{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,9]],"date-time":"2026-07-09T04:16:29Z","timestamp":1783570589154,"version":"3.55.0"},"reference-count":89,"publisher":"MDPI AG","issue":"16","license":[{"start":{"date-parts":[[2021,8,23]],"date-time":"2021-08-23T00:00:00Z","timestamp":1629676800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100012166","name":"National Key Research and Development Program of China","doi-asserted-by":"publisher","award":["2017YFA0603603"],"award-info":[{"award-number":["2017YFA0603603"]}],"id":[{"id":"10.13039\/501100012166","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["42001284"],"award-info":[{"award-number":["42001284"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100004608","name":"Natural Science Foundation of Jiangsu Province","doi-asserted-by":"publisher","award":["BK20200722"],"award-info":[{"award-number":["BK20200722"]}],"id":[{"id":"10.13039\/501100004608","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100010023","name":"Natural Science Research of Jiangsu Higher Education Institutions of China","doi-asserted-by":"publisher","award":["20KJB420001"],"award-info":[{"award-number":["20KJB420001"]}],"id":[{"id":"10.13039\/501100010023","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Horizon 2020","award":["834709, H2020-EU.1.1"],"award-info":[{"award-number":["834709, H2020-EU.1.1"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>The in situ leaf area index (LAI) measurement plays a vital role in calibrating and validating satellite LAI products. Digital hemispherical photography (DHP) is a widely used in situ forest LAI measurement method. There have been many software programs encompassing a variety of algorithms to estimate LAI from DHP. However, there is no conclusive study for an accuracy comparison among them, due to the difficulty in acquiring forest LAI reference values. In this study, we aim to use virtual (i.e., computer-simulated) broadleaf forests for the accuracy assessment of LAI algorithms in commonly used LAI software programs. Three commonly used DHP programs, including Can_Eye, CIMES, and Hemisfer, were selected since they provide estimates of both effective LAI and true LAI. Individual tree models with and without leaves were first reconstructed based on terrestrial LiDAR point clouds. Various stands were then created from these models. A ray-tracing technique was combined with the virtual forests to model synthetic DHP, for both leaf-on and leaf-off conditions. Afterward, three programs were applied to estimate PAI from leaf-on DHP and the woody area index (WAI) from leaf-off DHP. Finally, by subtracting WAI from PAI, true LAI estimates from 37 different algorithms were achieved for evaluation. The performance of these algorithms was compared with pre-defined LAI and PAI values in the virtual forests. The results demonstrated that without correcting for the vegetation clumping effect, Can_Eye, CIMES, and Hemisfer could estimate effective PAI and effective LAI consistent with each other (R2 &gt; 0.8, RMSD &lt; 0.2). After correcting for the vegetation clumping effect, there was a large inconsistency. In general, Can_Eye more accurately estimated true LAI than CIMES and Hemisfer (with R2 = 0.88 &gt; 0.72, 0.49; RMSE = 0.45 &lt; 0.7, 0.94; nRMSE = 15.7% &lt; 24.21%, 32.81%). There was a systematic underestimation of PAI and LAI using Hemisfer. The most accurate algorithm for estimating LAI was identified as the P57 algorithm in Can_Eye which used the 57.5\u00b0 gap fraction inversion combined with the finite-length averaging clumping correction. These results demonstrated the inconsistency of LAI estimates from DHP using different algorithms. It highlights the importance and provides a reference for standardizing the algorithm protocol for in situ forest LAI measurement using DHP.<\/jats:p>","DOI":"10.3390\/rs13163325","type":"journal-article","created":{"date-parts":[[2021,8,23]],"date-time":"2021-08-23T10:24:17Z","timestamp":1629714257000},"page":"3325","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":19,"title":["Comparative Evaluation of Algorithms for Leaf Area Index Estimation from Digital Hemispherical Photography through Virtual Forests"],"prefix":"10.3390","volume":"13","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5207-7614","authenticated-orcid":false,"given":"Jing","family":"Liu","sequence":"first","affiliation":[{"name":"Key Laboratory of Virtual Geographic Environment (Nanjing Normal University), Ministry of Education, Nanjing 210023, China"},{"name":"Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Longhui","family":"Li","sequence":"additional","affiliation":[{"name":"Key Laboratory of Virtual Geographic Environment (Nanjing Normal University), Ministry of Education, Nanjing 210023, China"},{"name":"Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Markku","family":"Akerblom","sequence":"additional","affiliation":[{"name":"Mathematics, Unit of Computing Sciences, Tampere University, 33720 Tampere, Finland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1138-8464","authenticated-orcid":false,"given":"Tiejun","family":"Wang","sequence":"additional","affiliation":[{"name":"Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7514 AE Enschede, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7446-8429","authenticated-orcid":false,"given":"Andrew","family":"Skidmore","sequence":"additional","affiliation":[{"name":"Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7514 AE Enschede, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Xi","family":"Zhu","sequence":"additional","affiliation":[{"name":"Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7514 AE Enschede, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Marco","family":"Heurich","sequence":"additional","affiliation":[{"name":"Department of Visitor Management and National Park Monitoring, Bavarian Forest National Park, 94481 Grafenau, Germany"},{"name":"Wildlife Ecology and Wildlife Management, University of Freiburg, 79098 Freiburg, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2021,8,23]]},"reference":[{"key":"ref_1","unstructured":"Fernandes, R., Plummer, S., Nightingale, J., Baret, F., Camacho, F., Fang, H., Garrigues, S., Gobron, N., Lang, M., and Lacaze, R. (2021, June 15). Global Leaf Area Index Product Validation Good Practices. Version 2.0. 2014; p. 76, Available online: https:\/\/lpvs.gsfc.nasa.gov\/LAI\/LAI_home.html."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"403","DOI":"10.1038\/523403a","article-title":"Agree on biodiversity metrics to track from space: Ecologists and space agencies must forge a global monitoring strategy","volume":"523","author":"Skidmore","year":"2015","journal-title":"Nature"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1431","DOI":"10.1175\/BAMS-D-13-00047.1","article-title":"The concept of essential climate variables in support of climate research, applications, and policy","volume":"95","author":"Bojinski","year":"2014","journal-title":"Bull. Am. Meteorol. Soc."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"207","DOI":"10.1016\/j.compag.2011.10.001","article-title":"CIMES: A package of programs for determining canopy geometry and solar radiation regimes through hemispherical photographs","volume":"79","author":"Gonsamo","year":"2011","journal-title":"Comput. Electron. Agric."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"97","DOI":"10.1016\/j.rse.2014.03.017","article-title":"Remote sensing of spring phenology in northeastern forests: A comparison of methods, field metrics and sources of uncertainty","volume":"148","author":"White","year":"2014","journal-title":"Remote Sens. Environ."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"15","DOI":"10.1016\/j.agrformet.2017.05.021","article-title":"Digital photography for tracking the phenology of an evergreen conifer stand","volume":"246","author":"Lang","year":"2017","journal-title":"Agric. For. Meteorol."},{"key":"ref_7","first-page":"36","article-title":"VALERI: A network of sites and a methodology for the validation of medium spatial resolution land satellite products","volume":"76","author":"Baret","year":"2005","journal-title":"Remote Sens. Environ."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"111935","DOI":"10.1016\/j.rse.2020.111935","article-title":"Evaluation of global leaf area index and fraction of absorbed photosynthetically active radiation products over North America using Copernicus Ground Based Observations for Validation data","volume":"247","author":"Brown","year":"2020","journal-title":"Remote Sens. Environ."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"68","DOI":"10.1016\/j.isprsjprs.2017.02.002","article-title":"Assimilating leaf area index of three typical types of subtropical forest in China from MODIS time series data based on the integrated ensemble Kalman filter and PROSAIL model","volume":"126","author":"Li","year":"2017","journal-title":"ISPRS J. Photogramm. Remote Sens."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"236","DOI":"10.1016\/j.isprsjprs.2020.08.003","article-title":"Estimating crop biomass using leaf area index derived from Landsat 8 and Sentinel-2 data","volume":"168","author":"Dong","year":"2020","journal-title":"ISPRS J. Photogramm. Remote Sens."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"111696","DOI":"10.1016\/j.rse.2020.111696","article-title":"A voxel matching method for effective leaf area index estimation in temperate deciduous forests from leaf-on and leaf-off airborne LiDAR data","volume":"240","author":"Zhu","year":"2020","journal-title":"Remote Sens. Environ."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1065","DOI":"10.1016\/j.rse.2010.12.011","article-title":"Airborne discrete-return LIDAR data in the estimation of vertical canopy cover, angular canopy closure and leaf area index","volume":"115","author":"Korhonen","year":"2011","journal-title":"Remote Sens. Environ."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"13","DOI":"10.1016\/j.isprsjprs.2017.12.004","article-title":"Large off-nadir scan angle of airborne LiDAR can severely affect the estimates of forest structure metrics","volume":"136","author":"Liu","year":"2018","journal-title":"ISPRS J. Photogramm. Remote Sens."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"131","DOI":"10.1016\/j.rse.2013.12.007","article-title":"Deriving and validating Leaf Area Index (LAI) at multiple spatial scales through lidar remote sensing: A case study in Sierra National Forest, CA","volume":"143","author":"Tang","year":"2014","journal-title":"Remote Sens. Environ."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"739","DOI":"10.1029\/2018RG000608","article-title":"An overview of global leaf area index (LAI): Methods, products, validation, and applications","volume":"57","author":"Fang","year":"2019","journal-title":"Rev. Geophys."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"14","DOI":"10.1038\/s43017-019-0001-x","article-title":"Characteristics, drivers and feedbacks of global greening","volume":"1","author":"Piao","year":"2020","journal-title":"Nat. Rev. Earth Environ."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"290","DOI":"10.3832\/ifor0775-005","article-title":"Digital hemispherical photography for estimating forest canopy properties: Current controversies and opportunities","volume":"5","author":"Chianucci","year":"2012","journal-title":"IForest-Biogeosci. For."},{"key":"ref_18","unstructured":"Frazer, G. (1999). Gap Light Analyzer (GLA) Imaging Software to Extract Canopy Structure and Gap Light Transmission Indices from True-Colour Fisheye Photographs, Users Manual and Program Documentation, Millbrook."},{"key":"ref_19","first-page":"56","article-title":"CAN_EYE V6. 4.91 User Manual","volume":"12","author":"Weiss","year":"2017","journal-title":"Recuperado"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"236","DOI":"10.1016\/j.agrformet.2007.02.004","article-title":"Correcting non-linearity and slope effects in the estimation of the leaf area index of forests from hemispherical photographs","volume":"144","author":"Schleppi","year":"2007","journal-title":"Agric. For. Meteorol."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"50","DOI":"10.1016\/j.rse.2006.04.019","article-title":"Estimation of LAI and fractional cover from small footprint airborne laser scanning data based on gap fraction","volume":"104","author":"Morsdorf","year":"2006","journal-title":"Remote Sens. Environ."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"931","DOI":"10.1046\/j.1365-2486.2002.00526.x","article-title":"Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem","volume":"8","author":"Hymus","year":"2002","journal-title":"Glob. Chang. Biol."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Rudic, T.E., McCulloch, L.A., and Cushman, K.C. (2020). Comparison of Smartphone and Drone Lidar Methods for Characterizing Spatial Variation in PAI in a Tropical Forest. Remote Sens., 12.","DOI":"10.3390\/rs12111765"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"950","DOI":"10.1016\/j.rse.2009.12.010","article-title":"Comparison and evaluation of Medium Resolution Imaging Spectrometer leaf area index products across a range of land use","volume":"114","author":"Canisius","year":"2010","journal-title":"Remote Sens. Environ."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"767","DOI":"10.1016\/j.rse.2010.11.004","article-title":"An assessment of the MODIS collection 5 leaf area index product for a region of mixed coniferous forest","volume":"115","author":"Disney","year":"2011","journal-title":"Remote Sens. Environ."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"119","DOI":"10.1038\/nature15539","article-title":"Death from drought in tropical forests is triggered by hydraulics not carbon starvation","volume":"528","author":"Rowland","year":"2015","journal-title":"Nature"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"644","DOI":"10.1016\/j.agrformet.2007.11.015","article-title":"Estimation of leaf area and clumping indexes of crops with hemispherical photographs","volume":"148","author":"Demarez","year":"2008","journal-title":"Agric. For. Meteorol."},{"key":"ref_28","first-page":"43","article-title":"Foliar and woody materials discriminated using terrestrial LiDAR in a mixed natural forest","volume":"64","author":"Zhu","year":"2018","journal-title":"Int. J. Appl. Earth Obs. Geoinf."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"5041","DOI":"10.1002\/2014WR016724","article-title":"Improved snow interception modeling using canopy parameters derived from airborne LiDAR data","volume":"51","author":"Moeser","year":"2015","journal-title":"Water Resour. Res."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"543","DOI":"10.1007\/s10342-009-0353-8","article-title":"Estimating leaf area index in different types of mature forest stands in Switzerland: A comparison of methods","volume":"129","author":"Thimonier","year":"2010","journal-title":"Eur. J. For. Res."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"68","DOI":"10.1016\/j.isprsjprs.2016.09.015","article-title":"Retrieval of forest leaf functional traits from HySpex imagery using radiative transfer models and continuous wavelet analysis","volume":"122","author":"Ali","year":"2016","journal-title":"ISPRS J. Photogramm. Remote Sens."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"365","DOI":"10.1016\/j.agrformet.2010.11.009","article-title":"Comparison of methods for measuring gap size distribution and canopy nonrandomness at J\u00e4rvselja RAMI (RAdiation transfer Model Intercomparison) test sites","volume":"151","author":"Pisek","year":"2011","journal-title":"Agric. For. Meteorol."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"749","DOI":"10.1016\/j.foreco.2008.05.032","article-title":"Methodology comparison for slope correction in canopy leaf area index estimation using hemispherical photography","volume":"256","author":"Gonsamo","year":"2008","journal-title":"For. Ecol. Manag."},{"key":"ref_34","doi-asserted-by":"crossref","unstructured":"Glatthorn, J., and Becksch\u00e4fer, P. (2014). Standardizing the Protocol for Hemispherical Photographs: Accuracy Assessment of Binarization Algorithms. PLoS ONE, 9.","DOI":"10.1371\/journal.pone.0111924"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"147","DOI":"10.17221\/76\/2009-JFS","article-title":"Comparison of output results from two programmes for hemispherical image analysis: Gap Light Analyser and WinScanopy","volume":"56","author":"Kucbel","year":"2010","journal-title":"J. For. Sci."},{"key":"ref_36","first-page":"36","article-title":"Comparison of four different programs for the analysis of hemispherical photographs using parameters of canopy structure and solar radiation transmittance","volume":"519","author":"Promis","year":"2011","journal-title":"Sierra"},{"key":"ref_37","doi-asserted-by":"crossref","unstructured":"Hall, R.J., C\u00f4t\u00e9, J.-F., Mailly, D., and Fournier, R.A. (2017). Comparison of software tools for analysis of hemispherical photographs. Hemispherical Photography in Forest Science: Theory, Methods, Applications, Springer.","DOI":"10.1007\/978-94-024-1098-3_7"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"59","DOI":"10.1007\/s00468-014-1058-2","article-title":"Evaluating optical measurements of leaf area index against litter collection in a mixed broadleaved-Korean pine forest in China","volume":"29","author":"Liu","year":"2015","journal-title":"Trees-Struct. Funct."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"48","DOI":"10.1016\/j.agrformet.2018.02.003","article-title":"Continuous estimation of canopy leaf area index (LAI) and clumping index over broadleaf crop fields: An investigation of the PASTIS-57 instrument and smartphone applications","volume":"253","author":"Fang","year":"2018","journal-title":"Agric. For. Meteorol."},{"key":"ref_40","unstructured":"Hyyppa, J. (2021, June 15). Virtual Forest. Available online: https:\/\/www.mdpi.com\/journal\/remotesensing\/special_issues\/Virtual_Forest."},{"key":"ref_41","doi-asserted-by":"crossref","unstructured":"Calders, K., Origo, N., Burt, A., Disney, M., Nightingale, J., Raumonen, P., Akerblom, M., Malhi, Y., and Lewis, P. (2018). Realistic Forest Stand Reconstruction from Terrestrial LiDAR for Radiative Transfer Modelling. Remote Sens., 10.","DOI":"10.3390\/rs10060933"},{"key":"ref_42","first-page":"57","article-title":"Virtual Forest Management: Possibilities and Challenges","volume":"12","author":"Uusitalo","year":"2001","journal-title":"Int. J. For. Eng."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"64","DOI":"10.1016\/j.agrformet.2014.03.016","article-title":"Hemispherical photography simulations with an architectural model to assess retrieval of leaf area index","volume":"194","author":"Leblanc","year":"2014","journal-title":"Agric. Forest Meteorol."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"1781","DOI":"10.1016\/j.agrformet.2009.06.001","article-title":"The computation of foliage clumping index using hemispherical photography","volume":"149","author":"Gonsamo","year":"2009","journal-title":"Agric. For. Meteorol."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"76","DOI":"10.1016\/j.isprsjprs.2019.09.015","article-title":"Comparison of terrestrial LiDAR and digital hemispherical photography for estimating leaf angle distribution in European broadleaf beech forests","volume":"158","author":"Liu","year":"2019","journal-title":"ISPRS J. Photogramm. Remote Sens."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"1958","DOI":"10.1109\/LGRS.2015.2440438","article-title":"Comparison of Five Slope Correction Methods for Leaf Area Index Estimation From Hemispherical Photography","volume":"12","author":"Cao","year":"2015","journal-title":"IEEE Geosci. Remote. Sens. Lett."},{"key":"ref_47","unstructured":"Raumonen, P., Casella, E., Calders, K., Murphy, S., \u00c5kerbloma, M., and Kaasalainen, M. (2015, January 5\u201327). Massive-scale tree modelling from TLS data. Proceedings of the Pia15+hrigi15\u2014Joint Isprs Conference, Munich, Germany."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"181","DOI":"10.1016\/j.agrformet.2017.07.027","article-title":"Validating canopy clumping retrieval methods using hemispherical photography in a simulated Eucalypt forest","volume":"247","author":"Woodgate","year":"2017","journal-title":"Agric. For. Meteorol."},{"key":"ref_49","doi-asserted-by":"crossref","unstructured":"Zou, J., Zhuang, Y., Chianucci, F., Mai, C., Lin, W., Leng, P., Luo, S., and Yan, B. (2018). Comparison of Seven Inversion Models for Estimating Plant and Woody Area Indices of Leaf-on and Leaf-off Forest Canopy Using Explicit 3D Forest Scenes. Remote Sens., 10.","DOI":"10.3390\/rs10081297"},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"108101","DOI":"10.1016\/j.agrformet.2020.108101","article-title":"An assessment study of three indirect methods for estimating leaf area density and leaf area index of individual trees","volume":"292","author":"Wei","year":"2020","journal-title":"Agric. For. Meteorol."},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"373","DOI":"10.1016\/j.rse.2017.01.032","article-title":"Estimation of 3D vegetation density with Terrestrial Laser Scanning data using voxels. A sensitivity analysis of influencing parameters","volume":"191","author":"Grau","year":"2017","journal-title":"Remote Sens. Environ."},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"491","DOI":"10.3390\/rs5020491","article-title":"Fast Automatic Precision Tree Models from Terrestrial Laser Scanner Data","volume":"5","author":"Raumonen","year":"2013","journal-title":"Remote Sens."},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"208","DOI":"10.1016\/j.isprsjprs.2019.01.005","article-title":"Variation of leaf angle distribution quantified by terrestrial LiDAR in natural European beech forest","volume":"148","author":"Liu","year":"2019","journal-title":"ISPRS J. Photogramm. Remote Sens."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"20170045","DOI":"10.1098\/rsfs.2017.0045","article-title":"Non-intersecting leaf insertion algorithm for tree structure models","volume":"8","author":"Akerblom","year":"2018","journal-title":"Interface Focus"},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"37","DOI":"10.1016\/j.agrformet.2003.08.001","article-title":"Review of methods for in situ leaf area index (LAI) determination: Part II. Estimation of LAI, errors and sampling","volume":"121","author":"Weiss","year":"2004","journal-title":"Agric. For. Meteorol."},{"key":"ref_56","doi-asserted-by":"crossref","first-page":"249","DOI":"10.1016\/0168-1923(92)90040-B","article-title":"Foliage area and architecture of plant canopies from sunfleck size distributions","volume":"60","author":"Chen","year":"1992","journal-title":"Agric. For. Meteorol."},{"key":"ref_57","unstructured":"Ross, J. (2012). The Radiation Regime and Architecture of Plant Stands, Springer Science & Business Media."},{"key":"ref_58","doi-asserted-by":"crossref","first-page":"141","DOI":"10.1071\/BT9670141","article-title":"A formula for average foliage density","volume":"15","author":"Miller","year":"1967","journal-title":"Aust. J. Bot."},{"key":"ref_59","doi-asserted-by":"crossref","first-page":"207","DOI":"10.1016\/j.compag.2011.10.001","article-title":"CIMES-FISHEYE \u00a9 A Package of Programs for the Assessment of Canopy Geometry and Solar Radiation Regimes through Hemispherical Photographs","volume":"79","author":"Walter","year":"2011","journal-title":"Comput. Electron. Agric."},{"key":"ref_60","doi-asserted-by":"crossref","first-page":"173","DOI":"10.1016\/0168-1923(90)90030-A","article-title":"Derivation of an angle density function for canopies with ellipsoidal leaf angle distributions","volume":"49","author":"Campbell","year":"1990","journal-title":"Agric. For. Meteorol."},{"key":"ref_61","doi-asserted-by":"crossref","first-page":"106","DOI":"10.1016\/j.agrformet.2006.12.003","article-title":"Comparison of leaf angle distribution functions: Effects on extinction coefficient and fraction of sunlit foliage","volume":"143","author":"Wang","year":"2007","journal-title":"Agric. For. Meteorol."},{"key":"ref_62","doi-asserted-by":"crossref","unstructured":"Norman, J.M., and Campbell, G.S. (1989). Canopy structure. Plant Physiological Ecology, Springer.","DOI":"10.1007\/978-94-009-2221-1_14"},{"key":"ref_63","doi-asserted-by":"crossref","first-page":"229","DOI":"10.1016\/0168-1923(86)90033-X","article-title":"Estimation of leaf area index from transmission of direct sunlight in discontinuous canopies","volume":"37","author":"Lang","year":"1986","journal-title":"Agric. For. Meteorol."},{"key":"ref_64","doi-asserted-by":"crossref","first-page":"6211","DOI":"10.1364\/AO.34.006211","article-title":"Plant canopy gap-size analysis theory for improving optical measurements of leaf-area index","volume":"34","author":"Chen","year":"1995","journal-title":"Appl. Optics."},{"key":"ref_65","doi-asserted-by":"crossref","first-page":"7667","DOI":"10.1364\/AO.41.007667","article-title":"Correction to the plant canopy gap-size analysis theory used by the Tracing Radiation and Architecture of Canopies instrument","volume":"41","author":"Leblanc","year":"2002","journal-title":"Appl. Optics."},{"key":"ref_66","doi-asserted-by":"crossref","first-page":"187","DOI":"10.1016\/j.agrformet.2004.09.006","article-title":"Methodology comparison for canopy structure parameters extraction from digital hemispherical photography in boreal forests","volume":"129","author":"Leblanc","year":"2005","journal-title":"Agric. For. Meteorol."},{"key":"ref_67","first-page":"801","article-title":"The computation of forest leaf area index on slope using fish-eye sensors","volume":"323","author":"Walter","year":"2000","journal-title":"Comptes Rendus l\u2019Acad\u00e9mie Sci. Ser. III-Sci. Vie"},{"key":"ref_68","doi-asserted-by":"crossref","first-page":"125","DOI":"10.1016\/S0168-1923(01)00284-2","article-title":"A practical scheme for correcting multiple scattering effects on optical LAI measurements","volume":"110","author":"Leblanc","year":"2001","journal-title":"Agric. For. Meteorol."},{"key":"ref_69","doi-asserted-by":"crossref","first-page":"833","DOI":"10.1016\/j.agrformet.2011.01.019","article-title":"Classification method of mixed pixels does not affect canopy metrics from digital images of forest overstorey","volume":"151","author":"Macfarlane","year":"2011","journal-title":"Agric. For. Meteorol."},{"key":"ref_70","doi-asserted-by":"crossref","first-page":"83","DOI":"10.1016\/j.agrformet.2015.02.012","article-title":"Understanding the variability in ground-based methods for retrieving canopy openness, gap fraction, and leaf area index in diverse forest systems","volume":"205","author":"Woodgate","year":"2015","journal-title":"Agric. For. Meteorol."},{"key":"ref_71","doi-asserted-by":"crossref","first-page":"179","DOI":"10.1016\/0168-1923(87)90078-5","article-title":"Simplified estimate of leaf area index from transmittance of the sun\u2019s beam","volume":"41","author":"Lang","year":"1987","journal-title":"Agric. For. Meteorol."},{"key":"ref_72","unstructured":"LiCOR (2009). LAI-2200 Plant Canopy Analyzer. Instruction Manual, Li-cor Cor."},{"key":"ref_73","unstructured":"Schleppi, P. (2021, June 15). Hemisfer v2.2 User Manual. Available online: https:\/\/www.schleppi.ch\/hemisfer\/."},{"key":"ref_74","doi-asserted-by":"crossref","first-page":"197","DOI":"10.1016\/j.agrformet.2017.09.024","article-title":"A robust leaf area index algorithm accounting for the expected errors in gap fraction observations","volume":"248","author":"Gonsamo","year":"2018","journal-title":"Agric. For. Meteorol."},{"key":"ref_75","doi-asserted-by":"crossref","first-page":"1287","DOI":"10.1016\/j.agrformet.2011.05.009","article-title":"Leaf area index uncertainty estimates for model-data fusion applications","volume":"151","author":"Richardson","year":"2011","journal-title":"Agric. For. Meteorol."},{"key":"ref_76","doi-asserted-by":"crossref","first-page":"176","DOI":"10.1016\/j.agrformet.2006.10.013","article-title":"Estimation of leaf area index in eucalypt forest using digital photography","volume":"143","author":"Macfarlane","year":"2007","journal-title":"Agric. For. Meteorol."},{"key":"ref_77","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/j.agrformet.2016.05.009","article-title":"Quantifying the impact of woody material on leaf area index estimation from hemispherical photography using 3D canopy simulations","volume":"226","author":"Woodgate","year":"2016","journal-title":"Agric. For. Meteorol"},{"key":"ref_78","doi-asserted-by":"crossref","first-page":"1069","DOI":"10.1093\/treephys\/tpp042","article-title":"Woody-to-total area ratio determination with a multispectral canopy imager","volume":"29","author":"Zou","year":"2009","journal-title":"Tree Physiol."},{"key":"ref_79","doi-asserted-by":"crossref","first-page":"231","DOI":"10.1016\/j.agrformet.2018.01.029","article-title":"Variability and bias in active and passive ground-based measurements of effective plant, wood and leaf area index","volume":"252","author":"Calders","year":"2018","journal-title":"Agric. For. Meteorol."},{"key":"ref_80","doi-asserted-by":"crossref","first-page":"457","DOI":"10.1016\/j.agrformet.2017.09.004","article-title":"Estimation of plant area index and phenological transition dates from digital repeat photography and radiometric approaches in a hardwood forest in the Northeastern United States","volume":"249","author":"Toda","year":"2018","journal-title":"Agric. For. Meteorol."},{"key":"ref_81","doi-asserted-by":"crossref","first-page":"4133","DOI":"10.1111\/gcb.13787","article-title":"Inconsistencies of interannual variability and trends in long-term satellite leaf area index products","volume":"23","author":"Jiang","year":"2017","journal-title":"Glob. Chang. Biol."},{"key":"ref_82","doi-asserted-by":"crossref","first-page":"19","DOI":"10.1016\/j.agrformet.2003.08.027","article-title":"Review of methods for in situ leaf area index determination\u2014Part I. Theories, sensors and hemispherical photography","volume":"121","author":"Jonckheere","year":"2004","journal-title":"Agric. For. Meteorol."},{"key":"ref_83","doi-asserted-by":"crossref","first-page":"191","DOI":"10.1046\/j.1466-822X.2003.00026.x","article-title":"Global synthesis of leaf area index observations: Implications for ecological and remote sensing studies","volume":"12","author":"Asner","year":"2003","journal-title":"Glob. Ecol. Biogeogr."},{"key":"ref_84","doi-asserted-by":"crossref","first-page":"118","DOI":"10.1016\/j.rse.2011.12.008","article-title":"Global clumping index map derived from the MODIS BRDF product","volume":"119","author":"He","year":"2012","journal-title":"Remote Sens. Environ."},{"key":"ref_85","doi-asserted-by":"crossref","first-page":"111296","DOI":"10.1016\/j.rse.2019.111296","article-title":"Global 500 m clumping index product derived from MODIS BRDF data (2001\u20132017)","volume":"232","author":"Wei","year":"2019","journal-title":"Remote Sens. Environ."},{"key":"ref_86","doi-asserted-by":"crossref","first-page":"112056","DOI":"10.1016\/j.rse.2020.112056","article-title":"A simulation method to infer tree allometry and forest structure from airborne laser scanning and forest inventories","volume":"251","author":"Fischer","year":"2020","journal-title":"Remote Sens. Environ."},{"key":"ref_87","doi-asserted-by":"crossref","first-page":"390","DOI":"10.1016\/j.agrformet.2018.11.033","article-title":"Review of indirect optical measurements of leaf area index: Recent advances, challenges, and perspectives","volume":"265","author":"Yan","year":"2019","journal-title":"Agric. For. Meteorol."},{"key":"ref_88","doi-asserted-by":"crossref","first-page":"761","DOI":"10.1016\/j.envsoft.2010.12.008","article-title":"An architectural model of trees to estimate forest structural attributes using terrestrial LiDAR","volume":"26","author":"Cote","year":"2011","journal-title":"Environ. Modell. Softw."},{"key":"ref_89","doi-asserted-by":"crossref","first-page":"111274","DOI":"10.1016\/j.rse.2019.111274","article-title":"Simulating solar-induced chlorophyll fluorescence in a boreal forest stand reconstructed from terrestrial laser scanning measurements","volume":"232","author":"Liu","year":"2019","journal-title":"Remote Sens. Environ."}],"container-title":["Remote Sensing"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2072-4292\/13\/16\/3325\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T06:49:35Z","timestamp":1760165375000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2072-4292\/13\/16\/3325"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,8,23]]},"references-count":89,"journal-issue":{"issue":"16","published-online":{"date-parts":[[2021,8]]}},"alternative-id":["rs13163325"],"URL":"https:\/\/doi.org\/10.3390\/rs13163325","relation":{},"ISSN":["2072-4292"],"issn-type":[{"value":"2072-4292","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,8,23]]}}}