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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 and Yang, Wei and Kobayashi, Hideki and Biermann, Tobias and Carrara, Arnaud and Cremonese, Edoardo and Cuntz, Matthias and Fares, Silvano and Gerosa, Giacomo and Grunwald, Thomas and Hase, Niklas and Heliasz, Michael and Ibrom, Andreas and Knohl, Alexander and Kruijt, Bart and Lange, Holger and Limousin, Jean-Marc and Loustau, Denis and Lukes, Petr and Marzuoli, Riccardo and Molder, Meelis and Montagnani, Leonardo and Neirynck, Johannes and Peichl, Matthias and Rebmann, Corinna and Schmidt, Marius and Serrano, Francisco Ramon Lopez and Soudani, Kamel and Vincke, Caroline and Pisek, Jan (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. Ecological Indicators. 128 , 107841
[Research article]

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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.

Authors/Creators:George, Jan-Peter and Yang, Wei and Kobayashi, Hideki and Biermann, Tobias and Carrara, Arnaud and Cremonese, Edoardo and Cuntz, Matthias and Fares, Silvano and Gerosa, Giacomo and Grunwald, Thomas and Hase, Niklas and Heliasz, Michael and Ibrom, Andreas and Knohl, Alexander and Kruijt, Bart and Lange, Holger and Limousin, Jean-Marc and Loustau, Denis and Lukes, Petr and Marzuoli, Riccardo and Molder, Meelis and Montagnani, Leonardo and Neirynck, Johannes and Peichl, Matthias and Rebmann, Corinna and Schmidt, Marius and Serrano, Francisco Ramon Lopez and Soudani, Kamel and Vincke, Caroline and Pisek, Jan
Title: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
Series Name/Journal:Ecological Indicators
Year of publishing :2021
Volume:128
Article number:107841
Number of Pages:11
Publisher:ELSEVIER
ISSN:1470-160X
Language:English
Publication Type:Research article
Article category:Scientific peer reviewed
Version:Published version
Copyright:Creative Commons: Attribution 4.0
Full Text Status:Public
Subjects:(A) Swedish standard research categories 2011 > 1 Natural sciences > 106 Biological Sciences (Medical to be 3 and Agricultural to be 4) > Ecology
Keywords:Forest background reflectivity, Fractional vegetation cover, Leaf area index, NDVI, Simple ratio, Diversity, Understory layer
URN:NBN:urn:nbn:se:slu:epsilon-p-112751
Permanent URL:
http://urn.kb.se/resolve?urn=urn:nbn:se:slu:epsilon-p-112751
Additional ID:
Type of IDID
DOI10.1016/j.ecolind.2021.107841
Web of Science (WoS)000663325100009
ID Code:24840
Faculty:S - Faculty of Forest Sciences
Department:(S) > Dept. of Forest Ecology and Management
Deposited By: SLUpub Connector
Deposited On:12 Jul 2021 08:25
Metadata Last Modified:12 Jul 2021 08:31

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