Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe

Leaf area index (LAI) is a key ecological indicator for describing the structure of canopies and for modelling energy exchange between atmosphere and biosphere. While LAI of the forest overstory can be accurately assessed over large spatial scales via remote sensing, LAI of the forest understory (LA...

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Autores principales:	Jan-Peter George, Wei Yang, Hideki Kobayashi, Tobias Biermann, Arnaud Carrara, Edoardo Cremonese, Matthias Cuntz, Silvano Fares, Giacomo Gerosa, Thomas Grünwald, Niklas Hase, Michael Heliasz, Andreas Ibrom, Alexander Knohl, Bart Kruijt, Holger Lange, Jean-Marc Limousin, Denis Loustau, Petr Lukeš, Riccardo Marzuoli, Meelis Mölder, Leonardo Montagnani, Johan Neirynck, Matthias Peichl, Corinna Rebmann, Marius Schmidt, Francisco Ramon Lopez Serrano, Kamel Soudani, Caroline Vincke, Jan Pisek
Formato:	article
Lenguaje:	EN
Publicado:	Elsevier 2021
Materias:	Forest background reflectivity Fractional vegetation cover Leaf area index NDVI Simple ratio Diversity Ecology QH540-549.5
Acceso en línea:	https://doaj.org/article/5e0392aefe9a42f596044ebf1d3cda6d
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spelling	oai:doaj.org-article:5e0392aefe9a42f596044ebf1d3cda6d2021-12-01T04:54:18ZMethod comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe1470-160X10.1016/j.ecolind.2021.107841https://doaj.org/article/5e0392aefe9a42f596044ebf1d3cda6d2021-09-01T00:00:00Zhttp://www.sciencedirect.com/science/article/pii/S1470160X21005069https://doaj.org/toc/1470-160XLeaf area index (LAI) is a key ecological indicator for describing the structure of canopies and for modelling energy exchange between atmosphere and biosphere. While LAI of the forest overstory can be accurately assessed over large spatial scales via remote sensing, LAI of the forest understory (LAIu) is still largely ignored in ecological studies and ecosystem modelling due to the fact that it is often too complex to be destructively sampled or approximated by other site parameters. Additionally, so far only few attempts have been made to retrieve understory LAI via remote sensing, because dense canopies with high LAI are often hindering retrieval algorithms to produce meaningful estimates for understory LAI. Consequently, the forest understory still constitutes a poorly investigated research realm impeding ecological studies to properly account for its contribution to the energy absorption capacity of forest stands. This study aims to compare three conceptually different indirect retrieval methodologies for LAIu over a diverse panel of forest understory types distributed across Europe. For this we carried out near-to-surface measurements of understory reflectance spectra as well as digital surface photography over the extended network of Integrated Carbon Observation System (ICOS) forest ecosystem sites. LAIu was assessed by exploiting the empirical relationship between vegetation cover and light absorption (Beer-Lambert- Bouguer law) as well as by utilizing proposed relationships with two prominent vegetation indices: normalized difference vegetation index (NDVI) and simple ratio (SR). Retrievals from the three methods were significantly correlated with each other (r = 0.63–0.99, RMSE = 0.53–0.72), but exhibited also significant bias depending on the LAI scale. The NDVI based retrieval approach most likely overestimates LAI at productive sites when LAIu > 2, while the simple ratio algorithm overestimates LAIu at sites with sparse understory vegetation and presence of litter or bare soil. The purely empirical method based on the Beer-Lambert law of light absorption seems to offer a good compromise, since it provides reasonable LAIu values at both low and higher LAI ranges. Surprisingly, LAIu variation among sites seems to be largely decoupled from differences in climate and light permeability of the overstory, but significantly increased with vegetation diversity (expressed as species richness) and hence proposes new applications of LAIu in ecological modelling.Jan-Peter GeorgeWei YangHideki KobayashiTobias BiermannArnaud CarraraEdoardo CremoneseMatthias CuntzSilvano FaresGiacomo GerosaThomas GrünwaldNiklas HaseMichael HeliaszAndreas IbromAlexander KnohlBart KruijtHolger LangeJean-Marc LimousinDenis LoustauPetr LukešRiccardo MarzuoliMeelis MölderLeonardo MontagnaniJohan NeirynckMatthias PeichlCorinna RebmannMarius SchmidtFrancisco Ramon Lopez SerranoKamel SoudaniCaroline VinckeJan PisekElsevierarticleForest background reflectivityFractional vegetation coverLeaf area indexNDVISimple ratioDiversityEcologyQH540-549.5ENEcological Indicators, Vol 128, Iss , Pp 107841- (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Forest background reflectivity Fractional vegetation cover Leaf area index NDVI Simple ratio Diversity Ecology QH540-549.5
spellingShingle	Forest background reflectivity Fractional vegetation cover Leaf area index NDVI Simple ratio Diversity Ecology QH540-549.5 Jan-Peter George Wei Yang Hideki Kobayashi Tobias Biermann Arnaud Carrara Edoardo Cremonese Matthias Cuntz Silvano Fares Giacomo Gerosa Thomas Grünwald Niklas Hase Michael Heliasz Andreas Ibrom Alexander Knohl Bart Kruijt Holger Lange Jean-Marc Limousin Denis Loustau Petr Lukeš Riccardo Marzuoli Meelis Mölder Leonardo Montagnani Johan Neirynck Matthias Peichl Corinna Rebmann Marius Schmidt Francisco Ramon Lopez Serrano Kamel Soudani Caroline Vincke Jan Pisek Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe
description	Leaf area index (LAI) is a key ecological indicator for describing the structure of canopies and for modelling energy exchange between atmosphere and biosphere. While LAI of the forest overstory can be accurately assessed over large spatial scales via remote sensing, LAI of the forest understory (LAIu) is still largely ignored in ecological studies and ecosystem modelling due to the fact that it is often too complex to be destructively sampled or approximated by other site parameters. Additionally, so far only few attempts have been made to retrieve understory LAI via remote sensing, because dense canopies with high LAI are often hindering retrieval algorithms to produce meaningful estimates for understory LAI. Consequently, the forest understory still constitutes a poorly investigated research realm impeding ecological studies to properly account for its contribution to the energy absorption capacity of forest stands. This study aims to compare three conceptually different indirect retrieval methodologies for LAIu over a diverse panel of forest understory types distributed across Europe. For this we carried out near-to-surface measurements of understory reflectance spectra as well as digital surface photography over the extended network of Integrated Carbon Observation System (ICOS) forest ecosystem sites. LAIu was assessed by exploiting the empirical relationship between vegetation cover and light absorption (Beer-Lambert- Bouguer law) as well as by utilizing proposed relationships with two prominent vegetation indices: normalized difference vegetation index (NDVI) and simple ratio (SR). Retrievals from the three methods were significantly correlated with each other (r = 0.63–0.99, RMSE = 0.53–0.72), but exhibited also significant bias depending on the LAI scale. The NDVI based retrieval approach most likely overestimates LAI at productive sites when LAIu > 2, while the simple ratio algorithm overestimates LAIu at sites with sparse understory vegetation and presence of litter or bare soil. The purely empirical method based on the Beer-Lambert law of light absorption seems to offer a good compromise, since it provides reasonable LAIu values at both low and higher LAI ranges. Surprisingly, LAIu variation among sites seems to be largely decoupled from differences in climate and light permeability of the overstory, but significantly increased with vegetation diversity (expressed as species richness) and hence proposes new applications of LAIu in ecological modelling.
format	article
author	Jan-Peter George Wei Yang Hideki Kobayashi Tobias Biermann Arnaud Carrara Edoardo Cremonese Matthias Cuntz Silvano Fares Giacomo Gerosa Thomas Grünwald Niklas Hase Michael Heliasz Andreas Ibrom Alexander Knohl Bart Kruijt Holger Lange Jean-Marc Limousin Denis Loustau Petr Lukeš Riccardo Marzuoli Meelis Mölder Leonardo Montagnani Johan Neirynck Matthias Peichl Corinna Rebmann Marius Schmidt Francisco Ramon Lopez Serrano Kamel Soudani Caroline Vincke Jan Pisek
author_facet	Jan-Peter George Wei Yang Hideki Kobayashi Tobias Biermann Arnaud Carrara Edoardo Cremonese Matthias Cuntz Silvano Fares Giacomo Gerosa Thomas Grünwald Niklas Hase Michael Heliasz Andreas Ibrom Alexander Knohl Bart Kruijt Holger Lange Jean-Marc Limousin Denis Loustau Petr Lukeš Riccardo Marzuoli Meelis Mölder Leonardo Montagnani Johan Neirynck Matthias Peichl Corinna Rebmann Marius Schmidt Francisco Ramon Lopez Serrano Kamel Soudani Caroline Vincke Jan Pisek
author_sort	Jan-Peter George
title	Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe
title_short	Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe
title_full	Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe
title_fullStr	Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe
title_full_unstemmed	Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe
title_sort	method comparison of indirect assessments of understory leaf area index (laiu): a case study across the extended network of icos forest ecosystem sites in europe
publisher	Elsevier
publishDate	2021
url	https://doaj.org/article/5e0392aefe9a42f596044ebf1d3cda6d
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Method comparison of indirect assessments of understory leaf area index (LAIu): A case study across the extended network of ICOS forest ecosystem sites in Europe

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