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Leaf developmental categories contribute to the top-of-canopy reflectance and should be considered when taking ground truth for remote sensing studies (RS). Deciduous tree species, Betula pendula, Populus tremula, and Alnus incana, were sampled from May to October 2018 in an Estonian hemiboreal forest. Chlorophyll and carotenoid content were detected biochemically; leaf anatomical traits (leaf, palisade, and spongy mesophyll thickness) were measured on leaf cross-sections; leaf reflectance was measured by a spectroradiometer with an integrating sphere (350\u20132500 nm). Biophysical and anatomical leaf traits were related to 64 vegetation indices (VIs). Linear models based on VIs for all tested leaf traits were more robust if both juvenile and mature leaves were included. This study provides information on which VIs are interchangeable or independent. Pigment and leaf thickness sensitive indices formed PC1; water and structural trait related VIs formed an independent group associated with PC3. Type of growth and leaf age could affect the validation of biophysical and anatomical leaf trait retrieval from the optical signal. It is, therefore, necessary to sample both leaf developmental categories\u2014young and mature\u2014in RS, especially if sampling is only once within the vegetation season.<\/jats:p>","DOI":"10.3390\/rs13071353","type":"journal-article","created":{"date-parts":[[2021,4,1]],"date-time":"2021-04-01T10:44:01Z","timestamp":1617273841000},"page":"1353","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":19,"title":["Leaf Age Matters in Remote Sensing: Taking Ground Truth for Spectroscopic Studies in Hemiboreal Deciduous Trees with Continuous Leaf Formation"],"prefix":"10.3390","volume":"13","author":[{"given":"Eva","family":"Neuwirthov\u00e1","sequence":"first","affiliation":[{"name":"Department of Experimental Plant Biology, Faculty of Science, Charles University, Vinicna 5, 128 44 Prague 2, Czech Republic"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7458-1467","authenticated-orcid":false,"given":"Andres","family":"Kuusk","sequence":"additional","affiliation":[{"name":"Tartu Observatory, Faculty of Science and Technology, University of Tartu, Observatoorium 1, T\u00f5ravere, N\u00f5o Parish, 61602 Tartu County, Estonia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3060-641X","authenticated-orcid":false,"given":"Zuzana","family":"Lhot\u00e1kov\u00e1","sequence":"additional","affiliation":[{"name":"Department of Experimental Plant Biology, Faculty of Science, Charles University, Vinicna 5, 128 44 Prague 2, Czech Republic"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7565-1244","authenticated-orcid":false,"given":"Joel","family":"Kuusk","sequence":"additional","affiliation":[{"name":"Tartu Observatory, Faculty of Science and Technology, University of Tartu, Observatoorium 1, T\u00f5ravere, N\u00f5o Parish, 61602 Tartu County, Estonia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6912-1992","authenticated-orcid":false,"given":"Jana","family":"Albrechtov\u00e1","sequence":"additional","affiliation":[{"name":"Department of Experimental Plant Biology, Faculty of Science, Charles University, Vinicna 5, 128 44 Prague 2, Czech Republic"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2327-3197","authenticated-orcid":false,"given":"Lea","family":"Hallik","sequence":"additional","affiliation":[{"name":"Tartu Observatory, Faculty of Science and Technology, University of Tartu, Observatoorium 1, T\u00f5ravere, N\u00f5o Parish, 61602 Tartu County, Estonia"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2021,4,1]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1177","DOI":"10.1126\/science.284.5417.1177","article-title":"Net Primary Production of a Forest Ecosystem with Experimental CO2 Enrichment","volume":"284","author":"DeLucia","year":"1999","journal-title":"Science"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"940","DOI":"10.1126\/science.1192666","article-title":"Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 through 2009","volume":"329","author":"Zhao","year":"2010","journal-title":"Science"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"2413","DOI":"10.1111\/nph.16866","article-title":"Integrating the Evidence for a Terrestrial Carbon Sink Caused by Increasing Atmospheric CO2","volume":"229","author":"Walker","year":"2020","journal-title":"New Phytol."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"988","DOI":"10.1126\/science.1201609","article-title":"A Large and Persistent Carbon Sink in the World\u2019s Forests","volume":"333","author":"Pan","year":"2011","journal-title":"Science"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"2861","DOI":"10.1111\/gcb.12916","article-title":"Effects of Climate Extremes on the Terrestrial Carbon Cycle: Concepts, Processes and Potential Future Impacts","volume":"21","author":"Frank","year":"2015","journal-title":"Glob. 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