The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology

Plant functional traits play a key role in the assessment of ecosystem processes and properties. Optical remote sensing is ascribed a high potential in capturing those traits and their spatiotemporal patterns. In vegetation remote sensing, reflectance-based retrieval methods are either statistical (...

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Autores principales: Felix Schiefer, Sebastian Schmidtlein, Teja Kattenborn
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Publicado: Elsevier 2021
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spelling oai:doaj.org-article:49ec9bed3fbb4676836159e6153f7afd2021-12-01T04:33:54ZThe retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology1470-160X10.1016/j.ecolind.2020.107062https://doaj.org/article/49ec9bed3fbb4676836159e6153f7afd2021-02-01T00:00:00Zhttp://www.sciencedirect.com/science/article/pii/S1470160X20310013https://doaj.org/toc/1470-160XPlant functional traits play a key role in the assessment of ecosystem processes and properties. Optical remote sensing is ascribed a high potential in capturing those traits and their spatiotemporal patterns. In vegetation remote sensing, reflectance-based retrieval methods are either statistical (relying on empirical observations) or physically-based (based on inversions of a radiative transfer model, RTM). Both trait retrieval approaches remain poorly investigated regarding phenology. However, within the phenology of a plant, its leaf constituents, canopy structure, and the presence of phenology-related organs (i.e., flowers or inflorescence) vary considerably – and so does its reflectance. We, therefore, addressed the question of how plant phenology affects the predictive performance of both statistical and RTM-based methods and how this effect differs between traits. For a complete growing season, we weekly measured traits of 45 herbaceous plant species together with hyperspectral canopy reflectance (ASD FieldSpec III). Plants were grown in an experimental setup. The investigated traits comprised Leaf Area Index (LAI) and the leaf traits chlorophyll, anthocyanins, carotenoids, equivalent water thickness, and leaf mass per area. We compared the predictive performances of PLSR models and three variants of PROSAIL inversions based on (1) all observations and based on (2) a phenological subset where flowering plants were excluded and only those observations most suitable for modeling were kept. Our results show that both statistical and RTM-based trait retrievals were largely affected by phenology. For carotenoids for example, R2 decreased from 0.58 at non-flowering canopies to 0.25 at 100% flowering canopies. Temporal trends were diverse. LAI and equivalent water thickness were best estimated earlier in the growing season; chlorophyll and carotenoids towards senescence. PLSR models showed generally higher bias than the PROSAIL-based retrieval approaches. Lookup-table inversion of PROSAIL in combination with a continuous wavelet transformation of reflectance showed highest accuracies. We found RTM-based retrieval not to be as accurate and transferable as previously indicated. Our results suggest that phenology is essential for accurate retrieval of plant functional traits and varies depending on the studied species and functional traits, respectively.Felix SchieferSebastian SchmidtleinTeja KattenbornElsevierarticleVegetation remote sensingLeaf traitsPROSAILPartial least squares regressionFloweringRadiative transfer modelEcologyQH540-549.5ENEcological Indicators, Vol 121, Iss , Pp 107062- (2021)
institution DOAJ
collection DOAJ
language EN
topic Vegetation remote sensing
Leaf traits
PROSAIL
Partial least squares regression
Flowering
Radiative transfer model
Ecology
QH540-549.5
spellingShingle Vegetation remote sensing
Leaf traits
PROSAIL
Partial least squares regression
Flowering
Radiative transfer model
Ecology
QH540-549.5
Felix Schiefer
Sebastian Schmidtlein
Teja Kattenborn
The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology
description Plant functional traits play a key role in the assessment of ecosystem processes and properties. Optical remote sensing is ascribed a high potential in capturing those traits and their spatiotemporal patterns. In vegetation remote sensing, reflectance-based retrieval methods are either statistical (relying on empirical observations) or physically-based (based on inversions of a radiative transfer model, RTM). Both trait retrieval approaches remain poorly investigated regarding phenology. However, within the phenology of a plant, its leaf constituents, canopy structure, and the presence of phenology-related organs (i.e., flowers or inflorescence) vary considerably – and so does its reflectance. We, therefore, addressed the question of how plant phenology affects the predictive performance of both statistical and RTM-based methods and how this effect differs between traits. For a complete growing season, we weekly measured traits of 45 herbaceous plant species together with hyperspectral canopy reflectance (ASD FieldSpec III). Plants were grown in an experimental setup. The investigated traits comprised Leaf Area Index (LAI) and the leaf traits chlorophyll, anthocyanins, carotenoids, equivalent water thickness, and leaf mass per area. We compared the predictive performances of PLSR models and three variants of PROSAIL inversions based on (1) all observations and based on (2) a phenological subset where flowering plants were excluded and only those observations most suitable for modeling were kept. Our results show that both statistical and RTM-based trait retrievals were largely affected by phenology. For carotenoids for example, R2 decreased from 0.58 at non-flowering canopies to 0.25 at 100% flowering canopies. Temporal trends were diverse. LAI and equivalent water thickness were best estimated earlier in the growing season; chlorophyll and carotenoids towards senescence. PLSR models showed generally higher bias than the PROSAIL-based retrieval approaches. Lookup-table inversion of PROSAIL in combination with a continuous wavelet transformation of reflectance showed highest accuracies. We found RTM-based retrieval not to be as accurate and transferable as previously indicated. Our results suggest that phenology is essential for accurate retrieval of plant functional traits and varies depending on the studied species and functional traits, respectively.
format article
author Felix Schiefer
Sebastian Schmidtlein
Teja Kattenborn
author_facet Felix Schiefer
Sebastian Schmidtlein
Teja Kattenborn
author_sort Felix Schiefer
title The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology
title_short The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology
title_full The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology
title_fullStr The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology
title_full_unstemmed The retrieval of plant functional traits from canopy spectra through RTM-inversions and statistical models are both critically affected by plant phenology
title_sort retrieval of plant functional traits from canopy spectra through rtm-inversions and statistical models are both critically affected by plant phenology
publisher Elsevier
publishDate 2021
url https://doaj.org/article/49ec9bed3fbb4676836159e6153f7afd
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