We calculated the Spearman's correlation coefficient Rs, a non-parametric statistic chosen because of the non-normal distribution of canopy parameters. All studied indices depended on the LAI (0.50 ≤ Rs ≤ 0.71), but the dependence on the MTA was of similar magnitude (-0.83 ≤ Rs ≤ -0.53) with EVI, EVI2, OSAVI and MTVI2 depending more strongly on MTA than on LAI. All studied indices were good proxies (0.78 ≤ Rs ≤ 0.88) for vegetation fractional cover (Fcover) which, for homogeneous crop canopies, is a nonlinear function of LAI and MTA. EVI2 and MTVI2 were the most strongly correlated with Fcover, although the difference to the other studied indices was small. This first study involving a large range of crop structures confirms the results from canopy reflectance simulations and emphasizes the necessity of leaf angle information for the successful mapping of LAI with Earth observation data.

    AB - Leaf inclination angle distribution is an important canopy structure characteristic which directly impacts the fraction of the intercepted solar radiation. Together with the leaf area index (LAI) it determines the structure and fractional cover of a homogeneous crop canopy. Unfortunately, this key canopy parameter has usually been ignored when applying common multispectral vegetation indices to the mapping of LAI, although its impact is known from model simulations. Therefore, we measured leaf angles and determined their distribution (quantified using the leaf mean tilt angle, MTA) for six crop species with different structures growing on 162 plots with a broad range of LAI (1.1-5.0) and leaf chlorophyll content (26-94 μg cm-2). Next, we calculated six vegetation indices widely used for LAI monitoring-the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), the two band enhanced vegetation index (EVI2), the modified triangular vegetation index (MTVI2), the optimized soil adjusted vegetation index (OSAVI) and the wide dynamic range vegetation index (WDRVI)-from airborne imaging spectroscopy data. We calculated the Spearman's correlation coefficient Rs, a non-parametric statistic chosen because of the non-normal distribution of canopy parameters. All studied indices depended on the LAI (0.50 ≤ Rs ≤ 0.71), but the dependence on the MTA was of similar magnitude (-0.83 ≤ Rs ≤ -0.53) with EVI, EVI2, OSAVI and MTVI2 depending more strongly on MTA than on LAI. All studied indices were good proxies (0.78 ≤ Rs ≤ 0.88) for vegetation fractional cover (Fcover) which, for homogeneous crop canopies, is a nonlinear function of LAI and MTA. EVI2 and MTVI2 were the most strongly correlated with Fcover, although the difference to the other studied indices was small. This first study involving a large range of crop structures confirms the results from canopy reflectance simulations and emphasizes the necessity of leaf angle information for the successful mapping of LAI with Earth observation data.

    KW - Canopy structure

    KW - EVI

    KW - EVI2

    KW - Imaging spectroscopy

    KW - Leaf angle distribution

    KW - Mean tilt angle

    KW - MTVI2

    KW - NDVI

    KW - OSAVI

    KW - PROSAIL

    KW - WDRVI

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    DO - 10.3390/rs9100994

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