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Research article - Peer-reviewed, 2021

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

George, Jan-Peter; Yang, Wei; Kobayashi, Hideki; Biermann, Tobias; Carrara, Arnaud; Cremonese, Edoardo; Cuntz, Matthias; Fares, Silvano; Gerosa, Giacomo; Grunwald, Thomas; Hase, Niklas; Heliasz, Michael; Ibrom, Andreas; Knohl, Alexander; Kruijt, Bart; Lange, Holger; Limousin, Jean-Marc; Loustau, Denis; Lukes, Petr; Marzuoli, Riccardo;
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Abstract

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 (LAI(u)) 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 LAI(u) 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. LAI(u) 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 LAI(u) > 2, while the simple ratio algorithm overestimates LAI(u) 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 LAI(u) values at both low and higher LAI ranges. Surprisingly, LAI(u) 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 LAI(u) in ecological modelling.

Keywords

Forest background reflectivity; Fractional vegetation cover; Leaf area index; NDVI; Simple ratio; Diversity; Understory layer

Published in

Ecological Indicators
2021, volume: 128, article number: 107841
Publisher: ELSEVIER

Authors' information

George, Jan-Peter
University of Tartu
Yang, Wei
Chiba University
Kobayashi, Hideki
Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Biermann, Tobias
Lund University
Carrara, Arnaud
CEAM
Cremonese, Edoardo
Regional Environmental Protection Agency - Italy
Cuntz, Matthias
Universite de Lorraine
Fares, Silvano
Consiglio Nazionale delle Ricerche (CNR)
Gerosa, Giacomo
Catholic University of the Sacred Heart
Grunwald, Thomas
Technische Universitat Dresden
Hase, Niklas
Helmholtz Center for Environmental Research (UFZ)
Heliasz, Michael
Lund University
Ibrom, Andreas
Technical University of Denmark
Knohl, Alexander
University of Gottingen
Kruijt, Bart
Wageningen University and Research
Lange, Holger
Norwegian Inst Bioecon Res
Limousin, Jean-Marc
Universite de Montpellier
Loustau, Denis
INRAE
Lukes, Petr
Global Change Research Centre of the Czech Academy of Sciences
Marzuoli, Riccardo
Catholic University of the Sacred Heart
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UKÄ Subject classification

Ecology

Publication Identifiers

DOI: https://doi.org/10.1016/j.ecolind.2021.107841

URI (permanent link to this page)

https://res.slu.se/id/publ/112751