Retrieval and validation of forest background reflectivity from daily Moderate Resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function (BRDF) data across European forests

Biogeosciences, 18, 621–635, 2021 h ps://doi.o g/10.5194/bg-18-621-2021 © Au ho (s) 2021. This wo k is dis ibu ed unde he C ea i e Commons A ibu ion 4.0 License. Re ie al and alida ion o o es backg ound e lec i i y om daily Mode a e Resolu ion Imaging Spec o adiome e (MODIS) bidi ec ional e lec ance dis ibu ion unc ion (BRDF) da a ac oss Eu opean o es s Jan Pisek1, Angela E b2, Lau i Ko honen3, Tobias Bie mann4, A naud Ca a a5, Edoa do C emonese6, Ma hias Cun z7, Sil ano Fa es8, Giacomo Ge osa9, Thomas G ünwald10, Niklas Hase11, Michal Heliasz4, And eas Ib om12, Alexande Knohl13, Johannes Koble 14, Ba K uij 15, Holge Lange16, Leena Leppänen17, Jean-Ma c Limousin18, F ancisco Ramon Lopez Se ano19, Denis Lous au20, Pe Lukeš21, La s Lundin22, Ricca do Ma zuoli9, Meelis Mölde 4, Leona do Mon agnani23,31, Johan Nei ynck24, Ma hias Peichl25, Co inna Rebmann11, E a Rubio19, Ma ga ida San os-Reis26, C ys al Schaa 2, Ma ius Schmid 27, Guillaume Simioni28, Kamel Soudani29, and Ca oline Vincke30 1Ta u Obse a o y, Uni e si y o Ta u, Tõ a e e, Ta umaa, Es onia 2School o he En i onmen , Uni e si y o Massachuse s Bos on, Bos on, Massachuse s, USA 3School o Fo es Sciences, Uni e si y o Eas e n Finland, Joensuu, Finland 4Lund Uni e si y, Lund, Sweden 5Fundación CEAM, Pa e na, Valencia, Spain 6ARPA Valle d’Aos a, Sain -Ch is ophe, I aly 7Uni e si é de Lo aine, Ag oPa isTech, INRAE, UMR Sil a, Nancy, F ance 8CNR – Na ional Resea ch Council, Rome, I aly 9Depa men o Ma hema ics and Physics, Uni e si à Ca olica del Sac o Cuo e, B escia, I aly 10Ins i u e o Hyd ology and Me eo ology, Depa men o Hyd o Sciences, Technische Uni e si ä D esden, D esden, Ge many 11Helmhol z Cen e o En i onmen al Resea ch – UFZ, Leipzig, Ge many 12Depa men o En i onmen al Enginee ing, Technical Uni e si y o Denma k, Kongens Lyngby, Denma k 13Facul y o Fo es Sciences and Fo es Ecology, Uni e si y o Gö ingen, Gö ingen, Ge many 14Umwel bundesam GmbH, Vienna, Aus ia 15Depa men o En i onmen al Sciences, Wageningen Uni e si y & Resea ch, Wageningen, he Ne he lands 16No wegian Ins i u e o Bioeconomy Resea ch, Ås, No way 17Space and Ea h Obse a ion Cen e, Finnish Me eo ological Ins i u e, Sodankylä, Finland 18CEFE, Uni e si é Mon pellie , CNRS, EPHE, IRD, Uni e si é Paul-Valé y Mon pellie , Mon pellie , F ance 19IER-ETSIAM, Uni e sidad de Cas illa-La Mancha, Albace e, Spain 20INRAE, Bo deaux, F ance 21Global Change Resea ch Ins i u e, Academy o Sciences o he Czech Republic, B no, Czech Republic 22Depa men o Soil and En i onmen , Swedish Uni e si y o Ag icul u al Sciences, Uppsala, Sweden 23Facul y o Science and Technology, F ee Uni e si y o Bolzano, Bolzano, I aly 24INBO, Ge aa dsbe gen, Belgium 25Depa men o Fo es Ecology and Managemen , Swedish Uni e si y o Ag icul u al Sciences, Umeå, Sweden 26cE3c – Cen e o Ecology, E olu ion and En i onmen al Changes, Lisbon, Po ugal 27Fo schungszen um Jülich, Jülich, Ge many 28INRAE URFM, A ignon, F ance 29Uni e si é Pa is-Saclay, CNRS, Ag oPa isTech, Ecologie, Sys éma ique e E olu ion, O say, F ance 30Facul y o Bioscience Enginee ing, Ea h and Li e Ins i u e, Uni e si é Ca holique de Lou ain, Lou ain-la-Neu e, Belgium Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union. 622 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y 31Fo es Se ices, Au onomous P o ince o Bolzano, Bolzano, I aly Co espondence: Jan Pisek ([email p o ec ed]) Recei ed: 30 Sep embe 2020 – Discussion s a ed: 4 No embe 2020 Re ised: 12 Janua y 2021 – Accep ed: 12 Janua y 2021 – Published: 27 Janua y 2021 Abs ac . In o ma ion abou o es backg ound e lec ance is needed o accu a e biophysical pa ame e e ie al om o es canopies (o e s o y) wi h emo e sensing. Sepa a ing unde - and o e s o y signals would enable mo e accu a e modeling o o es ca bon and ene gy luxes. We e ie ed alues o he no malized di e ence ege a ion index (NDVI) o he o es unde s o y wi h he mul i-angula Mode a e Resolu ion Imaging Spec o adiome e (MODIS) bidi ec- ional e lec ance dis ibu ion unc ion (BRDF)/albedo da a (g idded 500m daily Collec ion 6 p oduc ), using a me hod o iginally de eloped o bo eal o es s. The o es loo back- g ound e lec ance es ima es om he MODIS da a we e compa ed wi h in si u unde s o y e lec ance measu emen s ca ied ou a an ex ensi e se o o es ecosys em expe i- men al si es ac oss Eu ope. The e lec ance es ima es om MODIS da a we e, hence, es ed ac oss di e se o es con- di ions and phenological phases du ing he g owing season o examine hei applicabili y o ecosys ems o he han bo- eal o es s. He e we epo ha he me hod can deli e good e ie als, especially o e di e en o es ypes wi h open canopies (low oliage co e ). The pe o mance o he me hod was ound o be limi ed o e o es s wi h closed canopies (high oliage co e ), whe e he signal om unde s o y be- comes oo a enua ed. The spa ial he e ogenei y o indi id- ual ield si es and he limi a ions and documen ed quali y o he MODIS BRDF p oduc a e shown o be impo an o he co ec assessmen and alida ion o he e ie als ob ained wi h emo e sensing. 1 In oduc ion The e lec ance om he o es canopy backg ound/ o es loo can o en con ound and e en domina e he adiome - ic signal om he uppe o es canopy laye o he a mo- sphe e. Fo es unde s o y is de ined he e as all he compo- nen s ound unde he o es canopy, including unde s o y ege a ion, lea li e , moss, lichen, ock, soil, snow, o a mix u e he eo (Pisek and Chen, 2009). I unaccoun ed o , o es unde s o y can in oduce po en ial bias in he es ima- ion o o e s o y biophysical pa ame e s (e.g., lea a ea in- dex, LAI, and ac ion o abso bed pho osyn he ically ac i e adia ion, APAR) and, subsequen ly, p oduc i i y es ima es (e.g., he ne p ima y p oduc i i y – NPP) as he con ibu ion o he unde s o y o he o al ene gy abso p ion capaci y o a o es s and can be qui e signi ican (Cla k e al., 2001; Law e al., 2001). The unde s o y ege a ion in o es ecosys ems should be ea ed di e en ly om he o e s o y in ca bon cy- cle modeling because o he di e en esidence imes o ca - bon ixed h ough NPP in di e en ecosys em componen s (Vogel and Gowe , 1998; Ren ch e al., 2003; Ma ques and Oli ei a, 2004; Kim e al., 2016). Cu en ly, he unde s o y is o en ea ed as an unknown quan i y in ca bon models due o he di icul ies in measu ing i p ope ly and consis- en ly ac oss la ge scales (Luyssae e al., 2007). The p e- dic ions ega ding he spec al a ia ion in o es backg ound ha e posed a pe sis en challenge (McDonald e al., 1998; Gemmell, 2000) because o he high a iabili y in incoming adiance below he o es canopy, challenges wi h he spec- al cha ac e iza ion, and weak signal in some pa s o he spec um o bo h o e s o y and unde s o y (Schaepman e al., 2009), and he gene al a ying na u e o he unde s o y (Mille e al., 1997). Mul i-angle emo e sensing can cap u e signals o di e - en o es laye s because he obse ed p opo ions o di - e en o es laye s a y wi h he iewing angle, making i possible o sepa a e o es o e s o y and unde s o y signal. He e, we aim o consolida e p e ious e o s o acking un- de s o y e lec ance and i s dynamics wi h mul i-angle Ea h obse a ion da a (Canisius and Chen, 2007; Pisek and Chen, 2009; Pisek e al., 2010, 2012, 2015a, 2015b, 2016; Jiao e al., 2014) by es ing he alidi y o his app oach, using Mod- e a e Resolu ion Imaging Spec o adiome e bidi ec ional e- lec ance dis ibu ion unc ion (MODIS BRDF)/albedo da a (g idded 500m daily Collec ion; 6 MCD43 p oduc ), agains in si u unde s o y e lec ance measu emen s o e an ex- ended se o In eg a ed Ca bon Obse a ion Sys em (ICOS) o es ecosys em si es. The alida ion p ocedu e was de- ined o comply as much as possible wi h he bes p ac- ices p oposed by he Commi ee on Ea h Obse a ion Sa el- li es (CEOS) Wo king G oup on Calib a ion and Valida ion (WGCV) Land P oduc Valida ion (LPV) subg oup (Ga - igues e al., 2008; Ba e e al., 2006). I co esponds o S age 1 alida ion, as de ined by he CEOS (Nigh ingale e al., 2011; Weiss e al., 2014), whe e p oduc accu acy shall be assessed o e a small ( ypically <30) se o loca ions and ime pe iods by compa ison wi h in si u o o he sui able e - e ence da a. Using he ex ended se o ICOS o es ecosys- ems as alida ion si es, we asked he ollowing ques ions: 1. Can ICOS o es ecosys em si es se e as a sui able al- ida ion da a se wi h espec o hei oo p in and he pixel esolu ion o Ea h obse a ion (EO) p oduc s? Biogeosciences, 18, 621–635, 2021 h ps://doi.o g/10.5194/bg-18-621-2021 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y 623 Figu e 1. Dis ibu ion o s udy si es ac oss Eu ope; o u he de- ails, e e o Table 1. 2. Can we e ie e eliable no malized di e ence ege a- ion index (NDVI; Rouse e al., 1973; Tucke , 1979) dy- namics o unde s o y wi h MODIS BRDF da a ac oss di e se o es condi ions du ing he g owing season? 3. A e he e impo an di e ences be ween he o al (o e - s o y and unde s o y) and unde s o y-only NDVI sig- nals? 2 Ma e ials and me hods 2.1 S udy si es The ICOS is a dis ibu ed pan-Eu opean esea ch in as- uc u e p o iding in si u s anda dized, in eg a ed, long- e m and high-p ecision obse a ions o lowe a mosphe e g een- house gas (GHG) concen a ions and land–a mosphe e and ocean–a mosphe e GHG in e ac ions (Gielen e al., 2017). In his s udy, we ca ied ou he e alua ion o e he ne wo k o 31 ICOS-a ilia ed o es ecosys em si es, complemen ed wi h addi ional si es in Spain, Po ugal, Aus ia, and Finland. Toge he , hese selec ed 40 s udy si es comp ise a la ge a i- e y o o es o e - and unde s o y ypes, spanning a wide la - i udinal g adien om almos 38◦N (Yes e, Spain) o 68◦N (Ken ä o a, Finland). Si e loca ions a e shown in Fig. 1, and ege a ion cha ac e is ics a e summa ized in Table 1. 2.2 Unde s o y spec a and o es canopy co e /closu e in si u measu emen s Following he e minology by Schaepman-S ub e al. (2006), we e e o he e lec ance ac o s mea- su ed by he ield spec ome e s as he sa elli e-de i ed hemisphe ical–di ec ional e lec ance ac o s (HDRFs). The gi en spec ome e ’s ield o iew is app oxima ed as being angula (cone) and na owe han a whole hemi- sphe e, wi h some aniso opy cap u ed which co esponds o no mal emo e sensing iewing geome y. An o e iew o he unde aken in si u campaigns a each si e and hei cha ac e is ics a e gi en in Table 1. The indi idual si es we e isi ed be ween Ap il 2016 and Augus 2019, mos ly du ing he g owing season. Following he p o ocol by Rau iainen e al. (2011), he unde s o y spec- a we e measu ed wi h he Sun comple ely obscu ed by he clouds o a a ound sunse (di use ligh condi ions), co e - ing he isible/nea in a ed (NIR) egion, depending on he spec ome e (see Table 1 o mo e de ails). A o al o h ee unde s o y spec a we e measu ed e e y 2m along wo 50m long ansec s laid a each si e, esul ing in 50 measu emen poin s (150 indi idual measu emen s). T ansec s co e ed and cha ac e ized condi ions wi hin he measu emen oo p in o he gi en owe . I should be no ed ha he owe oo - p in migh be di e en om he exac MODIS pixel oo - p in (see Sec . 2.5 o he spa ial homogenei y assessmen o MODIS pixels). The measu emen s conce ned condi ions on he o es loo and low he baceous and sh ubby species o ee seedlings and saplings, as he a ea sampled by each spec- al measu emen was es ima ed o co espond o a ∼50cm diame e ci cle on he g ound. The downwa d-poin ing spec- o adiome e (no o e op ics we e used) was held by he op- e a o ’s ou s e ched hand. A o al o h ee spec a abo e a 10in. (0.254m) Spec alon SRT-99–100 whi e panel we e eco ded a he beginning, a e e e y ou unde s o y spec- a measu emen poin s (e e y 8m), and a end o each an- sec . A hemisphe ical, conical e lec ance ac o was ob- ained wi h an uncalib a ed Spec alon e lec ance spec um and he linea ly in e pola ed i adiance. Finally, b oadband HDRFs o ed (620–670nm) and NIR (841–876nm) wa e- leng hs we e compu ed wi h ela i e spec al esponse unc- ions o he MODIS senso onboa d Te a. The unde s o y NDVI alue o gi en si e was calcula ed om he ed and NIR-band alues and a e aged o e he wo ansec s. Es ima es o o e s o y oliage co e and c own co e we e ob ained om digi al co e pho og aphs (DCPs). O e s o y oliage co e was de ined as he pe cen age o g ound co - e ed by he e ical p ojec ion o oliage and b anches and c own co e as he pe cen age o g ound co e ed by he e - ical p ojec ions o he ou e mos pe ime e s o he c owns on he ho izon al plane (wi hou double-coun ing he o e - lap; Gschwan ne e al., 2009). The DCPs we e aken om below he canopy e e y 8m along ansec s a each si e. The came a (Nikon CoolPix4500; 2272×1704 esolu ion) was h ps://doi.o g/10.5194/bg-18-621-2021 Biogeosciences, 18, 621–635, 2021 624 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y Table 1. S udy si e cha ac e is ics and hei spa ial ep esen a i eness s a us. ICOS – In eg a ed Ca bon Obse ing Sys em si es; LTER – Long Te m Ecological Resea ch Ne wo k si es. No e ha he sampling da es a e shown in he o ma yyyy/mm/dd. Si e code Si e name La (◦) Long (◦) Sampling da e Spec ome e model Unde s o y ege a ion Rep esen a i eness AT-Zbn Zöbelboden (LTER) 47.842 14.442 2017/11/18 ASD FieldSpec 4 Calamag os is a ia, B achypodium syl a icum, Ho delymus eu opaeus, and Senecio o a us No ep esen a i e BE-B a B asschaa (ICOS) 51.304 4.519 2019/01/12 Ocean Op ics; FLAME-S-VIS-NIR-ES Be ula spec., Que cus obu , and So bus aucupa ia Rep esen a i e BE-Vie Vielsalm (ICOS) 50.3 5.983 2018/08/16 Ocean Op ics; FLAME-S-VIS-NIR-ES Spa se e n and moss co e Rep esen a i e a 0.5km CH-Da Da os (ICOS) 46.817 9.85 2018/07/12 Ocean Op ics; FLAME-S-VIS-NIR-ES Dwa sh ubs, bluebe y, and mosses Rep esen a i e CZ-BK1 Bílý Kˇ íž (ICOS) 49.502 18.539 2016/04/17 ASD FieldSpec 4 Vaccinium my illus L. Rep esen a i e a 1.5km CZ-Lnz Lanžho (ICOS) 48.682 16.948 2017/04/27 ASD FieldSpec 4 Allium u sinum and Asa um eu opeum Rep esen a i e DE-Hai Hainich (ICOS) 51.079 10.453 2018/04/12 Ocean Op ics; FLAME-S-VIS-NIR-ES Anemone nemo osa and Allium u sinum Rep esen a i e DE-HoH Hohes Holz (ICOS) 52.083 11.217 2018/04/11 Ocean Op ics; FLAME-S-VIS-NIR-ES Anemone nemo osa Rep esen a i e DE-RuW Wüs ebach (ICOS) 50.505 6.331 2018/08/16 Ocean Op ics; FLAME-S-VIS-NIR-ES Spa se Deschampsia lexuosa, Deschampsia cespi osa and Molinia cae ulea No ep esen a i e DE-Tha Tha and (ICOS) 50.967 13.567 2018/04/12 Ocean Op ics; FLAME-S-VIS-NIR-ES Fagus syl a ica,Abies alba, and De- schampsia lexuosa Rep esen a i e DK-So So oe (ICOS) 55.486 11.645 2018/09/26 Ocean Op ics; FLAME-S-VIS-NIR-ES Beech saplings and seedlings; P e id- ium aquilinum Rep esen a i e ES-AP1 Almodó a del Pina 39.677 −1.848 2017/11/09 ASD FieldSpec HandHeld 2 Que cus ilex ssp. ballo a,Rosma i- nus o icinalis, Thymus ulga is, La an- dula la i olia, Que cus cocci e a, and Genis a sco pius Rep esen a i e ES-CMu Cuenca del Majadas 40.252 −1.965 2017/11/12 ASD FieldSpec HandHeld 2 Junipe us communis,Junipe us oxyced us, and C a aegus monogyna Rep esen a i e a 0.5km ES-CPa Co es de Pallas 39.224 −0.903 2017/11/08 Ocean Op ics; FLAME-S-VIS-NIR-ES Rosma inus o icinalis,Ulex pa i lo us, and B achypodium e usum Rep esen a i e >0.5km ES-Ys Yes e 38.339 −2.351 2018/07/28 Ocean Op ics; FLAME-S-VIS-NIR-ES Rosma inus o icinalis L.,Thymus ul- ga is L., and Cis us clusii Dunal Rep esen a i e a 0.5km FI-Hal Halssiaapa 67.368 26.654 2017/06/13 ASD FieldSpec P o Sedge ege a ion Rep esen a i e FI-Hyy Hyy iälä (ICOS) 61.847 24.295 2018/06/28 Ocean Op ics; FLAME-S-VIS-NIR-ES Vaccinium spec. and No way sp uce seedlings Rep esen a i e a 0.5km Biogeosciences, 18, 621–635, 2021 h ps://doi.o g/10.5194/bg-18-621-2021 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y 625 Table 1. Con inued. Si e Code Si e Name La (◦) Long (◦) Sampling da e Spec ome e model Unde s o y ege a ion Rep esen a i eness FI-Ken Ken ä o a (ICOS) 67.987 24.243 2017/06/13 ASD FieldSpec P o Vaccinium my illus,Empe um nig um, and Vaccinium i is-idaea and he o es mosses Pleu ozium sch ebe i, Hylocomium splendens, and Dic anum polyse um Rep esen a i e a 2km FI-Kns Kale ansuo 60.647 24.356 2017/06/15 ASD FieldSpec P o Dwa sh ubs and mosses Rep esen a i e a 0.275km FI-Le Le osuo (ICOS) 60.642 23.96 2017/06/15 ASD FieldSpec P o Dwa sh ubs, mosses, and he bs Rep esen a i e <1.0km FI-Sod Sodankylä (ICOS) 67.362 26.638 2017/06/13 ASD FieldSpec P o Lingonbe y, Calluna ulga is, and lichens Sphe oid does no i <1.5km; no ep esen a i e a >1.5km FI-Va Vä iö (ICOS) 67.757 29.616 2017/06/14 ASD FieldSpec P o Mosses, lichens, and dwa sh ubs Rep esen a i e FR-Bil Bilos – Salles (ICOS) 44.494 −0.956 2018/06/14 Ocean Op ics; FLAME-S-VIS-NIR-ES Molinia coe ulea Moench.,P e idium aquilineum, and Ulex eu opaeus Rep esen a i e <0.5km FR-FBn Fon Blanche (ICOS) 43.241 5.679 2018/06/12 Ocean Op ics; FLAME-S-VIS-NIR-ES Que cus cocci e a,Philly ea la i olia, and o he species Rep esen a i e a 1.5km FR-Fon Fon ainebleau–Ba beau (ICOS) 48.476 2.780 2018/06/16 Ocean Op ics; FLAME-S-VIS-NIR-ES Ca pinus be ulus Rep esen a i e a 0.5km FR-Hes Hesse (ICOS) 48.674 7.066 2018/08/18 Ocean Op ics; FLAME-S-VIS-NIR-ES Fagus syl a ica seedlings and black- be y Sphe oid does no i FR-MsS Mon ie s (ICOS) 48.537 5.312 2019/01/14 Ocean Op ics; FLAME-S-VIS-NIR-ES Spa se Sphagnum spec. ege a ion Rep esen a i e >1.0km FR-Pue Puéchabon (ICOS) 43.741 3.596 2018/06/13 Ocean Op ics; FLAME-S-VIS-NIR-ES Buxus sempe i ens,Pis acia len iscus, Philly ea la i olia,Sal ia osma inus, and Ruscus aculea us Rep esen a i e IT-BF Bosco Fon ana (ICOS) 45.202 10.743 2018/07/10 Ocean Op ics; FLAME-S-VIS-NIR-ES Hede a helix,Co ylus spec., and Rus- cus aculea us Rep esen a i e a 1.5km IT-Cp2 Cas elpo ziano 2 (ICOS) 41.704 12.357 2019/01/25 Ocean Op ics; FLAME-S-VIS-NIR-ES Phylli ea la i olia and Pis acia len is- cus Rep esen a i e a 1.5km IT-Ren Renon (ICOS) 43.732 10.291 2018/07/11 Ocean Op ics; FLAME-S-VIS-NIR-ES Deschampsia lexuosa L., Vaccinium my illus L., and Rhododen- d on e ugineum L. Rep esen a i e a 0.5km IT-SR2 San Rosso e (ICOS) 61.847 24.295 2018/06/28 Ocean Op ics; FLAME-S-VIS-NIR-ES Ligus um ulga e Rep esen a i e <1.5km IT-T To gnon 45.833 7.567 2018/07/07 Ocean Op ics; FLAME-S-VIS-NIR-ES Junipe us communis,Rhododend on e ugineum, and Fes uca a ia No ep esen a i e h ps://doi.o g/10.5194/bg-18-621-2021 Biogeosciences, 18, 621–635, 2021 626 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y Table 1. Con inued. Si e Code Si e Name La (◦) Long (◦) Sampling da e Spec ome e model Unde s o y ege a ion Rep esen a i eness NL-Loo Loobos (ICOS) 52.167 5.744 2018/08/13 Ocean Op ics; FLAME-S-VIS-NIR-ES P unus se o ina,Vaccinium my illus, Deschampsia lexuosa, and mosses Rep esen a i e a 0.5km NO-Hu Hu dal (ICOS) 60.372 11.078 2018/09/27 Ocean Op ics; FLAME-S-VIS-NIR-ES Vaccinium spec. and No way sp uce seedlings Rep esen a i e PT-Co Co uche (LTER) 39.138 -8.333 2016/10/08 Ocean Op ics; FLAME-S-VIS-NIR-ES Rumex ace osella,Tube a ia gu a a, Tolpis ba ba a Plan ago co onopus, Ag os is pou e ii,B iza maxima, Vulpia b omoides, and Vulpia genicu- la a Sphe oid does no i SE-H m Hyl emossa (ICOS) 56.098 13.419 2018/09/28 Ocean Op ics; FLAME-S-VIS-NIR-ES Con inuous moss co e Sphe oid does no i SE-Knd Kindla (LTER) 59.754 14.908 2016/07/16 ASD FieldSpec P o E icaceous dwa sh ubs, mosses, and lichens Rep esen a i e SE-No No unda (ICOS) 60.086 17.48 2018/10/22 ASD FieldSpec P o Bilbe y, lingonbe y, and moss Rep esen a i e <1.5km SE-S b S a be ge (ICOS) 64.256 19.775 2019/08/23 ASD FieldSpec P o Bilbe y, lingonbe y, and moss Rep esen a i e se o au oma ic exposu e, ape u e-p io i y mode, minimum ape u e, and F2 lens (Mac a lane e al., 2007). The came a was le eled a he heigh o 1.4m abo e he g ound, and he lens was poin ed owa ds he zeni h. This se up p o ides a iew zeni h angle om 0 o 15◦, which is compa able wi h he i s ing o he LAI-2000 ins umen (Mac a lane e al., 2007). We used he algo i hm by Nobis and Hunzike (2005) o h eshold he majo i y o he DCP images. Howe e , some o he images we e isibly o e exposed, i.e., he 8bi digi al numbe s (DNs) o he backg ound sky we e 255, and pa s o any po ion o he sky we e black ( ypically a 240–250 DN). Nex , a me hod based on ma hema ical image mo phology (Ko honen and Heikkinen, 2009) was applied o es ima e he oliage and c own co e ac ions. In his me hod, black and whi e canopy images a e p ocessed wi h mo phological clos- ing and opening ope a ions ha a e well known in digi al im- age p ocessing (Gonzalez and Woods, 2002). As a esul , a il e o la ge gaps was ob ained. When a uning pa ame- e (called he s uc u ing elemen in image p ocessing) was se so ha la ge gaps only occu ed be ween indi idual ee c owns (Ko honen and Heikkinen, 2009), he p opo ions o gaps inside and be ween indi idual c owns could be calcu- la ed. 2.3 Backg ound signal e ie al me hod wi h EO da a The o al e lec ance o a pixel (R) esul s om he weigh ed linea combina ion o e lec ance alues by he o es canopy, o es backg ound, and hei sunli and shaded componen s (Li and S ahle , 1985; Chen e al., 2000; Bacou and B éon, 2005; Chopping e al., 2008; Roujean e al., 1992) as ollows: R=kTRT+kGRG+kZT RZT +kZGRZG,(1) which includes he e lec i i ies o he sunli c owns (RT), sunli unde s o y (RG), shaded c owns (RZT ), and shaded unde s o y (RZG). RGma ks he bidi ec ional e lec ance ac o (BRF) o he a ge (unde s o y). The kja e he p o- po ions o hese componen s a he chosen iewing angle o in he ins an aneous ield o iew o he senso a he gi en i - adia ion geome y. Following Canisius and Chen (2007), we de i e he unde s o y e lec i i y (RG) wi h he assump ion ha he e lec i i ies o he o e s o y and unde s o y a he gi en illumina ion geome y di e li le be ween he chosen iewing angles. While he componen s may no ully mee he de ini ion o Lambe ian e lec o s (i.e., e lec ing elec- omagne ic adia ion equally in all di ec ions), se e al p e- ious s udies (e.g., Bacou and B éon, 2005; Dee ing e al., 1999; Pel oniemi e al., 2005) ound o wa d-sca e ing e- lec ance ac o s o a ious a ge s o he p incipal plane o be ai ly cons an . The mos sui able iewing con igu a ion o he e ie al has been iden i ied by Pisek e al. (2015a), using a high angula esolu ion BRF da a se o Kuusk e al. (2014) and accompanying in si u measu emen s o unde - s o y e lec ance ac o s (Kuusk e al., 2013). The con igu- Biogeosciences, 18, 621–635, 2021 h ps://doi.o g/10.5194/bg-18-621-2021 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y 627 a ion consis s o he BRF a nadi (Rn=0◦), wi h he sola zeni h angle (SZA) co esponding o he Sun’s posi ion a 10:00 local ime (LT) o gi en day and ano he zeni h angle (Ra=40◦) wi h ela i e azimu h angle PHI=130◦. I can be exp essed by Eqs. (2) and (3) as ollows: Rn=kT nRT+kGnRG+kZT nRZT +kZGnRZG (2) Ra=kT aRT+kGaRG+kZT aRZT +kZGaRZG.(3) The p opo ions o he componen s (kj) we e ob ained using he ou -scale model (Chen and Leblanc, 1997) wi h pa am- e e s o gene alized deciduous and coni e ous ee s ands as an inpu (see Table 2; Kuusk e al., 2013). The unde s o y e lec ance a he desi ed wa eleng hs can be calcula ed by combining and sol ing Eqs. (2) and (3) and he inse ion o Rnand Raes ima es de i ed om app op ia e EO da a. The indi idual componen s (sunli /shaded o e s o y and unde - s o y) canno be esol ed wi h he MODIS spa ial esolu ion. The e lec ances o shaded ee c owns (RZT ) and unde s o y (RZG) a e ela ed o sunli ones ia Mas RZT =M×RTand RZG =M×RG, whe e M=RZ/R o a e e ence a ge , which can be measu ed in he ield o p ede e mined wi h he ou -scale model. He e, he same Mis assumed o o e s o y ees and he unde s o y. Based on his ield wo k in Canadian bo eal o es s, Whi e (1999) sugges ed ha angula ly con- s an , wa eleng h-dependen M alues may be app op ia e, a leas du ing he g owing season. The inpu s and pa ame- e s om Table 2 may no be always p ecisely known while e ie ing he unde s o y signal o e la ge a eas. Figu e 2 shows he ela ionships be ween he a ailable in si u da a o ee heigh s o ee densi ies o e ou s udy si es wi h he 1km2 esolu ion es ima es om he global maps o Sima d e al. (2011) and C ow he e al. (2015). The weak ela ionships indica e he cu en unsui abili y o he si e-speci ic a iable es ima es o in e es ( ee heigh and ee densi y) om cu - en ly a ailable global maps a a gi en spa ial esolu ion o ou pu pose. A he same ime, he calcula ed mean alues o he ee heigh s o needlelea ed (17.5m) and b oadlea ed ee s ands (22.7m) om Sima d e al. (2011) o e he s udy si es we e easonably close o ou o iginal gene alized inpu pa ame e alues in Table 2. Following Gemmell (2000), we op ed o epo a ange o unde s o y NDVI (NDVIu) al- ues ob ained wi h he combina ion o pa ame e alues om Table 2 o each si e and da e. Speci ying he co ec con- s ain s (window) o backg ound alone has been p e iously ound o g ea ly educe he e o s in he es ima ion o o e - s o y pa ame e s (Gemmell, 2000). 2.4 MODIS BRDF da a The MCD43A1 V6 bidi ec ional e lec ance dis ibu ion unc ion and albedo (BRDF/albedo) model pa ame e da a se is a 500m g idded daily p oduc . MCD43A1 is gene - a ed by in e ing mul i-da e, mul i-angula , cloud- ee, a mo- sphe ically co ec ed, and su ace e lec ance obse a ions acqui ed by MODIS ins umen s onboa d he Te a and Aqua Figu e 2. (a) Rela ionship be ween a ailable in si u es ima es o ee heigh (in me e s) wi h Sima d e al.’s (2011) es ima e. (b) Re- la ionship be ween a ailable in si u es ima es o ee densi y ( ees pe hec a e) wi h C ow he e al.’s (2015) es ima es. DBF – decidu- ous b oadlea o es ; EBF – e e g een b oadlea o es ; DNF – de- ciduous needlelea o es ; ENF – e e g een needlelea o es ; MF – mixed o es . sa elli es o e a 16d pe iod (Wang e al., 2018). The Julian da e ep esen s he nin h day o he 16d e ie al pe iod, and consequen ly, he obse a ions a e u he weigh ed o es i- ma e he BRDF/albedo o ha pa icula day o in e es . The MCD43A1 algo i hm uses all high-quali y obse a ions ha adequa ely sample he iewing hemisphe e o i an app o- p ia e semiempi ical BRDF model ( he RossThickLiSpa se- Recip ocal model; Roujean e al., 1992; Luch e al., 2000) o ha loca ion and da e o in e es . We compu ed he bidi- ec ional e lec ance ac o (BRF) a he op o he canopy wi h he iso opic pa ame e and wo ( olume ic and geo- me ic) ke nel unc ions (Roujean e al., 1992) o MODIS band 1 ( ed – 620–670nm) and band 2 (NIR – 841–876 nm). We used he Ross and Li ke nels o econs uc he BRF al- ues o equi ed geome ies (see Sec . 2.3) o each da e, and hen we de i ed he unde s o y signal, using he o mulas de- sc ibed in Sec . 2.3. The associa ed da a quali y (MCD43A2) p oduc was employed o assess he e ec o he e ie al quali y on he accu acy o he calcula ed unde s o y signal. All MODIS da a ha e been accessed and p ocessed h ough he Google Ea h Engine (Go elick e al., 2017). 2.5 Spa ial ep esen a i eness assessmen o he alida ion si es A me hod de eloped by Román e al. (2009), and e ined by Wang e al. (2012, 2014, 2017) was adop ed o e alua e he spa ial ep esen a i eness o in si u measu emen s o assess he unce ain ies a ising om a di ec compa ison be ween ield-measu ed o es unde s o y spec a and he co espond- ing es ima es wi h MODIS BRDF da a. To cha ac e ize he spa ial ep esen a i eness o a es si e o ep esen a sa el- li e e ie al, his me hod uses h ee a iog am model pa- ame e s ( he ange, sill, and nugge ), ob ained by he anal- ysis o nea -nadi su ace e lec ances om cloud- ee 30m Landsa /Ope a ional Land Image (OLI) da a (Román e al., 2009) collec ed as close o he sampling da e as possible. h ps://doi.o g/10.5194/bg-18-621-2021 Biogeosciences, 18, 621–635, 2021 628 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y Table 2. S and pa ame e s o he ou -scale model. S and Deciduous Coni e ous S and densi y ( eesha−1) 500, 1000, and 2000 500, 1000, and 2000 T ee heigh (m) 25 16 Leng h o li e c own (m) 9.2 4.2 Radius o c own p ojec ion (m) 1.87 1.5 Lea a ea index (m2m−2) 1, 2, and 3 1, 2, and 3 Figu e 3. Sho wa e BRF composi es cen e ed a ICOS si es o (a) No unda in Sweden and (c) Wüs ebach in Ge many. (b, d) Va iog am es ima o s (poin s), sphe ical model esul s (do ed cu es), and sample a iances (solid s aigh lines) ob ained o e he si es wi h Ope a ional Land Image (OLI) subse s and spa ial elemen s o 0.275, 0.5, 1.0, 1.5, and 2km as a unc ion o he dis ance be ween obse a ions. Va iog am legend explana ions: a – a iog am ange; a – sample a iance; c – a iog am sill; c0 – nugge a iance. Whe e alid image y was no a ailable wi hin a easonable window o he sampling da e, image y om he co espond- ing season o a di e en yea was used. As such, he analy- sis was done o illus a e he ep esen a i eness o he owe si e wi h espec o a pa icula poin in ime. Campagnolo e al. (2016) showed ha he e ec i e spa ial esolu ion o 500m g idded MODIS BRDF p oduc a mid-la i udes is a ound 833×618m because o he a ied oo p in s o he sou ce mul i-angula su ace e lec ance obse a ions. We analyzed each si e wi h i e di e en spa ial ex en s (0.275, 0.5, 1, 1.5, and 2km) o assess and illus a e he changes in spa ial ep esen a i eness wi h di e en spa ial esolu ions. 3 Resul s and discussion 3.1 Spa ial ep esen a i eness Table 1 p o ides he assessmen o spa ial he e ogenei y o all si es included in his s udy, using OLI subse s acqui ed a ound he ime o in si u measu emen s. The example e- sul s o he ICOS si es o No unda (SE-No ) in Sweden and Biogeosciences, 18, 621–635, 2021 h ps://doi.o g/10.5194/bg-18-621-2021 J. Pisek e al.: Re ie al and alida ion o o es backg ound e lec i i y 629 Wüs ebach (DE-RuW) in Ge many, using h ee OLI subse s, a e shown in Fig. 3. The a iog am unc ions wi h ele an model pa ame e s o he wo si es a e displayed in Fig. 3b and d. The ange co esponds o he alue on he xaxis whe e he model la ens ou . The e is no u he co ela ion o a biophysical p ope y associa ed wi h ha poin beyond he ange alue. The sill is he o dina e alue o he ange. A smalle sill alue indica es a mo e homogenous su ace (less a ia ion in su ace e lec ance). A su ace can be conside ed spa ially ep esen a i e wi h espec o he MODIS oo p in when he sill alue is <5.0e−4(Román e al., 2009; Wang e al., 2017). The sill alues o all spa ial ex en s a e well below he alue o 5.0e−4, up o 1km spa ial esolu ion in he case o No unda (Fig. 3b), which indica es ha he ield measu e- men s a e ep esen a i e and allow compa ison wi h MODIS e ie als a a 500m spa ial esolu ion. While he Wüs ebach si e can be conside ed spa ially homogeneous wi hin he im- media e icini y o 275m a ound he owe , he sill alue ex- ceeds he c i e ia o 5.0e−4 a >0.5km spa ial esolu ion. Du ing la e summe /ea ly au umn o 2013, ees we e almos comple ely emo ed in an a ea o 9ha wes o he owe in o - de o p omo e he na u al egene a ion o a nea -na u al de- ciduous o es om a sp uce monocul u e o es . The clea - elling a ea can be seen in Fig. 3d. This ac ion esul ed in an inc ease in he spa ial he e ogenei y o his ICOS si e. In si u measu emen s collec ed wi hin he oo p in o he Wüs ebach owe , hus, canno be deemed ully compa able wi h he e ie als wi h MODIS a a 500m spa ial esolu ion. O e all, mos o he si es we e ound ep esen a i e a he spa ial esolu ion o MODIS BRDF g idded da a. The non- ep esen a i e cases and he e ec on he unde s o y signal e ie al and ag eemen wi h he co esponding in si u mea- su emen s ca ied wi hin he measu emen oo p in o he indi idual owe s a e u he discussed in Sec . 3.2 and 3.3. Román e al. (2009) p o ide u he de ails on he assessmen o spa ial ep esen a i eness, using a se o ou geos a is ical a ibu es de i ed om semi a iog ams. 3.2 NDVI anges The e is only a weak ela ionship be ween he o al (o e - s o y and unde s o y) NDVI signal e ie ed wi h MODIS BRDF da a and co esponding in si u unde s o y NDVI mea- su emen s (R2=0.19; Fig. 4). To al NDVI alues alone do no allow one o disen angle he co ec unde s o y signal. In con as , ou e ie al me hod could ack he unde s o y signal dynamics o e a b oad NDVI ange (Fig. 5). The p e- dic ed unde s o y NDVI anges we e beyond he unce ain y limi s o in si u unde s o y measu emen s (co esponding o ±1 s anda d de ia ion (SD) he e) in less han 15% o cases. These si es wi h poo e ie als we e ca e ully in es iga ed o iden i y he issues p ecluding good esul s. Below, we o- cus on a discussion o esul s whe e he p edic ed and in si u measu ed NDVI anges o he unde s o y laye did no ag ee. Figu e 4. Rela ionship be ween o al (o e s o y and unde s o y) NDVI alues compu ed om nadi NDVI alues, using MODIS BRDF/albedo da a and in si u measu ed unde s o y NDVI alues o e he s udy si es. The unde s o y domina ed he o e all signal o open sh ubland a he Co es de Pallas (ES-CPa) si e and he decid- uous b oadlea o es si e a Mon ie s (FR-MsS) du ing he lea -o pa o he season (Fig. 5). Bo h si es we e ound o be spa ially ep esen a i e o compa ison wi h MODIS oo - p in da a a he ime o he a ailable in si u measu emen s (Table 1). The e a e only e y ew ees sca e ed ac oss he Co es de Pallas si e, and g ound ege a ion is ully exposed. Ex emely low ee densi y does no ma ch wi h any o he o iginal gene alized inpu pa ame e alues in Table 2, and he p edic ed unde s o y signal does no ma ch well wi h he in si u measu emen s. In si u measu emen s a Mon ie s we e ca ied ou du ing he lea -o pa o he season, which al- lowed a ull exposu e o he unde s o y. Despi e his, he p edic ed unde s o y NDVI ange om he MODIS da a did no o e lap wi h he in si u measu emen s a Mon ie s a all. Howe e , he MODIS BRDF alues o hese si es we e ma ked wi h lowe da a quali y lags (QA >1), which co - ec ly signals a dec ease in accu acy in he calcula ions o he unde s o y e lec ance as well. O e all, ou esul s con- i m ha , unde condi ions o e y low ee densi y/lea -o condi ions, he unde s o y signal can be assumed o be iden- ical o he o al scene NDVI. The pe o mance o he me hod u ns ou o be limi ed o e si es wi h a closed canopy, such as Bílý Kˇ íž (CZ- BK1), Hesse (FR-Hes), o Vielsalm (BE-Vie; Fig. 5). This is because he shadowing e ec makes di use sca e ing he dominan mechanism in such s ands, and he unde s o y ca - ies only a negligible in luence on he op-o -canopy signal. Bosco Fon ana (IT-BF ) is ano he b oadlea o es si e wi h e y high oliage co e (FC=0.91), ye he p edic ed unde - s o y NDVI ange en i ely o e laps wi h he collec ed in si u alues. I should be no ed ha , in con as o o he si es wi h h ps://doi.o g/10.5194/bg-18-621-2021 Biogeosciences, 18, 621–635, 2021