Retrieval and validation of vertical LAI profile derived from airborne and spaceborne LiDAR data at a deciduous needleleaf forest site

ABSTRACT

Leaf area index (LAI) is defined as one half of the total green leaf area per unit ground surface area. Its vertical profile is critical for understanding the remote sensing radiative transfer processes. LAI profile has been derived from airborne and spaceborne LiDAR data, such as the Global Ecosystem Dynamics Investigation (GEDI) installed on the International Space Station. However, the capability of various algorithms for the LAI profile estimation with airborne LiDAR is not clearly evaluated, and the estimated LAI profiles, including the GEDI LAI products, are not been fully validated. This study conducted a quantitative retrieval and validation of the LAI profiles using terrestrial and airborne laser scanning (TLS and ALS) and spaceborne GEDI data over a deciduous needleleaf forest site in northern China. The vertical LAI profile was estimated in the field using an upward digital hemispherical photography (DHP) attached to a portable measurement system in 2020 and 2021. A suite of new LiDAR indices combining both LiDAR return number and return intensity was explored for the LAI profile estimation. All LAI profiles obtained from the DHP, TLS, ALS, and GEDI during the leaf-on season and leaf-off season were compared. The DHP shows a good agreement with the TLS LAI profiles (R² = 0.97). The LAI profile derived from the ALS data using the combined light penetration index (LPI_RI) agrees well (R² ≥0.86) with the DHP, TLS, and GEDI estimates. In general, the LPI_RI is advantageous for regional LAI profile mapping from ALS. The GEDI cumulative LAI corresponds well with the DHP during the leaf-on season (R² = 0.90, RMSE = 0.23), but underestimates during the leaf-off season (R² = 0.70, RMSE = 0.14, bias=−0.13). The underestimation is attributed to the higher canopy and ground reflectance ratio (ρ_v/ρ_g) assigned in the algorithm and the height discrepancy between the GEDI and field measurements. For the GEDI LAI profile product, further validation and improvement are necessary for other biome types and landscape conditions, especially during the leaf-off season.

KEYWORDS:

• Leaf area index (LAI)
• vertical profile
• digital hemispherical photography (DHP)
• airborne laser scanning (ALS)
• Global Ecosystem Dynamics Investigation (GEDI)
• terrestrial laser scanning (TLS)

Highlights

• A portable field measurement system was developed to measure the LAI profile.

• The combined light penetration index (LPI_RI) is recommended for the LAI profile estimation from ALS.

• GEDI cumulative LAI performs well during the leaf-on season.

• GEDI cumulative LAI shows an underestimation during the leaf-off season.

Nomenclature

θ	=	Zenith angle
z	=	Height in the canopy
$G\left(\theta \right)$	=	Leaf projection function
J0	=	Transmitted laser pulse energy
$P\left(\theta \right)$	=	Gap fraction at zenith angle θ
$P\left(z\right)$	=	Gap fraction at height z
Iraw	=	The raw return intensity
Icorrected	=	The corrected return intensity
R0	=	The range between the ALS instrument and the return
Rs	=	The standard range (the average flying altitude)
ρg	=	The ground reflectance
ρv	=	The canopy reflectance
Rg	=	The laser energies from the ground return
Rv(0)	=	The laser energies from the canopy top to bottom
Rv(z)	=	The laser energies from the canopy top to height z
Rv1 and Rv2	=	The canopy return energies from two adjacent footprints
Rg1 and Rg2	=	The ground return energies from two adjacent footprints
R2	=	Coefficient of determination
ALS	=	Airborne Laser Scanning
DEM	=	Digital Elevation Model
DHP	=	Digital Hemispherical Photography
GEDI	=	Global Ecosystem Dynamics Investigation
GLAS	=	Geoscience Laser Altimeter System
LAI	=	Leaf Area Index
LAIcum	=	Cumulative Leaf Area Index
LAIi	=	Leaf Area Index for layer i
LiDAR	=	Light Detection and Ranging
PAI	=	Plant Area Index
RMSE	=	Root Mean Square Error
SNFP	=	Saihanba National Forest Park
TLS	=	Terrestrial Laser Scanning
VSP	=	Vertical Sampling Point
WAI	=	Woody Area Index
ABRI	=	Above and Below Ratio Index
ABRIR	=	Above and Below Ratio Index from Returns
ABRIInt	=	Above and Below Ratio Index from return Intensity
ABRIRI	=	Above and Below Ratio Index combining Returns and Intensity
FCI	=	canopy Fractional Cover Index
FCIR	=	canopy Fractional Cover Index from Returns
FCIInt	=	canopy Fractional Cover Index from return Intensity
FCIRI	=	canopy Fractional Cover Index combining Returns and Intensity
LPI	=	Light Penetration Index
LPIR	=	Light Penetration Index from Returns
LPIInt	=	Light Penetration Index from return Intensity
LPIRI	=	Light Penetration Index combining Returns and Intensity

Acknowledgments

This study was mainly supported by the National Natural Science Foundation of China (42171358) and the National Key Research and Development Program of China (2016YFA0600201) to H.F. The TLS data were provided by Dr. Jie Zou, Fuzhou University. The GEDI data are available from the NASA’s Land Processes Distributed Active Archive Center (LP DAAC). Drs. Lu Xu and Lixia Ma helped with the field data collection and the TLS data, respectively. The insightful comments provided by the anonymous reviewers helped improve the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The field measurement data in this study are available from the corresponding author, Y.W., upon reasonable request.

Supplementary Material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/15481603.2023.2214987

Additional information

Funding

The work was supported by the National Natural Science Foundation of China [42171358]; National Key Research and Development Program of China [2016YFA0600201].