Poplar Short Rotation Coppice Plantations under Mediterranean Conditions: The Case of Spain

Re iew Popla Sho Ro a ion Coppice Plan a ions unde Medi e anean Condi ions: The Case o Spain Ne ea Oli ei a 1,* , Césa Pé ez-C uzado 2, Isabel Cañellas 1, Roque Rod íguez-Soallei o 2 and Ho ensia Six o 1 1 Fo es Resea ch Cen e, Na ional Ins i u e o Ag icul u e and Food Resea ch and Technology (INIA-CIFOR), C a. de la Co uña km 7.5, E-28040 Mad id, Spain; [email p o ec ed] (I.C.); [email p o ec ed] (H.S.) 2Sus ainable Fo es Managemen G oup, Uni e si y o San iago de Compos ela, C/Benigno Ledo s/n, E-27002 Lugo, Spain; cesa [email p o ec ed] (C.P.-C.); oque. [email p o ec ed] (R.R.-S.) *Co espondence: oli ei a.ne [email p o ec ed] Recei ed: 15 No embe 2020; Accep ed: 14 Decembe 2020; Published: 17 Decembe 2020   Abs ac : De eloping a ci cula bioeconomy based on he sus ainable use o biological esou ces, such as biomass, seems o be he bes way o esponding o he challenges associa ed wi h global change. Among he many sou ces, sho o a ion o es c ops a e an essen ial ins umen o ob aining quali y biomass wi h a p edic able pe iodici y and yield, acco ding o he a eas o cul i a ion. This e iew aims o p o ide an o e iew o a ailable knowledge on sho o a ion coppice Populus spp. plan a ions unde Medi e anean condi ions and speci ically in Spain, in o de o iden i y no only he s a us, bu also he u u e p ospec s, o his ype o biomass p oduc ion. The analysis o a ailable in o ma ion was conduc ed by aking in o conside a ion he ollowing aspec s: Gene ic plan ma e ial; plan a ion design, including densi ies, o a ion leng hs and he numbe o o a ions, and mix u es; managemen ac i i ies, including i iga ion, e iliza ion, and weed con ol; yield p edic ion; biomass cha ac e iza ion; and inally, an e alua ion o he sus ainabili y o he plan a ion and ecosys em se ices p o ided. Despi e ad ances, he e is s ill much o be done i hese plan a ions a e o become a comme cial eali y in some Medi e anean a eas. To achie e his aim, di e en aspec s need o be econside ed, such as i iga ion, bea ing in mind ha wa e es ic ions ep esen a eal h ea ; he speci ic adap a ion o gene ic ma e ial o hese condi ions, in o de o ob ain a g ea e e iciency in esou ce use, as well as a g ea e esis ance o pes s and diseases o ole ance o abio ic s esses such as d ough and salini y; a ionalizing e iliza ion; quan i ying and aluing he ecosys em se ices; he ad ance o mo e eliable p edic i e models based on ecophysiology; he speci ic cha ac e iza ion o biomass o i s inal use (bioene gy/biop oduc s); echnological imp o emen s in managemen and ha es ing; and inally, imp o ing he c i icalaspec s de ec ed in en i onmen al, ene gy, and economic analyses o achie e p o i able and sus ainable plan a ions unde Medi e anean condi ions. Keywo ds: biomass; Populus; SRC(Sho Ro a ionCoppice); sho o a ionwoodyc ops; sus ainabili y; Medi e anean condi ions; managemen ; e iew 1. In oduc ion The challenges associa ed wi h clima e change, along wi h he changing pa adigm o bo h economic de elopmen and he ene gy model, ha e been c ys allized in o a G een Deal o Eu ope [ 1 ]. In his con ex , he use, p oduc ion, and u iliza ion o biomass a e undoub edly some o he main issues, e y much ela ed o he need o edi ec he linea economy owa ds a ci cula bioeconomy based on he use o sus ainable biological esou ces [ 2 ]. The Inno a ion S a egy o Sus ainable G ow h: A Bioeconomy o Eu ope [ 3 ] and i s la e e ision [ 4 ] es ablish he bioeconomy as he gene al amewo k and key ac o o achie ing g een, sus ainable g ow h in Eu ope. In ha mony wi h he Fo es s 2020,11, 1352; doi:10.3390/ 11121352 www.mdpi.com/jou nal/ o es s Fo es s 2020,11, 1352 2 o 43 Eu opean s a egy, while also aking in o accoun he na ional possibili ies, he Spanish S a egy o he Bioeconomy [ 5 ] and he cu en d a Law on Clima e Change and Ene gy T ansi ion [ 6 ] ha e been published. De elopmen in his a ea mus ake in o conside a ion he po en ial o na u al esou ces, including biomass. Theop ions o using exis ingbiomassin o es s, ag icul u al esidues, o hep oduc ion o new biomass om c ops plan ed speci ically o his pu pose a e all seen as key o he de elopmen o he bioeconomy [ 3 ], no only o economic easons, bu also wi h en i onmen al and social conside a ions in mind. In Spain, he S a egic Resea ch and Inno a ion Agenda in ela ion o biomass has also ecen ly been p esen ed [ 7 ]. In his con ex , biomass is a highly alued esou ce o bo h bioene gy p oduc ion and biop oduc s, hus con ibu ing owa ds add essing he abo emen ioned global challenges. The e a e a ious sou ces o o es biomass, which can be di e en ia ed in o he ollowing: (i) Tha de i ed om o es managemen ac i i ies associa ed wi h imbe exploi a ion; (ii) esidues esul ing om sil icul u al ope a ions apa om imbe exploi a ion; (iii) biomass de i ed om he o es y indus y; and las ly, (i ) dedica ed o es c ops speci ically designed o he p oduc ion o woody biomass. The i s ca ego y includes logging esidues and small diame e o c ooked logs which a e used o ene gy p oduc ion (pelle s, wood chips, o i ewood). The i s and hi d ca ego ies a e he ype o biomass mainly used in Spain oday. The Spanish Renewable Ene gy Associa ion (APPA) has epea edly poin ed o he unde u iliza ion o wood esou ces a a na ional le el, as only 41% o he annual wood inc emen is cu en ly being used, which is no ably below he a e age o 60–70% o Eu ope [8]. Howe e , i he amoun o wood esou ces has been cons an ly inc easing since he 1960 0 s, wha is he poin o de eloping speci ic o es y c ops? Fi s ly, hese c ops a e seen as being o pa icula impo ance in e ms o hei po en ial con ibu ion o he e icien di e si ica ion o biomass sou ces [ 9 – 11 ]. Secondly, o all biomass sou ces, o es c ops speci ically designed o woody biomass p oduc ion a e hose mos eadily managed in e ms o bo h ime and space, wi h a p edic able pe iodici y and yield. Thi dly, biomass om o es c ops can con ibu e o gene a ing and s abilizing he biomass ma ke . The aim o his s udy is o assess he ad ances made in popla sho o a ion coppice (SRC) plan a ions o e ecen yea s, ocusing on speci ic ad ances in Medi e anean condi ions unde i iga ion (Spain) wi hin he global con ex and o iden i y he a eas o esea ch whe e p og ess s ill needs o be made. The cu en s a e-o - he-a in biomass p oduc ion om popla SRC unde Medi e anean condi ions is add essed, highligh ing bo h he s eng hs and weaknesses. The e o e, his pape is s uc u ed a ound he ollowing: (i) A global ision o SRC plan a ions ocusing on popla as one o he mos sui able species in Medi e anean a eas; (ii) he s a e-o - he-a o hese plan a ions a a global le el, while ocusing on he p og ess made in Spain; and inally, (iii) conclusions d awn unde Medi e anean condi ions, wi h a pa icula emphasis on he weaknesses iden i ied and he sho - and long- e m lines o esea ch needed o add ess hem. 2. Sho Ro a ion Fo es C ops o he P oduc ion o Biomass: The Populus Genus Despi e he di e en dedica ed ene gy c ops, he cu en agenda [ 7 ] only conside s he baceous and woody lignocellulosic c ops o mo e di e se uses, which also include biop oduc s. Fas g owing species a e used in SRC, employing in ensi e o semi-in ensi e echniques [ 12 ], wi h coppicing cycles o be ween 2 and 8 yea s un il s ool p oduc i i y declines, which no mally occu s a e 15 yea s [13,14], depending on he si e quali y. Biomass om SRC may become essen ial as an addi ion o he biomass p o ided by o es s, con ibu ing owa ds mee ing he demands o Eu opean indus y and assu ing ma ke s abili y [ 15 ]. This is p obably linked o he need o ind spa io empo al complemen a i y in biomass esou ces, con ibu ing o ma ching he supply wi h he demand; his ci cums ance is al eady a comme cial eali y in many pa s o he wo ld [ 16 ]. The sui abili y o such biomass is also linked o he Fo es s 2020,11, 1352 3 o 43 in insic cha ac e is ics o i s p oduc ion and managemen [ 17 ], such as he abundance o imp o ed, highly adap able gene ic ma e ial; high a e o success ul oo ing; good ju enile g ow h; and i s esp ou ing capaci y, among o he s [ 18 – 20 ]. O he ai s associa ed wi h i s end use a e also deemed o be impo an , such as a low chlo ine (Cl) and sul u (S) con en , low ash con en , and high lignin con en [21–24], among o he s. SRC ha e been ound o p o ide ecosys em se ices seldom su icien ly quan i ied and con as ed, such as ai cleaning; he con ol o e osion o looding in ce ain a eas [ 25 – 27 ]; mi iga ion o he e ec s o clima e change h ough ca bon ixing in olia o oo biomass ac ions [ 14 , 28 , 29 ]; inc eases o he biodi e si y in ag icul u al en i onmen s [ 30 – 33 ]; and e en soil decon amina ion [ 34 – 36 ], including mining eclama ion [ 37 , 38 ]. F om a social pe spec i e, woody c ops con ibu e owa ds he c ea ion o employmen in u al a eas, gi en ha hese c ops p o ide an oppo uni y o make use o poo , ma ginal, o su plus ag icul u al land [ 39 ]. Finally, he use o biomass om SRC helps o educe he p essu e on na u al o es s by p o iding a aw ma e ial much demanded by socie y and he e o e by indus y. Despi e his, o achie e sus ainabili y in he implemen a ion o SRC, i is impo an o conside he impac o land use changes in a eas whe e hese plan a ions compe e wi h ag icul u al c ops o land, as well as aspec s ela ed o wa e consump ion in a eas wi h limi ed wa e esou ces. The e o e, i is necessa y o de ine he limi a ions in o de o gua an ee he sus ainabili y o hese c ops [ 40 ]. Many o hese aspec s can p obably be deal wi h h ough he use o bio echnology o achie e imp o emen s, o by using ci cula economy echniques such as was ewa e euse [41–43]. In Spain, he in e es in p oducing biomass om SRC da es back o he mid-80’s [ 44 – 46 ], coinciding o a la ge ex en wi h he c isis in he oil sec o . Howe e , i was a ound 2000 when ini ia i es o inc ease sou ces o enewable ene gy a a global scale, pa icula ly in Eu ope, p o ided he impe us o explo e possibili ies o he p oduc ion o biomass as a enewable esou ce. The clima ic, edaphic, and demog aphic cha ac e is ics o some pa s o he coun y a e sui able o he cul i a ion o SRC, wi h an expec ed high p oduc i i y [ 47 ] exceeding ha ob ained in o he Eu opean coun ies. Howe e , because o he Medi e anean clima ic condi ions, such plan a ions a e only iable wi h he use o i iga ion. In a eas wi h an A lan ic clima e, he condi ions o SRC a e also sui able, wi h limi a ions such as he o og aphic cha ac e is ics, which may complica e he in ensi e sil icul u e applied in his ype o plan a ion, o he limi a ions associa ed wi h ce ain species due o soil acidi y. Al hough many s udies ha e ocused on he selec ion o ege al ma e ial bes adap ed o gi en a eas o Spain, he e is s ill a long way o go wi h ega ds o iden i ying he in e ac ion be ween he geno ype and he en i onmen , which is a de e mining ac o in he success o ailu e o SRC. The e a e many woody species po en ially cul i able o biomass p oduc ion. In gene al, hey a e as g owing b oadlea species wi h a high e-sp ou ing capaci y. In Eu ope, he Salicaceae amily (Populus spp. and Salix spp.) p esen s he g ea es de elopmen s on an indus ial le el. Plan a ions based on species and hyb ids o Populus a e well-es ablished in bo h cen al and sou he n Eu ope, wi h examples in Ge many [ 48 ], he Uni ed Kingdom [ 13 ], he Czech Republic [ 49 ], Bulga ia [ 50 ], Se bia [ 51 ], Poland [ 52 ], and F ance [ 53 ], as well as in Medi e anean egions, mainly I aly[ 54 ]andSpain [ 55 ]. In no he nEu ope, whe eSalix spp. hasbeen hep edominan species o his pu pose, popla cul i a ion is beginning o a ac in e es , wi h di e en ials being es ablished o e alua e i s po en ial [56,57]. To a la ge ex en , Populus species a e a he o e on o biomass p oduc ion because o he highly e icien b eeding p og ams in di e en coun ies, many o which a e loca ed in Eu ope [ 58 ]. These p og ams a e a o ed due o he e y b oad gene ic base wi h which o wo k in e ms o ai s linked o cul i a ion and wood p ope ies [ 59 , 60 ], bu also due o knowledge o he genome sequence [ 61 ] and he ela i ely sho b eeding cycles. O he ac o s include a high capaci y o ege a i e ep oduc ion and hei apid g ow h a e. The impo ance o cul i a ing Salicaceae o biomass p oduc ion a a global scale is e lec ed by he abundance o in o ma ion ga he ed o e he las decade by he In e na ional Popla Commission Fo es s 2020,11, 1352 4 o 43 (s a u o y body wi hin he FAO), which has speci ically co e ed he subjec in one o i s wo king g oups [ 62 ]. The In e na ional Ca alogue o Base Ma e ials o Populus o ob aining o es ep oduc i e ma e ial con ains 358 en ies, among which se e al a e speci ically e e enced o biomass p oduc ion (‘Boiano-4’, ‘Baldo’, ‘Hunneghem’, and ‘Raspalje’). Many mo e a e a p elimina y s ages o inclusion and ha e been pu o wa d o his use (‘AF2’, ‘AF8’, ‘A4A’, ‘Mon iso’, ‘Muu ’, ‘O ion’, ‘Oudenbe g’, ‘Si io’, and ‘Ves en’). Wi hin he amewo k o he EU-POP p ojec , mo e han 17 geno ypes a e cu en ly being cha ac e ized as biomass p oduce s in a mul i-en i onmen ial in which en Eu opean coun ies a e aking pa , including Spain [63]. In addi ion o he geno ypes ha ha e been ca alogued (o a e in he ca aloguing phase) o his pu pose, he e a e o he s ha ha e been iden i ied o hei wood p oduc ion, bu which also seem o possess sui able cha ac e is ics o he p oduc ion o biomass. Figu e 1shows he main geno ypes ha a e being es ed o plan ed o comme cial pu poses in some Eu opean coun ies. Fo es s 2020, 11, x FOR PEER REVIEW 4 o 45 ep oduc i e ma e ial con ains 358 en ies, among which se e al a e speci ically e e enced o biomass p oduc ion (‘Boiano-4’, ‘Baldo’, ‘Hunneghem’, and ‘Raspalje’). Many mo e a e a p elimina y s ages o inclusion and ha e been pu o wa d o his use (‘AF2’, ‘AF8’, ‘A4A’, ‘Mon iso’, ‘Muu ’, ‘O ion’, ‘Oudenbe g’, ‘Si io’, and ‘Ves en’). Wi hin he amewo k o he EU-POP p ojec , mo e han 17 geno ypes a e cu en ly being cha ac e ized as biomass p oduce s in a mul i- en i onmen ial in which en Eu opean coun ies a e aking pa , including Spain [63]. In addi ion o he geno ypes ha ha e been ca alogued (o a e in he ca aloguing phase) o his pu pose, he e a e o he s ha ha e been iden i ied o hei wood p oduc ion, bu which also seem o possess sui able cha ac e is ics o he p oduc ion o biomass. Figu e 1 shows he main geno ypes ha a e being es ed o plan ed o comme cial pu poses in some Eu opean coun ies. Figu e 1. Some o he main geno ypes plan ed o biomass pu poses in each coun y a e ep esen ed on a Eu opean map. In Spain, he main species o in e es a a comme cial o p e-comme cial le el a e hose belonging o he gene a Populus spp. and Eucalyp us spp. The cul i a ion o species and hyb ids o Populus o indus ial wood ( enee ) is well-es ablished in many a eas o he coun y [64]. Al hough popla p oduc ion plan a ions in Spain ep esen less han 1% o he ee-co e ed o es a ea (app oxima ely 100,000 ha), in some p o inces, hese plan a ions accoun o mo e han 50% o he ha es ed wood, which is a ound 40% o he economic alue o oundwood cu ings. In he p o ince o Cas illa y Leon, o example, popla is he o es species wi h he highes economic alue [65,66]. Howe e , ega ding popla in SRC, he e is only a oken p esence in Spain, wi h i occupying a ound 50 ha a i s peak. The po en ial land a a na ional le el co esponds o i iga ed ag icul u al ma ginal land o ood p oduc ion wi h he edaphoclima ic equi emen s o he species [47]. Cu en ly, he i iga ed ag icul u al land in Spain is 3.8 Mha [67]. Figu e 1. Some o he main geno ypes plan ed o biomass pu poses in each coun y a e ep esen ed on a Eu opean map. In Spain, he main species o in e es a a comme cial o p e-comme cial le el a e hose belonging o he gene a Populus spp. and Eucalyp us spp. The cul i a ion o species and hyb ids o Populus o indus ial wood ( enee ) is well-es ablished in many a eas o he coun y [ 64 ]. Al hough popla p oduc ion plan a ions in Spain ep esen less han 1% o he ee-co e ed o es a ea (app oxima ely 100,000 ha), in some p o inces, hese plan a ions accoun o mo e han 50% o he ha es ed wood, which is a ound 40% o he economic alue o oundwood cu ings. In he p o ince o Cas illa y Leon, o example, popla is he o es species wi h he highes economic alue [ 65 , 66 ]. Howe e , Fo es s 2020,11, 1352 5 o 43 ega ding popla in SRC, he e is only a oken p esence in Spain, wi h i occupying a ound 50 ha a i s peak. The po en ial land a a na ional le el co esponds o i iga ed ag icul u al ma ginal land o ood p oduc ion wi h he edaphoclima ic equi emen s o he species [ 47 ]. Cu en ly, he i iga ed ag icul u al land in Spain is 3.8 Mha [67]. The geno ypes o plan ing should be included in he Eu opean Ca alogue o Base Ma e ial o Populus ep oduc i e ma e ial. The e is also a Spanish Na ional Ca alogue o Base Ma e ial o o es ep oduc i e ma e ial o he Populus genus in quali ied and con olled ca ego ies comp ising 24 comme cial quali y geno ypes (Table 1). Some o his ma e ial, which is well-adap ed o he speci ic Spanish Medi e anean condi ions, could also be o in e es o biomass p oduc ion. Mos o he geno ypes al eady es ed o cu en ly being es ed a e lis ed in Figu e 1, and include ma e ials om bo h he Eu opean and Spanish lis s, as well as some ha ha e no ye been ca alogued. The po en ial o Populus is explained o a la ge deg ee by he a ailabili y o ma e ial adap ed o exis ing condi ions, along wi h app op ia e knowledge ela ing o he managemen o he species in many pa s o he coun y [68,69]. Table 1. Geno ypes included in he Na ional Ca alogue o Base Ma e ials o he p oduc ion o o es ep oduc i e ma e ials ela ed o he genus Populus L. Pa en age Sec ion Geno ypes P. ×canadensis Mönch D ×N Aigei os ‘2000 Ve de’, ‘Aga he F’ a, ‘E-2980a, ‘B anagesi’, ‘B-1M’, ‘CanadáBlanco’, ‘Do skamp’, ‘Fle o’, ‘Gua di’, ‘I-2140b, ‘Campeado ’ b, ‘I-454/400, ‘Luisa A anzo’, ‘MC’, ‘T iplo’ P. del oides W. Ba am ex Ma shall D Aigei os ‘Lux’, ‘Vi ia o’ P. ×gene osa Hen y T ×D Tacamahaca ×Aigei os ‘Beaup e’, ‘Boela e’, ‘Raspalje’, ‘Unal’, ‘USA 49-177’ P. ×gene osa Hen y ×P. alba L. (T ×D) ×A(Tacamahaca ×Aigei os) ×Populus ‘I-114/690 P. nig a L. N Aigei os ‘T 56/75’, ‘Bo dils’, ‘Lomba do leones’ D is P. del oides; N is P. nig a; T is P. ichoca pa; A is P. alba; a ‘Aga he F’ =‘E-298 0 ; and b ‘I-214 0 =‘Campeado ’. O de o 24/06/1992, O de APA/544/2003 o 06/032003, Resolu ion o 07/07/2006 o he Di ecci ó n Gene al de Ag icul u a, and Resolu ion o 07/11/2011 o he Di ección Gene al de Recu sos Ag ícolas y Ganade os. In ela ion o Eucalyp us, he cul i a ion o some species and hyb ids o he p oduc ion o biomass has been signi ican in ce ain a eas o he coun y, eaching p oduc ion alues o 14.6 Mg ha −1 yea −1 in he case o Eucalyp us globulus Labill. and 21.5 Mg ha −1 yea −1 o Eucalyp us ni ens (Deane and Maiden) Maiden [ 70 ] in A lan ic a eas wi hou i iga ion, whe eas he de elopmen o simila yields in sou he n Spain would need i iga ion. Some geno ypes o Eucalyp us exhibi he addi ional ad an age o p esen ing an accep able deg ee o ole ance o d ough condi ions [ 71 , 72 ], al hough he high le els o p oduc ion a e associa ed wi h scena ios whe e d ough s do no occu o whe e i iga ion is used. One o he main di e ences wi h espec o popla is ha Eucalyp us is an e e g een species, so ha es ed ees include wigs and e en lea es, yielding biomass wi h a high ash con en . Fu he mo e, ials wi h o he po en ially usable species ha e aken place in ecen yea s, leading o di e ing esul s. In his ega d, a a Medi e anean scale, he e ha e been expe imen al ials wi h Ulmus pumila L. [ 73 ], Robinia pseudoacacia L. [ 74 – 77 ], hyb ids o Salix spp. [ 78 – 81 ], Pla anus × hispanica Mill. ex Münchh [ 77 ], and di e en cul i a s o Paulownia spp. [ 82 , 83 ], wi h he la e displaying se e e adap a ion p oblems o he clima ic condi ions o he Medi e anean a ea (ea ly os s and looding) [ 84 ]. Al hough some o hese may be o in e es in e ms o adding di e si y o he a ea o Fo es s 2020,11, 1352 6 o 43 c op de elopmen , pe haps he main limi a ion o mos o hese species s ems om he ac ha he e is a lack o imp o ed gene ic ma e ial, which is necessa y o allowing hei use in di e en en i onmen s. 3. Lines o P og ess: S a e o he A Sus ainable imp o emen in biomass p oduc ion om popla SRC c ops equi es ad ances o be made in di e en aspec s, which ha e been summa ized in Figu e 2. An o e all ision o he cu en si ua ion o each o hese aspec s is p o ided in his sec ion. Fo es s 2020, 11, x FOR PEER REVIEW 6 o 45 3. Lines o P og ess: S a e o he A Sus ainable imp o emen in biomass p oduc ion om popla SRC c ops equi es ad ances o be made in di e en aspec s, which ha e been summa ized in Figu e 2. An o e all ision o he cu en si ua ion o each o hese aspec s is p o ided in his sec ion. Li e a u e sea ches o he combina ion o he e ms Populus o popla , and sho o a ion o ene gy c op, we e conduc ed using Web o Science (WOS, Co e collec ion o Web o Science), which is one o he main jou nal da abases. The e we e no es ic ions in e ms o he yea o publica ion o language. Figu e 3 shows he numbe o documen s pe coun y (including only he op 30 coun ies) ound in WOS, o aling 2185 documen s. F om hese sea ches, documen s om Spain we e il e ed and i was ound ha some o hem we e no p ope ly classi ied in he da abase. Ou o he 112 documen s ela ing o Spain ( anked 10 h) ound in he WOS, only 83 we e included. Figu e 2. Lines o ac ion o imp o ing sho o a ion coppice-managed woody c ops. Figu e 2. Lines o ac ion o imp o ing sho o a ion coppice-managed woody c ops. Li e a u e sea ches o he combina ion o he e ms Populus o popla , and sho o a ion o ene gy c op, we e conduc ed using Web o Science (WOS, Co e collec ion o Web o Science), which is one o he main jou nal da abases. The e we e no es ic ions in e ms o he yea o publica ion o language. Figu e 3shows he numbe o documen s pe coun y (including only he op 30 coun ies) ound in WOS, o aling 2185 documen s. F om hese sea ches, documen s om Spain we e il e ed and i was ound ha some o hem we e no p ope ly classi ied in he da abase. Ou o he 112 documen s ela ing o Spain ( anked 10 h) ound in he WOS, only 83 we e included. In his e iew, we ook in o accoun no only he documen s ound in his da abase, bu also o he scien i ic publica ions belonging o Jou nal Ci a ions Repo (JCR) indexed jou nals, as well as nume ous pieces o a ailable g ay li e a u e. All his in o ma ion was iden i ied by acing back pape s ci ed in he e e ences o he iden i ied s udies and e iewing publica ions by scien is s who ha e wo ked o a e cu en ly wo king on popla sho o a ion in Spain. In any case, gi en he abundance o exis ing in o ma ion and he di icul ies associa ed wi h he use o di e se e minology, he e may be ce ain li e a u e ha we a e no awa e o , and he e o e has no been conside ed in his e iew. Fo es s 2020,11, 1352 7 o 43 Fo es s 2020, 11, x FOR PEER REVIEW 7 o 45 Figu e 3. Ranking o he numbe o publica ions pe coun y (including only he op 30 coun ies) ound in he Web o Science (WOS) jou nal da abase, o he combina ion o he e ms Populus o popla , and sho o a ion o ene gy c op, in July 2020. In his e iew, we ook in o accoun no only he documen s ound in his da abase, bu also o he scien i ic publica ions belonging o Jou nal Ci a ions Repo (JCR) indexed jou nals, as well as nume ous pieces o a ailable g ay li e a u e. All his in o ma ion was iden i ied by acing back pape s ci ed in he e e ences o he iden i ied s udies and e iewing publica ions by scien is s who ha e wo ked o a e cu en ly wo king on popla sho o a ion in Spain. In any case, gi en he abundance o exis ing in o ma ion and he di icul ies associa ed wi h he use o di e se e minology, he e may be ce ain li e a u e ha we a e no awa e o , and he e o e has no been conside ed in his e iew. A classi ica ion o all he in o ma ion a ailable a a na ional le el was pe o med acco ding o he six ca ego ies de ailed in Figu e 2. This in o ma ion was also b oken down acco ding o he a ailabili y o li e a u e, sepa a ing he so-called g ay li e a u e om ha con ained in science jou nals and books (Figu e 4). In Spain, biomass cha ac e iza ion and sus ainabili y (mainly ene gy, economic, and en i onmen al analysis) a e he lines o esea ch which ha e been explo ed he mos based on scien i ic publica ions, al hough, i we include g ay bibliog aphy, hen he mos explo ed lines a e p oduc ion, modeling, and gene ic ma e ial. App oxima ely 50.5% o he in o ma ion e alua ed co esponds o g ay li e a u e, 4.5% o books, and he es o scien i ic publica ions (45%). Figu e 3. Ranking o he numbe o publica ions pe coun y (including only he op 30 coun ies) ound in he Web o Science (WOS) jou nal da abase, o he combina ion o he e ms Populus o popla , and sho o a ion o ene gy c op, in July 2020. A classi ica ion o all he in o ma ion a ailable a a na ional le el was pe o med acco ding o he six ca ego ies de ailed in Figu e 2. This in o ma ion was also b oken down acco ding o he a ailabili y o li e a u e, sepa a ing he so-called g ay li e a u e om ha con ained in science jou nals and books (Figu e 4). In Spain, biomass cha ac e iza ion and sus ainabili y (mainly ene gy, economic, and en i onmen al analysis) a e he lines o esea ch which ha e been explo ed he mos based on scien i ic publica ions, al hough, i we include g ay bibliog aphy, hen he mos explo ed lines a e p oduc ion, modeling, and gene ic ma e ial. App oxima ely 50.5% o he in o ma ion e alua ed co esponds o g ay li e a u e, 4.5% o books, and he es o scien i ic publica ions (45%). 3.1. Sui abili y o Gene ic Ma e ial Clonal Selec ion Popla is he model ee o gene ic s udies and is he u hes ahead in e ms o biological knowledge and gene ic esou ces [ 58 ]. Using he bes adap ed ma e ial when de eloping plan a ions helps o ensu e he e icien use o si e esou ces and he e o e highe le els o p oduc ion. Pheno ypic plas ici y e e s o he capaci y o an o ganism o al e i s cha ac e is ics in esponse o en i onmen al condi ions [ 85 ]. The geno ype by en i onmen in e ac ion (G*E) is e idenced by he ins abili y o pheno ypic co ela ions de i ed om d as ic di e ences in biomass p oduc ion ound in popla plan a ions [86–88]. The G*E in e ac ion makes popla clonal ecommenda ions mo e di icul , al hough i can also p o ide an oppo uni y o maximize p oduc ion in hese plan a ions by ma ching he mos sui able ma e ial o speci ic si e condi ions. Geno ypic s abili y is unde s ood as he capaci y o a cul i a o p oduce in acco dance wi h he p oduc i e po en ial o each en i onmen , ha is, wi hou s aying om he beha io expec ed o he a e age geno ypic alue [ 89 ]. Howe e , he s a egies may di e , and his can be obse ed in di e en popla SRC plan a ions; hese s a egies include a emp ing o ind a b oad adap a ion o op imizing he adap a ion o speci ic si e condi ions, hence he necessi y o cha ac e ize he ma e ial [ 90 – 92 ]. This cha ac e iza ion no only has an impac on c op managemen , Fo es s 2020,11, 1352 8 o 43 bu also has epe cussions o imp o emen s a egies. In Spain, popla ma e ial des ined o biomass p oduc ion has been cha ac e ized in ecen yea s h ough he analysis o his in e ac ion [ 55 , 77 , 93 – 95 ]. Fo es s 2020, 11, x FOR PEER REVIEW 8 o 45 Figu e 4. Pe cen ages o Spanish publica ions in each o he six ca ego ies es ablished. The ligh colo co esponds o he g ay li e a u e on each subjec . 3.1. Sui abili y o Gene ic Ma e ial Clonal Selec ion Popla is he model ee o gene ic s udies and is he u hes ahead in e ms o biological knowledge and gene ic esou ces [58]. Using he bes adap ed ma e ial when de eloping plan a ions helps o ensu e he e icien use o si e esou ces and he e o e highe le els o p oduc ion. Pheno ypic plas ici y e e s o he capaci y o an o ganism o al e i s cha ac e is ics in esponse o en i onmen al condi ions [85]. The geno ype by en i onmen in e ac ion (G*E) is e idenced by he ins abili y o pheno ypic co ela ions de i ed om d as ic di e ences in biomass p oduc ion ound in popla plan a ions [86–88]. The G*E in e ac ion makes popla clonal ecommenda ions mo e di icul , al hough i can also p o ide an oppo uni y o maximize p oduc ion in hese plan a ions by ma ching he mos sui able ma e ial o speci ic si e condi ions. Geno ypic s abili y is unde s ood as he capaci y o a cul i a o p oduce in acco dance wi h he p oduc i e po en ial o each en i onmen , ha is, wi hou s aying om he beha io expec ed o he a e age geno ypic alue [89]. Howe e , he s a egies may di e , and his can be obse ed in di e en popla SRC plan a ions; hese s a egies include a emp ing o ind a b oad adap a ion o op imizing he adap a ion o speci ic si e condi ions, hence he necessi y o cha ac e ize he ma e ial [90–92]. This cha ac e iza ion no only has an impac on c op managemen , bu also has epe cussions o imp o emen s a egies. In Spain, popla ma e ial des ined o biomass p oduc ion has been cha ac e ized in ecen yea s h ough he analysis o his in e ac ion [55,77,93–95]. Wi h ega ds o he biomass, he de elopmen o geno ypes aimed a achie ing inc eased p oduc ion, a g ea e ole ance o pes s and diseases, and speci ic eeds ock p ope ies is ongoing h ough b eeding p og ams in he coun ies spea heading he esea ch, wi h he mos p ominen o hese being he USA, Canada, I aly, F ance, Ge many, and Sweden [58]. T adi ional app oaches based on ecu en selec ion as a way o inc ease hyb id igo [54,96] a e now suppo ed by genomics and phenomic echniques which con ibu e o accele a ing hese p ocesses [97,98]. Fo example, ools such as genome edi ing using C isp /Cas9 echnology [99–101], he de elopmen o ansgenic plan s [102], comple e genome associa ion analysis [103], and new pheno ypic ools [104], a e now leading o impo an ad ances in gene ic imp o emen o he Populus genus. A ele an e iew o he p og ess owa ds an imp o ed biomass quali y and esilience o p oduc ion using bo h adi ional app oaches and new echnology is p o ided by Cli on-B own e al. [58], speci ying he b eeding p og ams in which each o hese echniques has been used. New a enues o esea ch ha e also been opened up h ough conside ing epigene ic inhe i ance, also e med so inhe i ance. This ela es o inhe i able changes o he gene exp ession induced by en i onmen al changes [105,106]. Figu e 4. Pe cen ages o Spanish publica ions in each o he six ca ego ies es ablished. The ligh colo co esponds o he g ay li e a u e on each subjec . Wi h ega ds o he biomass, he de elopmen o geno ypes aimed a achie ing inc eased p oduc ion, a g ea e ole ance o pes s and diseases, and speci ic eeds ock p ope ies is ongoing h ough b eeding p og ams in he coun ies spea heading he esea ch, wi h he mos p ominen o hese being he USA, Canada, I aly, F ance, Ge many, and Sweden [58]. T adi ional app oaches based on ecu en selec ion as a way o inc ease hyb id igo [ 54 , 96 ] a e now suppo ed by genomics and phenomic echniques which con ibu e o accele a ing hese p ocesses [ 97 , 98 ]. Fo example, ools such as genome edi ing using C isp /Cas9 echnology [99–101], he de elopmen o ansgenic plan s [ 102 ], comple e genome associa ion analysis [ 103 ], and new pheno ypic ools [ 104 ], a e now leading o impo an ad ances in gene ic imp o emen o he Populus genus. A ele an e iew o he p og ess owa ds an imp o ed biomass quali y and esilience o p oduc ion using bo h adi ional app oaches and new echnology is p o ided by Cli on-B own e al. [ 58 ], speci ying he b eeding p og ams in which each o hese echniques has been used. New a enues o esea ch ha e also been opened up h ough conside ing epigene ic inhe i ance, also e med so inhe i ance. This ela es o inhe i able changes o he gene exp ession induced by en i onmen al changes [105,106]. In Spain, al hough p og ess in his line o esea ch is limi ed by he lack o speci ic b eeding p og ams, p og ess has been made in he de elopmen and es ing o di e en ans o med popla lines exp essingapineglu aminesyn he ase(GS) ansgenein ol edinNassimila ion[ 107 ]; heCsDMLgene ha induces bud o ma ion needed o he su i al o he apical me is em unde he ha sh condi ions o win e [ 108 ]; and a gene ela ed o ABI3/VP1 1 (CsRAV1) o inc ease syllepsis, and consequen ly biomass p oduc ion, in comme cial eli e ees [ 109 , 110 ]. The biomass po en ial o he di e en a ailable geno ypes has also been es ed, wi h mul iple clonal ials in ol ing adi ional geno ypes [ 95 , 111 – 116 ], as well as new geno ypes speci ically selec ed o biomass [ 77 , 117 ]. The esul s ha e e ealed hyb ids like ‘O ion’ and ‘AF2’ as si e-speci ic geno ypes, while ‘I-214’ and ‘Mon iso’ ha e been demons a ed o adap well o a wide a ie y o scena ios. Likewise, he abili y o di e en geno ypes o p oduce biomass in a eas deg aded by mining is also being es ed, wi h some geno ypes, such as ‘Raspalje’ and ‘AF2’, displaying accep able yields [ 118 ]. In gene al, a s ong co ela ion has been iden i ied be ween he esponse o en i onmen al a iables and he axonomic backg ound, wi h he eu ame ican geno ypes such as ‘2000 Ve de’ esponding be e o lowe la i udes, highe mean empe a u es o he ege a i e pe iod, and a lowe sandy soil con en , unlike in e ame ican geno ypes such as ‘Unal’ [ 93 ]. 3.2. Plan a ion Design The design o a plan a ion is also a highly in luen ial ac o in s and managemen and he op imiza ion o p oduc ion [ 119 ]. Many di e en ypes o design ha e been es ed in Spain, combining Fo es s 2020,11, 1352 9 o 43 di e en plan a ion densi ies [ 120 ], ee dis ibu ions [ 121 ], and o a ion leng hs [ 122 ]. The possibili y o mixing di e en species o di e en a ie ies o he same species has also been pu o wa d as a way o op imize p oduc ion. 3.2.1. Densi ies A b oad ange o ini ial s ocking a es ha e been employed o de ine he op imum plan a ion densi y, om 1000 cu ings ha −1 up o mo e han 40,000 cu ings ha −1 [ 123 , 124 ]. E en highe densi ies ha e been con empla ed, albei less equen ly, as in he case o DeBell e al. [ 125 ]. In Spain, a wide ange o densi ies in mul iple en i onmen s ha e been es ed [ 46 , 126 , 127 ]. In e ms o he yield, densi ies o a ound 20,000–25,000 cu ings ha −1 we e he mos p oduc i e in he i s yea , bu he e ec o compe i ion educed he di e ences be ween hese and less s ocked plan a ions as o a ion app oached h ee yea s [ 120 ]. O he ac o s, such as he cos s associa ed wi h he highe densi ies due o he g ea e numbe o cu ings, inc eased demand o wa e , and inc eased consump ion o consumables and ime equi ed o ope a ions such as i iga ion o ha es ing [ 128 ], along wi h impedimen s o mechaniza ion o he plan a ion, mean ha densi ies in excess o 15,000 cu ings ha−1a e no ecommended [120]. Fu he mo e, he posi ion o he s ools o a gi en densi y is also a ma e o deba e. The e o e, i is common o ials o include bo h single- ow layou s [ 78 , 92 ] and double ows [ 13 , 129 ]. No signi ican di e ences in yield we e ound be ween he wo layou s [ 130 ], al hough single- ow designs end o acili a e managemen ope a ions. In Spain, bo h layou s ha e been es ed [77,131], al hough a single ow is mo e commonly used. The densi y and dis ibu ion o s ools should be di ec ly linked o he o a ion leng hs applied, along wi h he main enance and ha es ing p ocesses, always a emp ing o keep he p oduc ion cos pe biomass uni ob ained as low as possible. 3.2.2. Ro a ion Leng hs and Numbe o Coppices The o a ion is a ac o closely linked o he densi y, as well as being in luenced by he en i onmen . Di e en o a ions ha e been employed in he managemen o popla SRC, om yea ly o longe o a ions o o he p oduc i e objec i es [ 132 – 135 ]. In ecen yea s, o a ions o 2 o 4 yea s ha e been he mos equen ly employed, wi h he inal ha es ing age depending on he geno ype and si e condi ions [90,129,136]. In Spain, o a ions om 2 o 9 yea s ha e been es ed [ 110 , 117 , 128 , 137 ], al hough ew s udies ha e compa ed di e en o a ion leng hs. He n á ndez Ga asa e al. [ 138 ] de e mined ha he maximum olume p oduc ion occu ed be ween 3 and 4 yea s, depending on he geno ype plan ed. The sui abili y o a 3- o 4-yea o a ion leng h o di e en geno ypes unde poo si e condi ions was e alua ed by Oli ei a e al. [ 139 ], who no ed ha biomass p oduc ion inc eases in mos geno ypes om 74% o 166% when he o a ion leng h is ex ended by 1 yea . Howe e , some geno ypes, such as ‘AF2’ and ‘Do skamp’, p esen ed no signi ican di e ences. These esul s a e consis en wi h hose ob ained unde a e age si e condi ions, whe e he maximum olume p oduc ion was no eached wi h a 3-yea o a ion leng h, excep in he cases o ce ain geno ypes, such as ‘A2A’, ‘A4A’, and ‘Lomba do Leones’ [ 140 ]. The e o e, he o a ion leng h chosen clea ly depends on bo h he geno ype and he si e condi ions, wi h longe o a ion leng hs being ad isable when he condi ions a e no op imal. Fu he s udies a e needed in o de o op imize he o a ion leng h o speci ic si es, geno ypes, and managemen condi ions. Ano he ques ion ha has spa ked con o e sy is whe he o no o coppice a e he i s es ablishmen yea o encou age mul iple shoo g ow h [141,142]. This is a common p ac ice in some coun ies, such as England [ 143 ]. In Spain, he use o managemen echniques bo h wi h o wi hou coppicing a e he i s es ablishmen yea has also been s udied [ 144 – 146 ], al hough no appa en ad an age was ound wi h ega d o he addi ional coppice a e he i s es ablishmen yea , ei he in e ms o biomass yield o quali y. Fo es s 2020,11, 1352 16 o 43 Table 2. Con . Species and Popla Hyb ids Geno ype Densi y Age Coun y Yield (Mg dm ha−1Yea −1) by Ro a ion Re e ence 1s 2nd 3 d P. ×canadensis ‘AF2’ 6061 2I aly 16 17 17 [92] * ‘I-214’ 11.5 18 13.5 P. ×gene osa ×P. nig a ‘AF6’ 15 17.5 15 ‘Mon iso’ 17.5 23.5 17.5 P. ×gene osa ×P. ichoca pa ‘AF8’ 15 24 19 P. maximowiczii × P. ichoca pa ‘NE-42’ 1333 7Poland 8 [52] P. ichoca pa ×P. ichoca pa ‘F i zi Pauley’ 8.1 P. ×canadensis ‘AF2’ 4.1 ‘Albelo’ 4 ‘Deg osso’ 6.8 ‘Kos e ’ 5.6 ‘Pola go’ 4.3 P. ×gene osa ×P. ichoca pa ‘AF8’ 2.5 P. nig a ×P. maximowiczii ‘Max 5’ 11,110 3 Poland 7.8 [227] P. ichoca pa ‘Columbia Ri e ’ 10,000 3Uni ed Kingdom 6.71 6.62 [13] ‘F i zi Pauley’ 8.59 8.24 ‘T ichobel’ 9.08 9.59 P. ichoca pa ×P. balsami e a ‘Balsam Spi e’ 7.24 7.03 P. ×canadensis ‘Ga e ’ 6.58 5.58 ‘Ghoy’ 6.45 5.77 ‘Gibecq’ 5.7 4.73 P. ×gene osa ‘Beaup é’ 7.34 4.87 ‘Boela e’ 6.23 4.2 ‘Hazendans’ 7.23 7.56 ‘Hoog o s ’ 8.84 8.12 ‘Raspalje’ 6.69 4.66 ‘Unal’ 7.55 5.25 P. balsami e a ×P. ichoca pa ‘Balsam Spi e’ 10,000 5 Uni ed Kingdom 14.42 [228] 4444 5 11.20 10,000 4 13.98 * Es ima ed alues om g aphs. Fo es s 2020,11, 1352 17 o 43 Table 3. Di e en examples o he biomass yield (Mg dm ha −1 yea −1 ) pe geno ype, age, and o a ion in popla sho o a ion plan a ions in Spain unde Medi e anean condi ions. Species and Popla Hyb ids Geno ype Densi y Age Loca ion Yield (Mg dm ha−1Yea −1) by Ro a ion Re e ence 1s 2nd 3 d P. ×canadensis ‘AF2’ 13,333 3A agón 24.72 [229] * ‘Ballo ino’ 25.08 ‘I-214’ 28.64 P. ×gene osa ×P. nig a ‘Mon iso’ 26.91 P. ×canadensis ‘Campeado ’ 10,000 5 So ia 12.10 [230] ‘Do skamp’ 6.3 P. ×gene osa ‘Boela e’ 16.6 ‘Raspalje’ 19.8 P. ×canadensis ‘Campeado ’ 10,000 6 So ia 9.3 [126,127] ‘I-214’ 9.9 P. ×gene osa ‘Boela e’ 12.9 ‘Raspalje’ 2500 8.0 5000 9.2 10,000 6 (2nd–3 d:5yea ) 16.5 17.2 14.8 10,000 3 9.8 20,000 12.7 40,000 15.6 P. ×canadensis ‘AF2’ 8000 7Lugo 0.84 [231] Wi hou i iga ion in a ansi ion zone be ween Medi e anean and A lan ic clima e ‘I-214’ 2.96 P. ×gene osa ‘Beaup é’ 3.07 ‘Raspalje’ 3.1 ‘Unal’ 1.97 P. ×gene osa ×P. nig a ‘AF6’ 1.14 ‘Mon iso’ 0.93 P. ichoca pa ‘T ichoca pa’ 3.93 P. ×canadensis ‘AF2’ and ‘I-214’ 33,333 3 So ia 7–12 2–9.5 1.5–4.5 [145] * P. ×canadensis ‘I-214’ 10,000 3 As u ias 9 11 6 [148] * Wi hou i iga ion in mining zone P. ×canadensis ‘I-214’ 37,037 2 Salamanca 12.41 9.99 24.83 [232] Fo es s 2020,11, 1352 18 o 43 Table 3. Con . Species and Popla Hyb ids Geno ype Densi y Age Loca ion Yield (Mg dm ha−1Yea −1) by Ro a ion Re e ence 1s 2nd 3 d P. ×canadensis ‘AF2’ 13,333 3A agón 15.53 [233] *, [234] ‘I-214’ 10.3 ‘Gua di’ 9.03 P. del oides ‘Vi ia o’ 14 P. ×gene osa ‘Unal’ 9.23 P. ×gene osa ×P. nig a ‘Mon iso’ 13.13 P. ×canadensis ‘I-214’ 33,333 2 Spain 15.1 [112] Populus spp. 4 geno ypes 13,333 3 G anada 13.7 [235] Populus spp. 3 geno ypes 19,700 4 So ia 12.0 Populus spp. 4 geno ypes 13,333 3 Zamo a 7.7 Populus spp. 4 geno ypes 33,333 3 León 6.9 Populus spp. 4 geno ypes 20,000 3 Na a a 16 P. nig a ‘Lomba do leones’ 17,316 3 So ia 9.1 17.1 [236] P. ×canadensis ‘A4A’ 20.0 12.2 ‘A2A’ 11.8 15.3 ‘I-214’ 22.0 30.1 ‘Luisa A anzo’ 17.3 27.1 ‘MC’ 16.8 18.4 ‘2000 Ve de’ 13.64 14.25 P. del oindes ‘Baldo’ 10,000 3Ge ona, Mad id, So ia 8.25–19.07 8.59–29.94 [77,80] P. ×canadensis ‘Ballo ino’ 8.25–15.94 11.45–19.20 ‘I-214’ 5.45–16.16 11.09–37.27 ‘O ion’ 12.53–22.65 12.70–34.56 ‘Oudenbe g’ 9.43–14.27 9.66–19.96 P. ×canadensis ‘I-214’ 6666-33,333 3 Ba celona 4.2 [47] Ge ona 5.7 Ge ona 11.6 G anada 18.3 León 4.7 León 5.8 León 15.2 Mad id 13.2 Na a a 18.5 So ia 8.1 So ia 14.2 Zamo a 8.2 Fo es s 2020,11, 1352 19 o 43 Table 3. Con . Species and Popla Hyb ids Geno ype Densi y Age Loca ion Yield (Mg dm ha−1Yea −1) by Ro a ion Re e ence 1s 2nd 3 d P. ×canadensis ‘I-214’ 5000 4 Mad id 12.4 [44] ‘Campeado ’ 10.0 ‘Canada Blanco’ 8.4 ‘I-214’ 2 10.90 ‘I-214’ 3 10.10 ‘I-214’ 5 17.30 ‘I-214’ 10,000 4 16.9 P. ×canadensis ‘AF2’ 13,333 3 G anada 17.1 [237] P. ×canadensis ‘AF2’ 33,333 3 León 16.75 [122] ‘Gua di’ 16.14 ‘I-214’ 15.14 ‘MC’ 15.01 ‘2000 Ve de’ 10.05 P. ×gene osa ‘Unal’ 11.38 ‘USA 49-177’ 15.92 P. ×gene osa ×P. nig a ‘Mon iso’ 15.16 ‘Pegaso’ 7.37 P. ×canadensis ‘AF2’ 33,333 3 León, Ge ona, Mad id, So ia 16.8 [55] ‘Gua di’ 12.44 ‘I-214’ 12.21 ‘MC’ 15.32 ‘2000 Ve de’ 15.38 P. ×gene osa ‘Unal’ 11.13 P. ×gene osa ×P. nig a ‘Mon iso’ 14.47 ‘Pegaso’ 6.65 P. del oides ‘Vi ia o’ 13,333 3 G anada 9.55 18.20 12.82 [150] P. ×canadensis ‘AF2’ 13.47 16.66 13.99 ‘Ballo ino’ 9.48 16.75 12.27 ‘I-214’ 9.11 13.44 15.81 P. ×gene osa ×P. nig a ‘Mon iso’ 12.29 11.57 13.92 Fo es s 2020,11, 1352 20 o 43 Table 3. Con . Species and Popla Hyb ids Geno ype Densi y Age Loca ion Yield (Mg dm ha−1Yea −1) by Ro a ion Re e ence 1s 2nd 3 d P. ×canadensis ‘AF2’ 20,000 3 (2nd o : 2 yea ) Na a a 15.59 [238] ‘A4A’ 23.80 22.12 ‘Gua di’ 15.59 6.43 ‘I-214’ 23.80 18.51 ‘MC’ 11.21 26.51 ‘T iplo’ 18.07 32.92 P. del oides ‘Vi ia o’ 16.68 38.18 P. ×gene osa ‘Beaup é’ 21.29 15.29 ‘Unal’ 25.86 12.15 P. ×gene osa ×P. nig a ‘Mon iso’ 15.41 ‘Pegaso’ 13.99 P. ×canadensis ‘I-214’ 15,000 3 (2nd o :2 yea ) Na a a 18.20 14.46 [239] 20,000 18.68 18.51 25,000 23.36 14.65 33,333 18.85 17.13 * Es ima ed alues om g aphs. Fo es s 2020,11, 1352 21 o 43 Modeling o G ow h, P oduc ion, and Biome ic Rela ionships in SRC Popla Plan a ions The use o es ima ion models o p edic a ailable biomass is becoming mo e equen as esea ch in o his ype o plan a ion p og esses, no only because o he cos s in ol ed in ca ying ou di ec es ima es [ 240 ], bu also due o he g ea e lexibili y and he possibili y o ex apola ing he esul s o la ge scales when he models a e based on a wide ange o empi ical da a. Allome ic models which ela e he ee diame e o ano he easily measu able a iable o he biomass a e hose mos commonly used in o es in en o ies o ecological s udies [ 241 , 242 ]. These models ha e also been employed in SRC plan a ions o es ima e he a ailable biomass [ 243 – 245 ]. These p edic ions a e pa icula ly impo an when e alua ing he economic iabili y o a c op [ 47 , 246 ]. Apa om he choice o eg ession model, he assump ions ha unde lie he eg ession p ocedu es, and he da a ans o ma ions used du ing he p ocedu es [ 247 ], we a e con on ed wi h a la ge numbe o ac o s which c ea e unce ain y and may a ec he inal esul s [ 248 ]. Simple equa ions which a e alid o a wide ange o condi ions and plan ma e ial a e usually p e e ed. The e a e se e al examples used o popla SRC plan a ions [ 18 , 30 , 78 , 136 , 171 , 226 , 249 – 253 ]. The p ecision o such allome ic equa ions is gene ally su icien o s em biomass componen s [ 254 ]. Howe e , o he examples o equa ions exis ha include mo e p edic i e a iables in he models, which can imp o e he p ecision o he es ima ions [ 11 , 47 , 205 , 255 – 258 ]. Al hough speci ic geno ype-le el models p o ide he g ea es p ecision, in cases whe e geno ype iden i ica ion is complica ed, models o geno ypes which a e axonomically close o axonomic-speci ic models a e o en used. Howe e , unde Medi e anean condi ions a leas , Oli ei a e al. [ 252 ] p o es ha he gene ic o igin does no explain he simila i ies in biomass allome y among geno ypes, so hese app oaches a e no always ad isable. O e he las decade, conside able ad ances ha e been made wi h ega ds o modeling o es ima e biomass om SRC popla plan a ions. No only ha e ad ances been made in allome ic models, bu also much e o has been channeled owa ds modeling o he aspec s o g ow h in hese plan a ions, such as he lea a chi ec u e [259], oo p oduc ion [260–262], and he use o p ocess models wi h his aim [263–267], in addi ion o o he managemen ools de i ed om models [47,268–270]. In ecen yea s, conside able p og ess has been made in he de elopmen o speci ic p edic i e models o SRC plan a ions unde Medi e anean condi ions. Imp o emen s in he model de elopmen me hodology o achie e mo e obus biomass p edic ions ha e been made [ 248 , 271 ], and he sui abili y o biomass models o localpopula ions, as wellas hei pe o mance o di e en samplesizes, ha ealso been e alua ed [ 253 ]. Di e en models ha e been de eloped, including a dynamic, whole-s and model o ‘I-214’ popla geno ype plan a ions in he no he n and cen al pla eau in Spain [ 272 ], along wi h indi idual ee models and gene al models o es ima ing bo h he abo e- and below-g ound biomass in popla SRC plan a ions unde Medi e anean condi ions [252,262]. O he managemen oolsde i ed ommodels,suchasmapso possiblezoneso p oduc ion [47,273,274], es ima ionsde i ed om heuseo new echnologies[ 270 , 275 , 276 ],and oolssuchas e e encediag ams, which a e pa icula ly use ul o bo h planning and managing his ype o c op [ 277 ], ha e also been de eloped in ecen yea s. 3.5. Biomass Cha ac e iza ion The use o lignocellulosic biomass is no limi ed o he p oduc ion o bioene gy. In ecen yea s, a wide ange o biop oduc - ela ed op ions ha e sp ung up, such as biopolyme s, bioplas ics, and suga e men a ion biop oduc s, among o he s [278–280]. Popla lignocellulosic biomass is mainly composed o cellulose (42–49%), hemicellulose (16–23%), and lignin (21–29%) [ 22 , 233 , 279 ]. I s biomass can supply aw ma e ial o p ocesses o he mochemical o mic obiological con e sion. Wo hy o no e among he o me is combus ion, ei he o domes ic o ag o-indus ial applica ions, bu also o indus ial cogene a ion o co- i ing [ 281 , 282 ]. The mochemical p ocesses also include gasi ica ion in downd a gasi ie s o luidized beds [ 283 ], slow py olysis o he p oduc ion o biocha [ 284 ], and as py olysis o p oduce bioc ude oil [ 285 ]. Wi h ega ds o he biochemical p ocesses, he con e sion o second-gene a ion bioe hanol is he mos s udied Fo es s 2020,11, 1352 22 o 43 p ocess [ 286 – 288 ], al hough lignocellulosic biomass can also be a sou ce o biobu anol [ 289 , 290 ] and o he second-gene a ion bio uels [22,279,291]. In Spain, se e al s udies ha e add essed he cha ac e iza ion o popla biomass o he mal use [ 148 , 231 , 292 , 293 ] as bio uels [ 294 – 297 ] o o new biop oduc s [ 298 , 299 ]. The lowe hea ing alue, which is used o calcula ed he a ailable ene gy, is usually a ound 18–20 MJ kg −1 in popla wood on a d y basis [ 300 , 301 ]. Values ound in Spain a e wi hin a ange e y simila o hose ob ained o o he Eu opean coun ies, al hough he cha ac e is ics a y, depending on he geno ype and he age, anging om 17.61 o 18.74 MJ kg−1[137,302,303]. The humidi y con en can be as high as 48–50% a win e ha es ing [ 231 , 303 ], bu popla biomass has shown a good abili y o ai d ying [ 303 ]. The speci ic densi y is known o be low [ 304 – 307 ], hus de i ing in low bulk densi y chips (150–260 kg m −3 , [ 231 ]), co esponding o low ene gy densi ies. Densi ica ion o p oduce pelle s is he e o e an op ion. The ash con en , which is nega i ely ela ed o he ene gy alue and associa ed wi h he isk o boile co osion [ 308 ], a ies b oadly (1–4%), depending on geno ype and si e condi ions [117,137,231,303]. The ouling and slagging isk de i ed om ash composi ions a e known o be e y low [308–310]. The p esence o ni ogen (N), which is ela ed o NOx emissions, is low (0.5%) in popla biomass [148,231,311], along wi h chlo ine (Cl, <0.02%) and sul u (S, <0.04%), which a e co osi e [145,312,313]. Thecombina iono ahigh ola ilema e con en (muchhighe han80%, [ 137 , 303 , 314 ]), so nesso he wood (which is easy o g ind), and a low lignin con en makes popla om SRC a p omisso y eeds ock o gasi ie s. I can also be used o he gene a ion o bioe hanol [287,315]. Lignocellulosic biomass has many o he uses in he con ex o he bioeconomy. Fo example, cellulose is used in he manu ac u ing o cosme ics, ex iles, and pha maceu ical p oduc s, among o he s [ 291 , 297 ]. Lignin can po en ially be used as a aw ma e ial in he manu ac u ing o p oduc s wi h high alue added, such as anillin, biopolyme s in pe ochemis y, and biopes icides, as well as a ma e ial o soil en ichmen [ 291 ]. Hyd-Popla lignin could be used o he p oduc ion o lame- e a dan ma e ials [ 299 ]. Fo example, Ma í n-Samped o e al. [ 234 ] and Iba a e al. [ 233 ] iden i ied he geno ype ‘Vi ia o’ as being e y p omising o use in he p oduc ion o bio uels, as well as in o he alue-added p oduc s, all o which poin o he sui abili y o popla aw ma e ial o di e en uses in he con ex o he bioeconomy. 3.6. Sus ainabili y and Ecosys em Se ices The impo ance o he sus ainabili y o c ops des ined o biomass p oduc ion has been highligh ed when e alua ing hei u u e de elopmen . Du ing he las decade, he Eu opean Commission has ca ied ou di e en analyses and much e o has been dedica ed o de ining c i e ia and indica o s o sus ainabili y (EC, Di ec i e 2009/28/EC, 2009; EU Pa liamen Resolu ion 2013; Di ec i e 2018/2001/EC; Eu opean Pa liamen 2017). The Wo ld Bioene gy Associa ion [ 40 ] has d awn up a documen which includes 24 olun a y sus ainabili y indica o s ela ed o bioene gy in gene al. This documen is pa icula ly impo an since i ep esen s he only mul ila e al ini ia i e wi h a b oad consensus among he di e en go e nmen s and in e na ional o ganiza ions, p o iding a amewo k o u u e policy de elopmen . Spain has been pa o his associa ion since 2008 h ough he Ins i u e o Di e si ica ion and Ene gy Sa ing (IDAE). Despi e his consensus, he e a e many di e en app oaches o ackling hese s udies, some imes because o geog aphical di e ences, which o en leads o app oaches ha a e no always homogeneous. Examples o hese ini ia i es include he analyses conduc ed by Dallemand e al. [316] and Dimi iou and Ru z [317], among many o he s. In any case, he c i e ia and indica o s mus be based on scien i ic e idence and con ain speci ica ions o each p oduc ion model o sou ce o biomass, as hese a y conside ably. In his ega d, he e a e nume ous aspec s which need o be add essed, such as li e cycle and wa e cycle analyses, soil quali y, and e osion con ol, among many o he s [ 318 ]. The en i onmen al impac s o es ablishmen , ha es ing, and anspo a ion ha e been conside ed negligible when he c op is g own Fo es s 2020,11, 1352 23 o 43 on ma ginal land in cen al Eu ope [ 319 ]. Sus ainabili y is also necessa y om economic and ene ge ic pe spec i es. Wi h ega ds o he la e , he adap a ion would appea o be a o able. Howe e , his may no be he case wi h ega d o he economic iabili y gi en he cu en p ices o biomass and absence o subsidies [ 320 , 321 ]. In Spain, se e al li e cycle analyses including no only en i onmen al, bu also ene ge ic and e en economic, issues [ 128 , 237 , 322 – 325 ] ha e been ca ied ou in o de o iden i y he mos ele an ac o s o hese c ops unde Medi e anean condi ions. These analyses concluded ha e iliza ion, anspo , and i iga ion a e some o he mos in luen ial ac o s [ 237 , 288 , 326 , 327 ]. The p ice o biomass, he p ice o land en , ha es ing, and i iga ion ha e been iden i ied as he mos in luen ial ac o s om an economic pe spec i e in Medi e anean en i onmen s [ 214 , 237 , 328 ]. The economic iabili y o popla SRC unde i iga ed Medi e anean condi ions can be achie ed ei he by ensu ing op imum p oduc i i y o h ough an inc ease in he ma ke p ices, associa ed wi h mo e di e si ied ene gy use o biop oduc s, along wi h he quan i ica ion o ecosys em se ices, which cu en ly do no ha e a ma ke p ice [ 214 ]. In addi ion o his, imp o emen s in clonal selec ion and i iga ion echnology, as well as he employmen o o he i iga ion me hods, such as making use o eused wa e , will be essen ial i hese plan a ions a e o be p o i able. A e iew by Li e al. [ 329 ] highligh s he necessi y o iden i y sus ainable sou ces o bioene gy. Wi h his pu pose in mind, he au ho s iden i ied popla c ops among he i e lignocellulosic c ops wi h he g ea es u u e po en ial. They compiled and desc ibed biomass yield in o ma ion ac oss a whole ange o loca ions and coun ies, and concluded ha hese c ops, which only accoun o 3% o he bioene gy in Eu ope [ 330 ], can be g own unde a wide ange o clima ic condi ions, hus allowing di ec compe i ion wi h ood c ops o be a oided. Mo eo e , i has been de e mined ha woody c ops cul i a ed in sho o a ion and he baceous lignocellulosic c ops emi be ween 40% and 99% less NO 2 han adi ional c ops and consequen ly ha e lowe e iliza ion equi emen s and a g ea e N use e iciency. They also seques e ca bon in he biomass ha emains in he soil (0.44 Mg C ha −1 yea −1 ) when hey a e plan ed on disused ag icul u al land, al hough he balance is no posi i e when hey a e plan ed on o me pas u eland. O he associa ed ecosys em bene i s include an inc eased biodi e si y (phy odi e si y and zoological di e si y) [ 32 , 331 ], e osion con ol and soil conse a ion [ 332 ], imp o emen s in he wa e quali y [ 333 ], and he ole o he c op in phy o emedia ion [ 334 – 336 ]. In he case o Spain, he po en ial o he genus in he phy o emedia ion o soils has been e alua ed h ough examples o he es o a ion o coal mining a eas in no he n Spain [131,337,338], o wa e phy odepu a ion [ 42 , 187 , 216 , 339 ]. Resea ch has also ocused on nu ien luxes, e alua ing he ole o annual lea li e in soil e ili y h oughou he o a ion [ 340 ]; he quan i ica ion o accumula ed ca bon in bo h abo e- and below-g ound ac ions ( eaching alues o a ound 6.5 Mg C ha −1 yea −1 in he abo e-g ound woody biomass, a ound 1.0 Mg C ha −1 yea −1 in he below-g ound biomass [ 262 ], and a ound 2.5 Mg C ha −1 yea −1 in he case o he li e [ 341 ]), as well as he economic implica ions o CO 2 cap u e [ 342 ]. The impac o hese o es y c ops on he landscape o he ag icul u al en i onmen s whe e hey a e g own has no ye been e alua ed in ou coun y. Thei e ec s on soil, biodi e si y, and i s unc ion o mi iga ing di use pollu ion a e examples o aspec s ha should be conside ed. Among he main conce ns in Eu ope in e ms o he sus ainabili y o c ops des ined o biomass p oduc ion and he e o e SRC plan a ions, a e hose linked o land use change. Acco ding o some s udies, he balance o g eenhouse gases and C in he soil indica es ha bioene gy as a whole plays a ole in he mi iga ion o clima e change [ 343 ]. Howe e , o he s udies ques ion his a i ma ion [ 330 ], o poin o a minimum cul i a ion pe iod, beyond which he C balance becomes posi i e [344–347]. The impac on wa e esou ces mus also be igo ously assessed [ 332 , 348 ]. In he Medi e anean a ea, he need o i iga ion is pe cei ed as one o he main weaknesses o hese plan a ions. Di e en op ions aimed a minimizing o achie ing mo e e icien i iga ion in Medi e anean en i onmen s ha e been men ioned in Sec ion 3.3.4. The applica ion o was ewa e in popla plan a ions no only cons i u es an a ac i e me hod o p oducing biomass h ough he egene a ion o was ewa e , bu also implies a educ ion o suspension o e ilize applica ion [42,187,188]. Hence, his app oach Fo es s 2020,11, 1352 24 o 43 no only p o ides a sus ainable way o minimize wa e use, bu also akes ad an age o he abili y o hese plan a ions as ege a ion il e s. Imp o ing he sus ainabili y o SRC c ops as con ibu o s o he biomass pool will p obably equi e global decisions ha ake in o accoun local speci ica ions. 4. Cu en S a us and Fu u e P ospec s in Spain The a ailabili y o abandoned ag icul u al land, he lack o economic al e na i es in u al a eas, and he possibili ies o complemen a i y a o ded by SRC c ops in e ms o supply along wi h he ecosys em se ices associa ed wi h hei es ablishmen on ag icul u e land, a e ac o s a o ing he implemen a ion o SRC o es c ops [ 349 – 351 ]. As wi h o he sou ces o biomass, i is impo an o analyse he biomass p oduced om o es c ops and assess i s sus ainabili y and he bes me hods o p oducing i . Howe e , he posi i e and nega i e aspec s o his ype o biomass p oduc ion, indeed o biomass p oduc ion in gene al, mus always be aken in o accoun and he e will always be a ce ain deg ee o con o e sy su ounding i s iabili y. Biomass om o es c ops in sho o a ion has a ac ed a lo o in e es in Spain, p obably due o he egula ions ega ding elec ici y p oduc ion in 2004 (RD 436/2004) and la e in 2007 (RD 661/2007). A ha ime, many la ge and medium-sized companies conside ed es ablishing o es c ops in SRC. Howe e , he amendmen o his egula ion in 2013 emo ed he incen i es a a ime when many aspec s associa ed wi h he es ablishmen o c ops in Medi e anean en i onmen s had s ill no been cla i ied, he ecosys em se ices had no been assessed, and he economic iabili y was a om gua an eed. This si ua ion esul ed in a declining in e es in he sec o , which con inues oday. Despi e he expec ed g adual inc ease in he con ibu ion o enewable ene gy o he inal g oss ene gy consump ion, in 2017, Spain was s ill 2.5% poin s om i s 2020 na ional a ge [ 352 ]. O he o al con ibu ion o enewable ene gy, biomass accoun s o 13% [ 353 ]. Wi h ega ds o he amoun o biomass des ined o biop oduc s, as a as we know, he e a e no s a is ics o he coun y gi en ha he e a e only a esidual numbe o bio e ine ies p oducing such biop oduc s [ 280 ]. The e o e, many aspec s mus s ill be esol ed be o e he economic iabili y o enewable biomass esou ces can be de e mined. Fo example, he p ice o biomass in Spain is lowe han in neighbo ing coun ies, so pa o he biomass is expo ed. This would appea o be p oblema ic in e ms o sus ainabili y. Mo eo e , no s able, p edic able ma ke o biomass exis s in Spain. In ela ion o biomass om o es c ops in sho o a ion, many ad ances ha e been made in Spain o e ecen yea s, which ha e been desc ibed in his a icle. These ad ances include he ollowing: (i) Inc eased knowledge o he gene ic ma e ial in ela ion o he en i onmen , wi h a be e unde s anding o he adap abili y o geno ypes, including hei wa e -use e iciency; (ii) maximizing p oduc ion based on densi ies, geno ypes, and managemen p ac ices; (iii) e alua ing he sus ainabili y o plan a ions om di e en pe spec i es (en i onmen al, ene ge ic, and economic); (i ) explo ing al e na i e designs, such as mixed plan a ions; ( ) implemen ing plan a ions in ma ginal a eas p e iously used o mining; ( i) quan i ying accumula ed ca bon; ( ii) speci ic p edic i e models and managemen ools o SRC plan a ions unde Medi e anean condi ions; ( iii) physiological and molecula cha ac e iza ion o a ibu es, mainly hose ha a e ele an o cul i a ion in ma ginal zones; (ix) chemical composi ion and py oly ic beha io ; and (x) he use o bio echnology o de elop new ma e ials, which should lead o inc eased p oduc ion in he u u e. The ole o hese o es c ops in wa e pu i ica ion has also ecen ly eme ged as a ma e o in e es . Despi e hese ad ances, we s ill ha e a long way o go o make SRC plan a ions a comme cial eali y in Spain. I is also known ha biomass, including dedica ed c ops, would gene a e signi ican economic e u ns in u al a eas [ 354 ]. This may be especially ele an in a coun y like Spain, whe e a la ge pa o he coun y su e s om high le els o depopula ion, al hough some aspec s ha hinde he sho - e m economic iabili y ha e also been iden i ied, such as hose p e iously men ioned ega ding land en al, he need o i iga ion, and he echnological de elopmen o ha es ing machine y a easonable local p ices. Howe e , o he issues need o be add essed in ela ion o c op managemen , Fo es s 2020,11, 1352 25 o 43 such as (i) he use o i iga ion, which should only be eso ed o in a eas whe e i is sus ainable, by mode nizing he sys ems, using mo e wa e -use e icien plan ma e ial o by using ecycled wa e ; (ii) he con inual upda ing o gene ic ma e ial adap a ions o he si e, es ing o new ma e ials, and making use o new echnologies (genome edi ing, linkage maps, e c.), in o de o p oduce ma e ial ha is esis an o pes s and diseases, ole an o d ough o a high salini y, o can adap mo e e ec i ely o speci ic soil and clima e condi ions; (iii) explo ing new plan a ion designs; al hough he p elimina y esul s o using mix u es a e no pa icula ly encou aging, many di e en al e na i es emain o be explo ed; and (i ) a ionalizing e iliza ion by e alua ing he inpu s de i ed om lea li e and he expo s o nu ien s om wood, as well as by using al e na i e e ilize inpu s, such as hose de i ed om sewage sludge. O he ques ions should be explo ed based on he need o ( ) quan i y and alue ecosys em se ices in e ms o inc eased biodi e si y in he ag icul u al landscape; ca bon accumula ion in each o he biomass ac ions, bo h hose ha a e ex ac ed, as well as hose ha emain in he soil, such as olia and oo biomass; and hei addi ional ole in phy o emedia ion, o ( i) ad ance p edic i e modeling o Medi e anean condi ions by combining he bes ea u es o empi ical and p ocess models, he ad an ages o which ha e been well-documen ed. The inclusion o ecophysiological a iables enabling he p edic ion o indi idual ee-le el biome y unde di e en condi ions, along wi h imp o emen s in de e mining he mos impo an a iables, a e some o he u u e objec i es o modeling SRC plan a ions unde Medi e anean condi ions. In addi ion, as wa e es ic ions ep esen a eal h ea , his ac o is also being conside ed in he de elopmen o models ha enable he simula ion o di e en clima e change scena ios. Rega ding ( ii) he cha ac e iza ion o biomass, al hough many s udies ha e al eady add essed his aspec , a mo e in-dep h knowledge will be equi ed as he inal p oduc s and hei in ended uses become mo e de ined. Iden i ying he equi emen s o each o he inal p oduc s will be essen ial in o de o de e mine he mos sui able geno ypes o each c op. Gi en ha Spain is a di e se coun y, i is also necessa y ( iii) o analyse whe e and how his biomass is p oduced a a na ional scale. Imp o ing he c i ical aspec s de ec ed in en i onmen al, ene ge ic, and economic analyses is essen ial o achie ing p o i able and sus ainable plan a ions unde Medi e anean condi ions. Biomass p oduced ad hoc h ough plan a ions unde SRC sys ems may be o in e es in many a eas o he Medi e anean, p o iding a u he op ion which could con ibu e o he de elopmen o a ci cula bioeconomy while also gene a ing impo an en i onmen al se ices. Au ho Con ibu ions: Concep ualiza ion, N.O. and H.S.; unding acquisi ion, H.S., R.R.-S., and I.C.; in es iga ion, N.O., H.S., C.P.-C., R.R.-S., and I.C.; me hodology, H.S. and N.O.; p ojec adminis a ion, H.S., R.R.-S., and I.C.; supe ision, H.S., I.C., C.P.-C., and R.R.-S.; w i ing—o iginal d a , N.O. and H.S.; w i ing— e iew and edi ing, N.O., C.P.-C., I.C., R.R.-S., and H.S. All au ho s ha e ead and ag eed o he published e sion o he manusc ip . Funding: This esea ch was unded by he Minis y o Science and Inno a ion (Spain) h ough he amewo k o he INIA p ojec s RTA2017-00015-CO2, co- inanced wi h unds om FEDER. Acknowledgmen s: The au ho s wish o hank Adam Collins o his English language e iew o he manusc ip . Con lic s o In e es : The au ho s decla e no con lic o in e es . Re e ences 1. Eu opean Commission. The Eu opean G een Deal; Eu opean Commission: B ussels, Belgium, 2019; p. 24. 2. He emäki, L.; Hanewinkel, M.; Muys, B.; Ollikainen, M.; Palah í , M.; T asoba es, A. 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