Deep lea ning models o es ima ing olume and Lo ey's
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heigh ac oss No dic coun ies using op ical and SAR
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sa elli e images
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Zso ia Koma1 Oleg An opo 2 Oli e Ca us3 Jukka Mie inen2 and Johannes B eidenbach1
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1 No wegian Ins i u e o Bioeconomy (NIBIO), Di ision o Fo es and Fo es Resou ces, Na ional
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Fo es In en o y, Høgskole eien 7, 1433 Ås, No way
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2 VTT Technical Resea ch Cen e o Finland, P.O. Box 1000, 02044 Espoo, Finland
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3Gamma Remo e Sensing, Wo bs asse 225, 3073 Gümligen, Swi ze land
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Ta ge jou nal: In e na ional Jou nal o Applied Ea h Obse a ion and Geoin o ma ion
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Highligh s
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- A la ge-scale, ine- esolu ion o es esou ce map was p oduced in No way om mul isou ce
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op ical and SAR image y, ained on an ALS-based e e ence map using a UNe deep lea ning
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amewo k.
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- The bes UNe model achie ed coe icien s o de e mina ion (R²) o 0.63 and 0.55 o imbe
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olume and Lo ey’s heigh , espec i ely, compa ed o 0.39 and 0.37 om he e e ence
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app oach (k-nea es neighbou s wi h k = 7).
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- We also analyse he geog aphical ans e abili y o he p e- ained model on Finnish da a o
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ine- une o No wegian condi ions.
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Abs ac
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Spa ially explici in o ma ion on o es esou ces and s uc u e is essen ial o sus ainable o es
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managemen and e idence-based policy-making. In he No dic egion, la ge-scale o es mapping o en
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elies on in eg a ing Na ional Fo es In en o y (NFI) ield plo s wi h ai bo ne lase scanning (ALS) da a.
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Howe e , he in equen co e age o na ionwide ALS campaigns limi s hei use o con inuous
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moni o ing. Sa elli e image y, wi h i s high empo al and spa ial esolu ion, p o ides a p omising
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al e na i e. We e alua e UNe -based deep lea ning models ained on wall- o-wall ALS-de i ed o es
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esou ce maps o p edic ing olume and Lo ey’s heigh in No way using op ical (Sen inel-2) and SAR
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(Sen inel-1, PALSAR-2) da a. The UNe models, ained on bo h Finnish and No wegian ALS maps, a e
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benchma ked agains kNN models based on No wegian NFI da a. T ans e lea ning is u he explo ed
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by ine- uning models using No wegian NFI plo s. Model unce ain ies a e assessed using 541 ese ed
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NFI plo s and 44 independen alida ion s ands mainly including high- olume bo eal o es s (>200 m³
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ha⁻¹). The bes -pe o ming UNe model, ained on No wegian ALS da a, achie ed an R² o 0.63 o
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olume and 0.55 o Lo ey’s heigh , consis en ly ou pe o ming kNN. Fine- uning imp o ed model
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ans e abili y, wi h gains in R² o up o 0.13 o olume and 0.46 o Lo ey’s heigh , when adap ing he
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Finnish model o No wegian condi ions. U ilizing SAR da a alongside op ical da a enhanced model
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accu acy, wi h R2 imp o emen anging om 0.03 o 0.19. O e all, ou indings demons a e he
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po en ial o UNe models ained on wall- o-wall ALS maps, o e ing a ans e able app oach o o es
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esou ce mapping ac oss No dic coun ies.
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Keywo ds: UNe , deep lea ning, o es esou ce mapping, sa elli e image y, SAR, mul isou ce
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1. In oduc ion
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Quan i ying he s a us and ends o o es esou ces and s uc u e—such as imbe olume and Lo ey’s
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heigh —is c ucial o in o ming e idence-based policy and managemen decisions ac oss local,
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na ional, and in e na ional le els (Tomppo e al., 2010; Vidal e al., 2016a). Sampling-based Na ional
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Fo es In en o ies (NFIs) p o ide his in o ma ion a na ional and egional scales, o ming he
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ounda ion o s a egic decision and policy-making ega ding key ecosys em se ices, including
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bioene gy po en ial, biodi e si y, and changes in ca bon s ock (Vidal e al., 2016b; B eidenbach e al.,
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2020). Howe e , because NFIs a e sampling-based su eys, hei capaci y o p oduce eliable es ima es
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a local scales is es ic ed (B eidenbach and As up, 2012; As up e al., 2019). To b idge his gap, NFI
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ield plo s a e o en combined wi h emo ely sensed da a o model ela ionships be ween ield-
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obse ed o es a ibu es and emo e sensing-based p edic o a iables, enabling mapping be ween
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NFI plo loca ions (McRobe s and Tomppo, 2007; Kangas e al., 2018). This app oach aligns wi h he
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equi emen s o se e al in e na ional policy amewo ks—including he EU Fo es S a egy o 2030,
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he EU Biodi e si y S a egy, and he EU De o es a ion Regula ion—which emphasize he need o
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spa ially explici o es moni o ing a bo h na ional and local scales. Consequen ly, in eg a ing emo ely
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sensed da a has become essen ial o p oducing de ailed o es esou ce maps and in o ma ion.
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Some o he ea lies na ional o es maps we e based on a combina ion o NFI and Landsa da a
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(Gje sen, 2007; McRobe s and Tomppo, 2007). In many coun ies, na ional o es esou ce maps a e
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oday based on in eg a ing ield plo s om NFIs wi h coun y-wide Ai bo ne Lase Scanning (ALS) da a
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(Kangas e al., 2018). ALS o e s de ailed 3D in o ma ion on o es s uc u e, enabling accu a e
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es ima ion o key a ibu es such as imbe olume and Lo ey’s heigh . Howe e , na ional ALS
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campaigns lack he empo al equency needed o e ec i e o es moni o ing. Fo example, No way’s
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na ionwide ALS campaign equi ed app oxima ely en yea s o comple e ull co e age (Hauglin e al.,
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2021), compa ed o a se en-yea ime ame in Sweden (Nilsson e al., 2017). In Finland, he u u e ALS
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campaigns a e planned o be conduc ed in nine-yea cycles (NLS, 2025). In con as , Ea h Obse a ion
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(EO) da ase s—such as op ical image y om Sen inel-2 and Syn he ic Ape u e Rada (SAR) da a om
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Sen inel-1 o ALOS-2 PALSAR-2—o e bo h high empo al and spa ial co e age. Se e al s udies ha e
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u ilized op ical emo e sensing o o es esou ce mapping, such as Sen inel-2 o LANDSAT (As ola e
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al., 2019; Lang e al., 2019; Mie inen e al., 2021) o ha e combined EO wi h ALS da a (Es eban e al.,
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2019; As ola e al., 2021). SAR da a wi h clea sensi i i y o o es s uc u e ha e also been used o
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o es mapping, p ima ily a s and-le el o coa se mapping uni s and in wide-a ea s udies (San o o e
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al., 2011; An opo e al., 2017; Ge e al., 2023b). Recen s udies ha e shown ha he combina ion o
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op ical and SAR da a imp o es he quan i ica ion o o es s uc u al cha ac e is ics (Wi ke e al., 2019;
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Pe sson e al., 2021; Ge e al., 2022, 2023a), especially i SAR ope a es in lowe equencies (e.g., L-
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band used on boa d ALOS-2), which opens new possibili ies o imp o ing he accu acy o maps o
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es ima ing o es a ibu es.
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EO-based o es esou ce maps ha e been c ea ed using s a is ical me hods such as linea eg ession,
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k-nea es neighbou s (kNN), and ensemble-based machine lea ning echniques such as andom o es s.
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S udies using only Sen inel-2 da a wi h hese app oaches ha e epo ed RMSEs anging om 27.2% o
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59.3% o es ima es o olume and Lo ey’s heigh in bo eal o es con ex s (As ola e al., 2019;
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Mie inen e al., 2021; Pe sson e al., 2021). When using SAR da a alone, RMSEs o olume ange om
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32% o 54.3% (Wi ke e al., 2019; Ge e al., 2023b; Pe sson e al., 2021). One s udy sys ema ically
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es ed he in eg a ion o op ical and SAR da ase s using kNN me hod and epo ed imp o ed accu acies
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in Sweden, wi h olume RMSE educed om 37.9% o 30.2% in bo eal o es s (Pe sson e al., 2021).
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Howe e , a ecu ing limi a ion is, o ins ance, a sa u a ion e ec in imbe olume p edic ions—
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pa icula ly abo e 300 m³/ha (San o o e al., 2011) —which con ibu es o lowe o e all accu acy
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compa ed o models based on ai bo ne lase scanning (ALS) da a (Wi ke e al., 2019). Recen ad ances
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in Deep Lea ning (DL) show p omise o imp o ing EO-based o es esou ce mapping. Ge e al. (2023a)
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demons a ed ha he p edic ion o Lo ey’s heigh using Sen inel-2 da a alone, as well as combined
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Sen inel-2 and SAR da a, imp o ed by 0.3% o 1.6% RMSE when applying he UNe DL me hod
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compa ed o adi ional app oaches such as kNN, andom o es , o linea eg ession. Simila ly, As ola
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e al. (2021) epo ed RMSE imp o emen s o 7.9% o olume and 2.5% o heigh when using DL
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models compa ed o e e ence me hods. Ne e heless, DL emains unde u ilized in eg ession asks o
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EO-based o es a ibu e es ima ion, especially when compa ed o i s mo e widesp ead applica ion in
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classi ica ion asks such as dis u bance mapping (Yuan e al., 2020).
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One o he key challenges in applying DL o o es esou ce mapping is he limi ed a ailabili y and
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quali y o sui able aining da a (Ball e al., 2017; Li e al., 2019). NFI da ase s a e o en conside ed weak
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labels in he con ex o DL because hey lack spa ial con ex due o a ela i ely small plo size. To add ess
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his challenge, ans e lea ning has eme ged as a p omising s a egy in a ious emo e sensing asks
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e.g. (Wu m e al., 2019; Zhou e al., 2023; Kuzu e al., 2024), bu only a ew s udies ha e been used o
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in he con ex o o es esou ce mapping (As ola e al., 2021; Ge e al., 2023a). Ins ead o aining a
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model solely on da a om he a ge egion, ans e lea ning allows he use o a p e- ained model—
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de eloped on a ela ed ask —which can hen be ine- uned using a smalle se o local aining da a.
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This app oach le e ages he gene alizable ea u es lea ned du ing p e- aining, enabling e ec i e
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model adap a ion e en wi h limi ed labelled da a. In addi ion, ALS-based o es esou ce maps can
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o e high-quali y, wall- o-wall aining da a ha can complemen o subs i u e adi ional plo -based
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labels (Ge e al., 2022, 2023a). Fo example, Ge e al. (2023a) demons a ed he success ul aining o
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models in Finnish Lapland using ALS-de i ed maps, along wi h op ical and SAR sa elli e image y,
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ollowed by ine- uning wi h plo -le el measu emen da a in sou he n Finland. To u he e alua e he
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e ec i eness o his app oach, applying ans e lea ning o neighbou ing coun ies—such as No way—
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p o ides an ideal es case.
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In his s udy, we de elop and e alua e he po en ial o UNe -based deep lea ning models in mul iple
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EO da a combina ions o es ima e imbe olume and Lo ey’s heigh using op ical (Sen inel-2) and SAR
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(Sen inel-1 and PALSAR-2) sa elli e image y. ALS-based o es esou ce maps se e as wall- o-wall
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aining da a o models ained in bo h Finland and No way. We in es iga e ans e lea ning by ine-
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uning ALS-based p e- ained models wi h No wegian NFI plo s, bo h by ans e ing he Finnish model
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o No wegian condi ions and explo ing u he op ions o imp o ing he No wegian model. The
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pe o mance and unce ain y o he UNe models ained on exis ing o es esou ce maps a e
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compa ed wi h kNN models ained on NFI ield plo s. Model unce ain y is assessed using bo h a es
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da ase o NFI plo s, which was no used in modelling, and an independen alida ion da ase o o es
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s ands. Ou esea ch ques ions a e: (1) Which modelling me hod -- UNe o kNN -- achie es he highes
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accu acy? (2) Can ans e lea ning, whe e NFI ield plo s a e inco po a ed in he model, imp o e he
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UNe accu acy, especially in he case o applying he Finnish model o No wegian condi ions? (3) Can
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he inclusion o SAR image y enhance he model’s p edic ion accu acy?
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2. Ma e ials and Me hods
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The o e all wo k low de eloped o his s udy (Fig. 1) consis s o ou main p ocessing s eps: (a) kNN
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modelling; (b) p e aining UNe models using ALS-based o es esou ce maps om Finland and
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No way; (c) ine- uning UNe models om Finland and No way o he No wegian s udy si e; and (d)
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assessing he unce ain y associa ed wi h each model ype (k-NN, p e ained UNe , and ine uned
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UNe ).
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Figu e 1. O e iew o he wo k low used o assessing he accu acy o di e en models (kNN, UNe , ine uned UNe ). The
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inpu da ase s a e ma ked wi h ec angles (sa elli e image y da a, o es esou ce maps, No wegian NFI plo s, and
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No wegian alida ion s ands), da k g ey indica ing inpu o aining he models and ligh g een o alida ion models. The
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in e media e models (kNN, UNe , and ine uned UNe ) a e ma ked wi h a snipped ec angle. The main p ocessing s eps a e
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shown wi h ounded ec angles (a-d co esponding o he main ex ). V is olume, and H is Lo ey’s heigh , S2 is Sen inel-2
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op ical images, S1 is Sen inel-1 SAR images, and P2 is PALSAR-2 SAR images. FI = Finland and NO = No way.
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2.1. Da ase s
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2.1.1. Sa elli e image y da a and p e-p ocessing
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ESA Cope nicus Sen inel-2 sa elli es ope a ing wi h he Mul i-Spec al Ins umen (MSI) p o ide op ical
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image y da a on 13 spec al bands a a 2–3-day e isi equency in he No dic coun ies, in isible,
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nea -in a ed, and he sho -wa e in a ed pa o he elec omagne ic spec um. In his s udy, we used
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he Le el-2 su ace e lec ance p oduc s p o ided by ESA a he isible and nea in a ed bands (B2, B3,
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B4, B5, B8) and sho -wa e in a ed bands (B11, B12). The 20 m esolu ion bands (B5, B11, B12) we e
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esampled in o 10 m esolu ion using he bilinea in e pola ion me hod o ma ching he esolu ion
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wi h he o he bands (10 m). The Sen inel-2 da a we e u he p ocessed wi h he Te amoni o
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composi ing sc ip (Mie inen e al., 2021), p oducing g owing season composi es by weigh ing based
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on cloudiness, haze, and shadows. The composi ing p ocess also deli e ed a quali y lag ha was used
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o il e ing cloud-a ec ed o bad-quali y pixels.
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In his s udy, we ha e used wo ypes o SAR da ase s. The i s consis ed o C-band backsca e image y
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acqui ed by Sen inel-1A in In e e ome ic Wide (IW) swa h mode, using VV and VH pola isa ion. The
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images in G ound-Range-De ec ed (GRD) o ma we e p e-p ocessed using he GAMMA Remo e
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Sensing so wa e (We ne e al., 2000), which esul ed in geocoded and adiome ic e ain co ec ed
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g0 backsca e images. The images we e subsequen ly esampled o 10 m o ma ch he Sen inel-2
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image y. The Sen inel-1 da a acqui ed wi hin a yea (30 o 31 images) we e hen used o calcula e he
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annual mean backsca e pe pola iza ion. Maps depic ing a eas o layo e o shadow we e applied o
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mask such a eas.
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The second ype o SAR da ase was based on he Japan Ae ospace Explo a ion Agency (JAXA), which
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eleases ee and open annual mosaics o he L-band da a acqui ed by he Phased-A ay ype L-band
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SAR (PALSAR-2) onboa d he Ad anced Land Obse ing Sa elli e-2 (ALOS-2).The backsca e mosaics,
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wi h a esolu ion o ~ 25 × 25 m², a e p oduced om image y acqui ed in Fine Beam Dual Pola iza ion
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mode (HH and HV), using s ipmap mode unde JAXA’s global basic obse a ion scena io. The images
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used o gene a e he mosaics we e adiome ic e ain-co ec ed, and geocoded o a geog aphic spa ial
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e e ence sys em, as wi h (Shimada and Oh aki, 2010). In his s udy, he HH and HV mosaics we e
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esampled o a 10 m g id and masked o layo e and shadow using he layo e /shadow maps p oduced
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by JAXA.
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All sa elli e image y da ase s we e p ocessed o he s udy a eas in Finland and No way. In Finland, he
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da ase s we e p ocessed o he yea 2020, and in No way o 2021, o align as closely as possible wi h
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he iming o he ALS-based o es esou ce maps.
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2.1.2. Finnish e e ence da ase s
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The Finnish UNe model was ained based on he wall- o-wall Finnish ALS-based o es esou ce map
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p oduced by he Finnish Fo es Cen e using ALS acquisi ions om 2020 and associa ed ield in en o y
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plo s as aining o k-NN based app oaches (Mal amo and Packalen, 2014). The map co e s he co e
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cen al and sou he n egions o Finland (Fig. 2), cap u ing bo eal o es s in 16 m spa ial esolu ion. The
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s udy a ea has ela i ely small ele a ion di e ences, wi h he highes poin s gene ally less han 300 m
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abo e sea le el. The o es in he e e ence a eas has a ound 40% pine, mo e han 30% sp uce, and he
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es deciduous ees. The a e age o es olume ac oss he s udy si es was 120 m3 ha-1, wi h an a e age
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heigh o 14 m.
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2.1.3. No wegian e e ence da ase s
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The No wegian s udy si e co e s app oxima ely 210,000 km² o land, encompassing he Sou h-Eas e n,
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Cen al, Sou he n, and Wes e n egions, ex ending om he coun y o T øndelag and sou h (Fig. 2). I
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is loca ed in he bo eal clima e zone and con ains mos o he p oduc i e o es o No way. The bo eal
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o es s a e domina ed by h ee main species: No way sp uce, Sco s pine, and deciduous ees, p ima ily
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bi ch, which a e o en mixed wi h coni e ous species. The s udy si e has a di e se opog aphical
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a ia ion.
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Figu e 2 The No wegian s udy a ea (highligh ed in g een on he No dic map) is whe e he di e en model ypes—kNN, UNe ,
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and UNe ine- uned —we e e alua ed. G een do s ep esen he NFI plo s used o es ing, while pink colou ed do s indica e
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he aining NFI plo s used o he kNN model and UNe ine- uning. B own a eas ma k he independen alida ion s ands.
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Panel A shows a zoomed-in iew o he s udy a ea, wi h panels B and C highligh ing wo example alida ion s ands used in
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he accu acy assessmen . The backg ound maps a e he ALS-based imbe olume map om Kilden (NIBIO, 2025) and
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Google sa elli e image y in panel A.
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Di e en ield obse a ion da ase s we e u ilized in his s udy (Fig.2, Table 1). Fi s , he No wegian NFI
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is a con inuously ope a ing in en o y based on a pe manen sample g id, whe e each ield plo is
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emeasu ed e e y i e yea s (B eidenbach e al., 2020). The NFI ield measu emen s a e ca ied ou
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wi hin ci cula 250 m2 plo s whe e he diame e a b eas heigh (dbh) and species o all ees wi h a
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dbh >= 5 cm a e eco ded, and he heigh o en ees is measu ed o calcula e imbe olume and
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Lo ey’s heigh . Only NFI plo s measu ed in 2021 wi hin a alid emo e sensing-based o es mask we e
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included in he s udy, which esul ed in 1566 NFI plo s. 541 NFI plo s we e se aside o es ing and no
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used o kNN o DL modelling, and he emaining 1025 NFI plo s we e used in model aining (Table 1).
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Addi ional s and-le el ield measu emen s (Fig. 2, Table 1), independen o he NFI, we e used o
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alida ion (Koma and B eidenbach, 2025). The da ase includes wo subse s: (a) ma u e s ands in Aske
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(eas e n egion) and Al e (wes e n egion), whe e 10–15 plo s pe s and we e sampled ollowing NFI
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p o ocols. Mean imbe olume and Lo ey’s heigh we e calcula ed o cha ac e ise each s and. (b) Long-
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e m ield ials assessing o es managemen me hods, consis ing o 1–30 adjacen ec angula s ands.
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Wi hin each s and, dbh and species we e eco ded o all ees, and heigh was measu ed o e e y
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ou h ee. S and-le el imbe olume and Lo ey’s heigh we e calcula ed and agg ega ed o ial-le el
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a e ages. A e applying a emo e sensing-based o es mask and excluding cloud-co e ed s ands om
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2021, 44 alida ion s ands co e ing 62 ha we e a ailable o unce ain y assessmen .
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Table 1 Summa y s a is ics o aining NFI, es ing NFI, and alida ion s and da ase s. The able includes he numbe o
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obse a ions (N), minimum (Min), maximum (Max), mean, and s anda d de ia ion (SD) o each a iable (Volume, Lo ey’s
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heigh , a ea o he plo , and he s ands).
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Da ase
Va iable
Uni
N
Min
Max
Mean
SD
T aining NFI plo s
Volume
m3ha-1
1025
0.18
1157.59
118.13
127.99
Lo ey's heigh
m
2.40
28.18
11.96
5.11
A ea
m2
250
Tes ing NFI plo s
Volume
m3ha-1
541
0.30
779.70
125.91
118.66
Lo ey's heigh
m
3.40
28.00
12.38
4.66
A ea
m2
250
Valida ion s ands
Volume
m3ha-1
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100.62
906.70
369.14
206.17
Lo ey's heigh
m
12.24
26.55
18.58
4.11
A ea
ha
0.11
4.32
1.40
1.06
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Thi d, he No wegian Fo es Resou ce Map (SR16) was used as a wall- o-wall aining da ase o he
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UNe model. These spa ially con inuous maps p o ide es ima es o imbe olume, Lo ey’s heigh , and
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o he o es a ibu es a a 16 m × 16 m esolu ion and a e ope a ionally upda ed each yea ollowing
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he wo k low desc ibed by Hauglin e al. (2021). They a e p oduced by combining NFI plo da a wi h
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p edic o a iables de i ed om na ionwide ALS acquisi ions conduc ed by he No wegian Mapping
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Agency. Species-speci ic uni a ia e linea mixed-e ec s eg ession models a e used o accoun o
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sys ema ic di e ences ac oss ALS p ojec s and egions. To ensu e consis ency, NFI plo alues a e
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p ojec ed o a common e e ence da e, and a emo e sensing-based o es mask is applied.
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2.2. Modelling
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2.2.1. kNN modeling
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The kNN me hod has been chosen as he baseline me hod o be compa ed wi h he DL me hod, since
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i has been success ully applied in es ima ing o es a iables and i is ope a ionally used, o example,
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in Finland (McRobe s and Tomppo, 2007). I is a non-pa ame ic algo i hm, whe e he p edic ions o
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he a ge a iables a e ob ained om as he a e ages o he k neighbou s wi h he smalles dis ances
226
o he a ge uni in he auxilia y space. The kNN model can be desc ibed as ollows:
227
𝑦𝑝=∑𝑤𝑖,𝑝
𝑖 𝜖 𝐼 𝑦𝑖
228
whe e he p edic ed ec o o each pixel is 𝑦𝑝 , 𝑦𝑖 is he ec o o obse a ions o he 𝑖- h con ibu ing
229
uni in he e e ence se , 𝐼 is he subse o con ibu ing uni s ha a e nea es wi h espec o he
230
dis ance me ic, and 𝑤𝑖,𝑝 is he weigh o i- h con ibu ing uni calcula ed as
231
𝑤𝑖,𝑝= {𝑑𝑖,𝑝
−𝑡
∑𝑑𝑖,𝑝
−𝑡
𝑖 𝜖 𝐼
0 𝑖 𝜖 𝐼
232
whe e 𝑡>0 weigh uni s in e sely o hei ea u e space dis ance o dis ance squa ed om pixel 𝑝. -
233
In his s udy, 𝑡 = 1 was used, wi h Euclidean dis ance and se en nea es neighbou s (k=7).
234
The kNN aining da abase was cons uc ed by in e sec ing aining NFI plo loca ions (Fig.2, Table 1)
235
wi h he p e-p ocessed EO da ase s, including Sen inel-2, Sen inel-1, and PALSAR-2 bands. Fo each
236
plo , all pixels o e lapping he 250 m² plo a ea we e conside ed, and hei con ibu ions we e
237
weigh ed by he ac ion o pixel a ea alling wi hin he plo . A weigh ed mean o he in e sec ing pixel
238
alues was hen compu ed, esul ing in a aining able ha combined ield-measu ed a ibu es wi h
239
co esponding EO ea u es. P edic ions we e gene a ed o each es NFI plo and alida ion s and
240
loca ions.
241
2.2.2. UNe model
242
A a ian o a UNe model (Ronnebe ge e al., 2015) has been adap ed o his s udy, which is a
243
con olu ional neu al ne wo k (CNN) widely used in biomedical image and o he seman ic segmen a ion
244
asks. Compa ed o a basic CNN, UNe models allow ex ac ion o deepe ea u e ep esen a ions om
245
he inpu image da ase s and keep he ea u e map size unchanged, which makes i sui able o pixel-
246
le el eg ession asks.
247
The UNe a chi ec u e applied (Fig.3.) in his s udy is a symme ic encode –decode ne wo k designed
248
o pixel-le el p edic ions. The encode con ains a double-con olu ion s uc u e, which includes a 2D
249
con olu ion, ba ch no maliza ion, and ReLU ac i a ion. A each s ep, 2×2 max-pooling hal es he
250
ea u e map. This allows he model o g adually ansi ion om cap u ing ine, local de ails o lea ning
251
mo e global spa ial pa e ns. The decode mi o s his p ocess by es o ing he o iginal esolu ion
252
h ough upsampling ope a ions. To eco e in o ma ion los du ing downsampling, skip connec ions
253
link co esponding encode and decode laye s, as he pooling ope a ion disca ded some de ails.
254
Applying skip-connec ion, he shallow ea u e maps a e conca ena ed o deep ea u es eco e ed om
255
up-sampling. Finally, a 1×1 con olu ion p ojec s he es o ed ea u e map in o he a ge space,
256
p oducing he desi ed pixel-wise p edic ions wi hou al e ing he spa ial dimensions.
257
258
259
Figu e 3 The UNe model a chi ec u e used in he s udy. Each box ep esen s a ea u e map wi h i s size indica ed nea by.
260
2.2.3. UNe model aining
261
Fi s , all inpu da ase s we e p epa ed o aining he UNe model (Fig. 4). Du ing p e- aining, he
262
Finnish and No wegian wall- o-wall ALS-based o es esou ce maps, desc ibed in Sec ions 2.1.2–2.1.3,
263
we e used as e e ence da ase s. The p e- aining da ase comp ised 1,068 ec angula pa ches o
264
Finland and 1,025 pa ches o No way, each wi h a size o 2.56 × 2.56 km² (256 × 256 pixels). These
265
pa ches we e andomly dis ibu ed ac oss he o es ed a eas co e ed by he ALS-based o es esou ce
266
maps. The esolu ion o he o es esou ce maps was aligned wi h he sa elli e image y da ase s by
267
esampling he na i e spa ial esolu ion o 16 × 16 m o 10 × 10 m. Fo ine- uning, 1,025 No wegian
268
NFI plo s selec ed o aining we e used. When p epa ing image pa ches ela ed o he aining NFI
269
plo s, only he 10 × 10 m pixels in e sec ing he ield plo loca ions con ained alid alues; all o he
270
pixels we e masked ou (se o no-da a).
271
P edic o a iables we e ex ac ed o he selec ed image pa ches ( o bo h p e- aining and ine-
272
uning) based on he p e-p ocessed and il e ed EO bands om Sen inel-2 only (S2), and om he
273
mul isou ce Sen inel-2, Sen inel-1, and PALSAR-2 da ase s (S2S1P2), as desc ibed in Sec ion 2.1.1. Fo
274
independen alida ion, EO image pa ches ela ed o he NFI es se —which included 541 ield plo s
275
se aside a he beginning o he modelling p ocess—and alida ion s ands we e p epa ed using he
276
same pa ch size, con aining he ele an plo s and s ands. Wi hin all image pa ches, non- o es pixels
277
we e masked ou . All EO pa ches we e u he no malised on a pe -band basis using z-sco e
278
no maliza ion, whe e each band’s mean was sub ac ed and he esul di ided by i s s anda d
279
de ia ion.
280
281
Figu e 4 O e iew o he UNe model aining wo k low. Inpu pa ches a e p epa ed o he Finnish and No wegian da ase s.
282
An independen es NFI plo s we e se aside be o e model aining and used only o inal unce ain y assessmen . Du ing
283
UNe p e- aining and ine- uning, he o es esou ce maps and aining NFI plo s used we e andomly spli 10 imes in o
284
aining, alida ion and es subse s. The in e nal alida ion and es a e used solely o un he DL model and a e no used o
285
unce ain y assessmen in his s udy.
286
Du ing UNe model p e- aining, he o e all da ase s we e andomly spli in o aining (70%), alida ion
287
(15%), and es ing (15%) pa ches. The Adam op imise was used wi h an ini ial lea ning a e o 1×10⁻⁴.
288
A ReduceLROnPla eau schedule was applied o adap i ely educe he lea ning a e by hal when he
289
alida ion loss pla eaued, wi h a pa ience o en epochs. The model was ained o 200 epochs wi h a
290
mild weigh decay o 1×10⁻⁵. The bes -pe o ming model was selec ed based on he lowes alida ion
291
loss, de e mined using he in e nal alida ion da a, and he co esponding weigh s we e sa ed o
292
subsequen ine- uning o each a ge a iable. T aining was conduc ed wi h a ba ch size o 16. The
293
inal model was used o p edic EO pa ches co esponding o he es NFI plo loca ions and alida ion
294
s ands.
295
esul s also showed ha in mos cases bias was educed, whe eas models wi hou ine- unning o en
439
exhibi ed highe bias, which aligns wi h Ge e al., (2023a).
440
Ou esul s indica e ha combining op ical and SAR p edic o s gene ally imp o ed model pe o mance
441
compa ed o op ical-only inpu s (Tables 2–3). Simila ends ha e been epo ed in o he bo eal o es
442
s udies. Fo example, Pe sson e al. (2021) demons a ed ha including SAR da a educed he RMSE
443
o olume es ima es in Sweden om 37.2% o 30.2%. Likewise, Ge e al. (2022, 2023a) epo ed an
444
imp o emen in Lo ey’s heigh p edic ion in Finland, wi h RMSE dec easing om 30.3% o 18.1%. In
445
ou s udy, he magni ude o imp o emen was smalle : he maximum educ ion in RMSE was om
446
61.3% o 57.1% o olume and om 26.4% o 25.3% o Lo ey’s heigh . O e all, we also ound ha he
447
addi ion o SAR, can imp o e cap u ing o es s uc u e, which aligns wi h se e al s udies (Wi ke e al.,
448
2019; Pe sson e al., 2021; Ge e al., 2022, 2023a). Two main ac o s can explain he mino
449
imp o emen in RMSE. Fi s , he mo e complex opog aphy in No way compa ed o Sweden o Finland
450
can in luence he SAR backsca e signal quali y. Second, Pe sson e al. (2021) u ilized TanDEM-X da a,
451
which o e s a ine spa ial esolu ion (10 m) and a mo e equen acquisi ion cycle (11-day epea
452
cycle), and, mos impo an ly, in e e ome ic cohe ence ha is inhe en ly sensi i e o e ical
453
s uc u e o o es s (Olesk e al., 2016). In con as , ou s udy also elied on PALSAR-2 da a addi ionally
454
o S1 wi h coa se esolu ion (25 m) and used a single annual mosaic.
455
5. Conclusion
456
Ou s udy demons a es he po en ial o using spa ially con inuous ALS-based o es esou ce maps as
457
aining da a o UNe -based deep lea ning models. In con as o mos p e ious bo eal o es mapping
458
app oaches—which mos commonly ely on end- o-end aining using ield plo s o speci ied sizes and
459
sa elli e image y wi hin a single coun y—we show ha ALS-de i ed maps can se e as an e ec i e
460
aining da ase ha also le e ages spa ial con ex and can be applied ac oss di e en No dic coun ies.
461
We ound ha deep lea ning models ou pe o m kNN me hod by educing o e es ima ion in low-
462
olume (<200 m³ ha⁻¹) o es s and unde es ima ion in high- olume (>200 m³ ha⁻¹) o es s. A key
463
ad an age o deep lea ning is i s ans e abili y wi h a limi ed se o local ield plo s. Ou esul s indica e
464
ha a model p e- ained on Finnish da a pe o med compa ably o kNN ained wi h No wegian da a.
465
Wi h he de elopmen o mo e comp ehensi e ounda ion models ained on mul i-coun y da ase s,
466
his app oach may enable obus , la ge-scale, c oss-coun y o es esou ce mapping.
467
Acknowledgmen s
468
This esea ch was unded by he Eu opean Space Agency (ESA) p ojec on Fo es Ca bon Moni o ing
469
unde con ac numbe 4000135015/21/I-NB—Fo es Ca bon Moni o ing, and by he EU unde
470
GA101056907 (Pa hFinde ). The sa elli e da a p ocessing was suppo ed by he ESA Ne wo k o
471
Resou ces Ini ia i e. Mic oso Copilo assis ed in enhancing he cla i y and co ec ness o he English
472
ex .
473
Au ho con ibu ions
474
ZK: Concep ualiza ion, Da a cu a ion, Fo mal analysis, Valida ion, W i ing o iginal d a , OA:
475
Concep ualiza ion, Da a cu a ion, Fo mal analysis, Me hodology, W i ing e iew and edi ing, OC: Da a
476
cu a ion, W i ing e iew and edi ing, JM: Fo mal analysis, P ojec adminis a ion, Funding acquisi ion,
477
W i ing e iew and edi ing, JB: P ojec adminis a ion, Funding acquisi ion, W i ing e iew and edi ing
478
Da a and code s a emen
479
The p e-p ocessed EO da ase is a ailable upon eques om he co esponding au ho . The used
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Appendix
596
Table S1 O e all unce ain y assessmen o imbe olume compa ing kNN, UNe , and ine- uned UNe models ained on
597
Finnish (FI) and No wegian (NO) da a. Resul s a e shown o NFI plo s, alida ion s ands, and wo ypes o p edic o a iable
598
combina ions (P ed. ype): S2 (Sen inel-2) and S2S1P2 (Sen inel-2 wi h Sen inel-1 and PALSAR
‑
2). Fine- uned models we e
599
p e- ained on ALS-based o es esou ce maps om Finland (FI) o No way (NO) and ine- uned u he using No wegian
600
aining NFI plo s.
601
Tes
da a
ype
P ed.
ype
Model
ype
kNN
UNe
Fine-
uned
UNe
kNN
UNe
Fine-
Tuned
UNe
kNN
UNe
Fine-
Tuned
UNe
RMSE [m3ha-1]
BIAS [m3ha-1]
R2
NFI
S2
FI
93.8
91.4
-20.4
-9.9
0.37
0.41
NFI
S2
NO
94.0
77.2
77.2
9.9
3.1
2.7
0.37
0.58
0.58
NFI
S2S1P2
FI
90.4
90.3
-5.1
-3.1
0.42
0.42
NFI
S2S1P2
NO
92.8
72.6
72.0
7.0
10.2
3.4
0.39
0.62
0.63
S and
S2
FI
190.1
176.7
-116.6
-98.5
0.04
0.17
S and
S2
NO
148.6
128.1
128.5
-67.6
-45
-46.0
0.42
0.57
0.56
S and
S2S1P2
FI
170.6
168.8
-74.2
-70.8
0.23
0.25
S and
S2S1P2
NO
156.8
119.8
123.5
-67
-4.6
-23.3
0.35
0.62
0.60
602
Table S2 O e all unce ain y assessmen o Lo ey’s heigh compa ing kNN, UNe , and ine- uned UNe models ained on
603
Finnish (FI) and No wegian (NO) da a. Resul s a e shown o NFI plo s, alida ion s ands, and wo ypes o p edic o a iable
604
combina ions (P ed. ype): S2 (Sen inel-2) and S2S1P2 (Sen inel-2 wi h Sen inel-1 and PALSAR
‑
2). Fine- uned models we e
605
p e- ained on ALS-based o es esou ce maps om Finland (FI) o No way (NO) and ine- uned u he using No wegian
606
aining NFI plo s.
607
Tes
da a
ype
P ed.
ype
Model
ype
kNN
UNe
Fine-
Tuned
UNe
kNN
UNe
Fine-
Tuned
UNe
kNN
UNe
Fine-
Tuned
UNe
RMSE [m]
BIAS [m]
R2
NFI
S2
FI
37.3
36.6
3.0
-2.0
0.36
0.38
NFI
S2
NO
36.9
33.0
32.6
3.4
5.2
1.7
0.37
0.50
0.51
NFI
S2S1P2
FI
45.9
36.7
7.5
4.3
0.03
0.38
NFI
S2S1P2
NO
37.0
31.9
31.3
3.2
6.2
1.8
0.37
0.53
0.55
S and
S2
FI
28.9
29.7
1.4
-5.6
0.48
0.46
S and
S2
NO
36.8
31.2
32.3
-15.1
-4.8
-9.2
0.17
0.40
0.36
S and
S2S1P2
FI
43.6
33.8
27.4
5.7
-0.17
0.29
S and
S2S1P2
NO
36.5
30.0
30.5
-13.7
0.8
-4.6
0.18
0.44
0.43
608
609
610