TY - JOUR
T1 - Evaluating the performance of PROSPECT in the retrieval of leaf traits across canopy throughout the growing season
AU - Gara, Tawanda W.
AU - Darvishzadeh, Roshanak
AU - Skidmore, Andrew K.
AU - Wang, Tiejun
AU - Heurich, Marco
PY - 2019/11
Y1 - 2019/11
N2 - Leaf traits and subsequently leaf spectral properties depend on the leaf
phenological stage and light conditions within a canopy. The PROSPECT
radiative transfer model has been extensively and successfully used to
retrieve leaf traits for mature, sunlit leaves at peak vegetation
growth, i.e. summer. However, research on the quantification of leaf
traits using PROSPECT across the canopy vertical profile throughout the
growing season is still lacking. Therefore, this study aims at examining
the effect of leaf position on the performance of the PROSPECT model in
modelling leaf optical properties and retrieving leaf chlorophyll
content (Cab), equivalent water thickness (EWT), and leaf
mass per area (LMA) throughout the growing season. To achieve this
objective, we collected 588 leaf samples from the upper and lower
canopies of deciduous stands over three seasons (i.e., spring, summer
and autumn) in Bavaria Forest National Park, Germany. Leaf traits
including Cab, EWT and LMA, were measured for all the
samples, and their reflectance spectra were obtained using an ASD
FieldSpec-3 Pro FR spectroradiometer coupled with an Integrating Sphere.
We initially assessed the performance of the PROSPECT model by
comparing reflectance spectra generated in forward mode against
reflectance spectra measured on leaf samples collected in the field. We
subsequently inverted the PROSPECT model to retrieve Cab, EWT
and LMA using the look-up-table (LUT) approach. Our results
consistently demonstrated that the measured reflectance of leaf samples
collected from the lower canopy had a stronger match with PROSPECT
simulated reflectance spectra, especially in the NIR spectrum compared
to leaf samples collected from the upper canopy throughout the growing
season. This observation concurred with the pattern of Cab
and EWT retrieval accuracies across the canopy i.e. the retrieval
accuracy for the lower canopy was consistently higher (NRMSE = 0.1-0.2
for Cab; NRMSE = 0.125-0.16 for EWT) when compared to the upper canopy (NRMSE = 0.122 - 0.269 for Cab;
NRMSE = 0.162 -0.0.258 for EWT) across all seasons. In contrast, LMA
retrieval accuracies for the upper canopy (NRMSE = 0.146 - 0.184) were
higher compared to the lower canopy (NRMSE = 0.162 - 0.239) for all
seasons except for the spring season. For all the leaf traits examined
in this study, the range in retrieval accuracy between the upper and
lower canopy was greater in summer (compared to other seasons). We
report for the first time that although the PROSPECT model provides
reasonable retrieval accuracy of Cab, EWT and LMA, variations
in leaf biochemistry and morphology through the vertical canopy profile
affects the performance of the model over the growing season. Findings
of this study have important implications on field sampling protocols
and upscaling leaf traits to canopy and landscape level using
multi-layered physical models coupled with PROSPECT.
AB - Leaf traits and subsequently leaf spectral properties depend on the leaf
phenological stage and light conditions within a canopy. The PROSPECT
radiative transfer model has been extensively and successfully used to
retrieve leaf traits for mature, sunlit leaves at peak vegetation
growth, i.e. summer. However, research on the quantification of leaf
traits using PROSPECT across the canopy vertical profile throughout the
growing season is still lacking. Therefore, this study aims at examining
the effect of leaf position on the performance of the PROSPECT model in
modelling leaf optical properties and retrieving leaf chlorophyll
content (Cab), equivalent water thickness (EWT), and leaf
mass per area (LMA) throughout the growing season. To achieve this
objective, we collected 588 leaf samples from the upper and lower
canopies of deciduous stands over three seasons (i.e., spring, summer
and autumn) in Bavaria Forest National Park, Germany. Leaf traits
including Cab, EWT and LMA, were measured for all the
samples, and their reflectance spectra were obtained using an ASD
FieldSpec-3 Pro FR spectroradiometer coupled with an Integrating Sphere.
We initially assessed the performance of the PROSPECT model by
comparing reflectance spectra generated in forward mode against
reflectance spectra measured on leaf samples collected in the field. We
subsequently inverted the PROSPECT model to retrieve Cab, EWT
and LMA using the look-up-table (LUT) approach. Our results
consistently demonstrated that the measured reflectance of leaf samples
collected from the lower canopy had a stronger match with PROSPECT
simulated reflectance spectra, especially in the NIR spectrum compared
to leaf samples collected from the upper canopy throughout the growing
season. This observation concurred with the pattern of Cab
and EWT retrieval accuracies across the canopy i.e. the retrieval
accuracy for the lower canopy was consistently higher (NRMSE = 0.1-0.2
for Cab; NRMSE = 0.125-0.16 for EWT) when compared to the upper canopy (NRMSE = 0.122 - 0.269 for Cab;
NRMSE = 0.162 -0.0.258 for EWT) across all seasons. In contrast, LMA
retrieval accuracies for the upper canopy (NRMSE = 0.146 - 0.184) were
higher compared to the lower canopy (NRMSE = 0.162 - 0.239) for all
seasons except for the spring season. For all the leaf traits examined
in this study, the range in retrieval accuracy between the upper and
lower canopy was greater in summer (compared to other seasons). We
report for the first time that although the PROSPECT model provides
reasonable retrieval accuracy of Cab, EWT and LMA, variations
in leaf biochemistry and morphology through the vertical canopy profile
affects the performance of the model over the growing season. Findings
of this study have important implications on field sampling protocols
and upscaling leaf traits to canopy and landscape level using
multi-layered physical models coupled with PROSPECT.
KW - Leaf traits
KW - Leaf position
KW - Seasonality
KW - PROSPECT model
KW - Retrieval accuracy
UR - http://www.scopus.com/inward/record.url?scp=85072193229&partnerID=8YFLogxK
U2 - 10.1016/j.jag.2019.101919
DO - 10.1016/j.jag.2019.101919
M3 - Article
SN - 1569-8432
VL - 83
SP - 1
EP - 15
JO - International Journal of Applied Earth Observation and Geoinformation
JF - International Journal of Applied Earth Observation and Geoinformation
M1 - 101919
ER -