Production and role of epicormic shoots in pruned hybrid poplar: effects of clone, pruning season and intensity

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1 Annls of Forest Science (21) 72: DOI.7/s ORIGINAL PAPER Production nd role of epicormic shoots in pruned hybrid poplr: effects of clone, pruning seson nd intensity Annie Desrochers & Vincent Murin & Emilie Trroux Received: 31 July 214 /Accepted: 28 November 214 /Published online: 19 December 214 # INRA nd Springer-Verlg Frnce 214 Abstrct Key messge Pruning one third of crown length in summer produced the lest number of epicormic brnches fter two growing sesons. Epicormic brnches cn be removed without compromising tree growth. Context The formtion of epicormic shoots is often observed following pruning tretments, but their role in the overll tree growth is unknown. Aims The objectives of this study were to exmine how pruningintensityndsesonffecttheproductionof epicormic brnches nd how their presence (or removl) ffects tree growth nd totl non-structurl crbohydrte reserves in the roots. Methods Trees from four hybrid poplr clones were pruned in fll, spring, nd summer t two intensities 1/3 nd 2/3 of the crown length nd n unpruned control. Produced epicormic brnches were removed from hlf the trees. Results Pruning intensity nd seson were the most importnt fctors ffecting the number nd biomss of epicormic shoots while clone ws not. Pruning 1/3 crown length in summer reduced the emergence of epicormic shoots compred Hndling Editor: Erwin Dreyer Contribution of the co-uthors Murin relized the field nd lbortory work, rn some of the dt nlysis, nd wrote the initil mnuscript. Desrochers designed the experiment, supervised the work, coordinted the reserch project, nd edited the mnuscript. Trroux rn the dt nlysis, produced grphs, nd wrote prts of the mnuscript. A. Desrochers (*): V. Murin : E. Trroux Forest Reserch Institute, Université du Québec en Abitibi-Témiscmingue, 341 rue Principle Nord, Amos, Québec J9T 2L8, Cnd e-mil: nnie.desrochers@uqt.c V. Murin e-mil: Vincent.murin@uqt.c E. Trroux e-mil: emilie.trroux@uqt.c to 2/3 nd spring or fll pruning. Two yers fter pruning, the removl of epicormic shoots did not ffect height or dimeter t brest height of trees, nd their presence did not restore root totl non-structurl crbohydrtes reserves to unpruned levels. Conclusion We concluded tht pruning should be done in summer t 1/3 of the crown length to reduce epicormic shoot formtion nd to void decreses in stem growth. If epicormic shoots pper, they cn be removed without compromising tree growth. Keywords Populus. Tree physiology. Silviculture. Brnch 1 Introduction Pruning tretments re used to dd vlue to crop trees in mnged stnds by incresing the mount of cler, knot-free wood (Hubert nd Courrud 1994; Wring nd O Hr 2). However, pruning might lso promote the production of epicormic shoots, brnches tht form long the trunk of trees fter they hve been pruned (Auchmoody 1972). Epicormic brnching is common in intensively mnged hybrid poplr plnttions (Shock et l. 23; Smith nd Blom 1966; Tbbush nd Beton 1998), most often following pruning tretments (DeBell et l. 22; Hubert nd Courrud 1994; Isebrnds et l. 27; vn Oosten 26). These epicormic brnches rise from buds found on or under the brk tht re often clled dormnt buds (Kozlowski nd Pllrdy 1997) nd cn be of proventitious or dventitious origin (Fink 1983; Meier et l. 212; Stone nd Stone 1943). Proventitious buds develop from existing buds nd re locted t the xil of leves (Stone nd Stone 1943) nd cn burst when the sequentil brnch to which they re ttched is removed or dying (Meier et l. 212), wheres dventitious buds rise outside the norml phyllotxy nd my be produced on the cllus locted on the cmbium lyer fter wounding of surrounding tissues

2 426 A. Desrochers et l. (Fink 1983; Meier et l. 212). Epicormic shoots re thus n impediment to the success of pruning opertions to produce high-qulity timber, becuse their pprition produce dditionl knots tht reduce timber qulity nd vlue (Fontine et l. 1999; Remphrey nd Dvidson 1992). Plnttion mngers thus need to know (i) how trees will rect to pruning in terms of epicormic shoot production, nd how the ltter will ffect trunk growth nd qulity, nd (ii) if it is useful to remove these epicormic shoots once they hve sprouted in terms of growth nd qulity of stems but lso in terms of n eventul re-sprouting of undesirble brnches. Epicormic shoot formtion is generlly thought to be response to sudden light increse (Gordon et l. 26; Wignll nd Browning 1988) or stress (Stone nd Stone 1943). Epicormic brnching cn lso be considered s mechnism for re-estblishment or mintennce of functionl crown (Del et l. 23), in response to physiologicl imblnce between photosynthetic nd nonphotosynthetic orgns (Nicolini et l. 21). However, significnt knowledge gps remin, especilly for the role of these brnches in overll tree physiology: Does their productionfterpruningledtoincresed(energysource) or decresed (energy sink) dimeters of the min stem? A source is n orgn which supplies crbon (net exporter), while sink is n orgn which uses crbon (net importer) (Kozlowski 1992). Since epicormic shoots re usully produced rpidly fter pruning nd cn hve very fst growth rtes nd lrge biomsses, they might initilly be very strong sinks nd cuse growth reductions or stgntion of the min stem. On the other hnd, since they rpidly produce new lef re on hevily pruned stems (Del et l. 23; Tkiyetl.2), they might rpidly compenste the loss of photosynthetic re of pruned crowns nd become n importnt energy source for the tree (Nicolini et l. 21). Since the imblnce between photosynthetic vs non-photosynthetic tissues creted by pruning increses with pruning intensity, it seems logicl tht the occurrence of epicormic brnches would increse with pruning intensity (O Hr nd Vlppil 2). However, this response could diverge ccording to the seson when pruning is done, since seson my lso ffect epicormic shoot formtion (Bchelrd 1969), most likely due to the hormonl nd nnul crbon lloction tht cn regulte epicormic brnching (Meier et l. 212). The objectives of this study were to exmine how pruning intensity nd seson ffect the production of epicormic brnches in hybrid poplr clones of different prentges nd how their presence (or removl) ffects tree growth nd totl non-structurl crbohydrte reserves in the roots. We pruned four hybrid poplr clones t three pruning intensities (unpruned, 1/3 nd 2/3 of crown length) nd three sesons (fll, spring, nd summer). We hypothesized tht severe pruning (2/3 crown length) done during the growing seson (summer) would result in more epicormic brnches produced, due to greter sudden light increse of the stems. Becuse epicormic brnches usully get very lrge rpidly (energy sink) while locted low on the stem (low light conditions), we hypothesized tht their production would negtively ffect stem growth nd tht their subsequent removl would lower root totl non-structurl crbohydrte reserves. The decision to remove epicormic brnches fter pruning brings supplementry costs to foresters wnting to produce knot-free timber nd requires more dt to mke sensible silviculturl choices. 2 Mteril nd methods 2.1 Study site The studied plnttion ws estblished in 27 t the New Liskerd Agriculturl Reserch Sttion of Guelph University in North-Estern Ontrio ( N, W). The regionl surficil geology is chrcterized by lcustrine clys nd snds from the post-glcil Lke Brlow (Rowe 1972), nd the soil type ws Humic Gleysol (Cnd Soil Survey Committee 1987). Between 1981 nd 2, nnul precipittions for the region verged 837 mm (rinfll 6 mm, snowfll 181 cm) with n verge dily temperture of 3.1 C nd 2,641 degreedys bove C (Environment Cnd 214; Ville Mrie wether sttion, bout 2 km NW of the study re). The site ws plowed in October 26 nd crosscultivted with disks followed by pre-emergent herbicide ppliction in spring 27 before plnting trees t 3.-m 3.-m spcing (816 stems h 1 ). The plnting stock consisted of dormnt 1-yer-old breroot trees. Plnting ws followed by fertiliztion with (N-P-K, 1gtree 1 ). Annul weed control consisted of crosscultivtion with disks followed by herbicide ppliction (RoundupTM). Four hybrid poplr clones were chosen bsed on their different prentge: 79 (Populus jckii (Populus blsmifer Populus deltoides)), 7472 (Populus blsmifer Populus trichocrp), (Populus mximowiczii Populus blsmifer), nd DN2 (Populus deltoides Populus nigr). Trees were plnted in split-split plot rndomized complete block design with three blocks (replictes), ech contining four plots of trees ( rows trees), one for ech clone. Ech plot ws rndomly divided into three pruning sesons (October 29 = fll, Mrch 2 = spring, or June 2 = summer) nd three intensities (unpruned, 1/3 or 2/3 of the crown length; Smith et l. 1997). One row of trees ws used for ech tretment combintion. To compre trees with nd without

3 Epicormic shoots in hybrid poplr 427 epicormic shoots, ech row ws divided in two sections; epicormic shoots tht sprouted fter pruning were periodiclly removed from the first five trees, while they were left to grow on the other five trees. For the study, we considered unpruned trees s trees without epicormic shoots. 2.2 Field smpling Height nd dimeter t brest height (dbh) of ll the trees were mesured t the end of ech growing seson from 29 to 211 (see Tble 1 from Murin nd DesRochers 213). At the end of August 2, root smples were collected for determintion of totl non-structurl crbohydrtes (TNC)bycuttingdiskfromoneoftheminroots.To compre TNC concentrtions between trees with or without epicormic shoots, we rndomly selected one tree per row to collect root smple per pruning tretment in pruned trees with nd without epicormic shoots nd unpruned trees from ech of the four clones (n=18, since unpruned trees were considered s hving no epicormic brnches). In September 2, we selected one tree with epicormic shoots per pruning tretment for ech clone (n=72). For those selected trees, ll epicormic shoots (with leves) formed in 2 (2) between the ground level nd pruning height were collected. The sme opertion ws repeted in September 211 on the sme trees, to collect newly formed epicormic shoots (211_new) if ny (n=72). At the sme dte, we lso selected 72 other trees to collect 2-yer-old epicormic shoots (211_old). We mesured brnch biomss, length, nd bsl dimeter s well s lef biomss on ech smple. Since some biomss dt went missing in the lb, we clculted brnch volume from its bsl dimeter nd length vlues nd used liner regression between brnch biomss nd volume (R 2 =.98) to estimte brnch biomss on missing smples. Brnch volume ws clculted from the following eqution: V ¼ 1=3 H π D 2 =4 ð1þ where V is the brnch volume in cubic centimeter, D the bsl dimeter of the brnch in centimeter, nd H the brnch length in centimeter then converted to biomss. 2.3 Lbortory work Shoots were brought to the lb in lrge plstic bgs nd stored in refrigertor t 2 C before they were oven-dried t 6 C until constnt mss ws obtined. Brnch biomss consisted of dry mss of shoots nd leves. Root smples for crbohydrte nlysis were ovendried t 6 C until constnt mss ws obtined nd ground to pss 4-mesh screen of Wiley mill (model 3383-L; Thoms Scientific, Swedesboro, NJ). Soluble sugrs were extrcted by boiling three times in 8 % ethnol t 9 C. Phenol sulfuric cid ws used to nlyze the ethnol extrct for totl sugr concentrtion. Strch ws digested using n enzyme mixture of α- mylse nd myloglucosidse followed by the colorimetric mesurement of the glucose hydrolyste using peroxidse-glucose oxidse-o-dinisidine regent (Chow nd Lndhäusser 24). 2.4 Sttisticl nlyses Sttisticl nlyses were done with R softwre version (R Development Core Tem 212). Liner mixed effects models were used to model tretment effects on ech response vrible. To reflect the plnttion design, ll models included rndom effects tht consisted of block nd clone nested in block. We used multiple comprisons of mens (Tukey s tests) to find differences mong tretments with the multcomp pckge (Hothorn et l. 28). The fit of the globl model ws ssessed for ech nlysis. The globl model ws composed of ll the fctors tested nd their interctions. The effect of pruning intensity (1/3, 2/3), clone (279, 7472, 91319, nd DN2), seson (fll, spring, nd summer), nd tretment (with or without epicormic brnches) on the number nd biomss of epicormic shoots produced by trees ws nlyzed using the lme function from the nlme pckge (Pinheiro et l. 212). A log trnsformtion ws pplied to normlize residuls nd improve homoscedsticity, but results nd predicted vlues re presented in k-trnsformed units. We used stepwise nd kwrd model selection to determine the most suitble model for the number of epicormic shoots. To nlyze the effects of epicormic shoots on tree height nd dbh growth in 2 nd 211, we built four models(dbh,height,dbh11,ndheight11) using the lme function from the nlme pckge (Pinheiro et l. 212). We included initil tree size mesurements (fll 29) s covrites. As ll the trees (with nd without epicormic shoots) were used for this nlysis, prmeter tretment in the models corresponds to the pruning intensity ssocited with the presence (1/3_epi or 2/3_epi) or bsence (1/3 or 2/3) of epicormic shoots, lso including the unpruned trees. Prmeter tree ws dded s rndom effect for this nlysis.

4 428 A. Desrochers et l. Tble 1 Globl models testing the number (NUMB) nd biomss (MASS) of epicormic shoots produced ccording to pruning intensity (1/3 nd 2/3 pruning of crown length), clone (79, 7472, 91319, nd DN2), seson (fll, spring, nd summer), nd yer (2, 211_new, nd 211_old) Tested fctors numdf dendf F vlue P vlue NUMB MASS NUMB MASS NUMB MASS NUMB MASS Intensity <.1 <.1 Seson <.1 <.1 Clone Yer <.1 Clone:Yer Intensity:Yer Intensity:Clone Seson:Yer <.1 Seson:Clone Intensity:Seson Intensity:Clone:Yer Seson:Clone:Yer Intensity:Seson:Yer Intensity:Seson:Clone Intensity:Clone:Yer:Seson Sttisticlly significnt vlues (P<.) re given in bold. The fctor yer 2 corresponds to epicormic brnches produced in 2, 211_new corresponds to epicormic brnches produced in 211, nd 211_old corresponds to epicormic brnches produced in 2 nd 211 nd collected t the end of 211 growing seson. NumDF nd dendf re the numertor nd denomintor of degrees of freedom, respectively The effect of pruning (unpruned, 1/3 pruning with or without epicormic shoots nd 2/3 with or without epicormic shoots), clone, nd seson on totl non-structurl crbohydrtes ws lso tested using lme function (Pinheiro et l. 212). We used stepwise nd kwrd model selection to determine the most suitble model. Tble 2 Selected model testing the effects of pruning intensity (1/3 nd 2/3 crown length), clone (79, 7472, 91319, nd DN2), seson (fll, spring, nd summer), nd yer (2, 211_ new, nd 211_old) on the number of epicormic shoots (NUMB model) Sttisticlly significnt vlues (P<.) re given in bold. The fctor yer 2 corresponds to epicormic brnches produced in 2, 211_new corresponds to epicormic brnches produced in 211, nd 211_old corresponds to epicormic brnches produced in 2 nd 211 nd collected t the end of 211 growing seson DF degrees of freedom Tested fctors Estimte vlue Stndrd error DF P vlue (Intercept) <.1 Intensity 2/ <.1 Seson spring Seson summer <.1 Clone Clone CloneDN yer211_new <.1 yer211_old Sesonspring:yer211_new Sesonsummer:yer211_new <.1 Sesonspring:yer211_old Sesonsummer:yer211_old Sesonspring:Clone Sesonsummer:Clone Sesonspring:Clone Sesonsummer:Clone Sesonspring:CloneDN Sesonsummer:CloneDN <.1

5 Epicormic shoots in hybrid poplr 429 ) Number of epicormic shoots per tree b) Biomss of epicormic shoots per tree (g) Results 3.1 Number nd biomss of epicormic shoots 1/3 2/3 Pruning intensity (crown length) Fig. 1 Predictions for the men number () nd biomss (b) ofepicormic shoots produced for ech pruning intensity (1/3 or 2/3 crown length), ll clones, sesons, nd growing sesons combined. Error brs re stndrd errors of the men. Brs lbeled with the sme letter within grph re not significntly different (Tukey s testtp<.) All pruned trees but one produced epicormic shoots. The number of epicormic shoots produced fter one growing seson (2) rnged from to 97. The model selected to explin the number of epicormic brnches produced by trees included pruning intensity, yer (1-yer-old brnches fter the 2 nd 211 [211_new] growing sesons nd 2-yer-old brnches [211_old]), pruning seson, clone, nd the interctions seson yer nd seson clone (Tbles 1 nd 2). As expected, the number of epicormic shoots incresed with pruning intensity (Fig. 1). The four clones hd similr responses to pruning intensity (Tble 1; P =.4), but clone DN2 produced more epicormic brnches fter summer pruning compred to clone 79 (interction clone seson; Fig. 2). Contrrily to our hypothesis, trees pruned in the fll nd spring produced on verge 3. times more epicormic shoots thn trees b b ) Number of epicormic shoots per tree b) Number of epicormic shoots per tree c DN2 Fll Spring Summer Fll Spring Summer b b 2 211_new 211_old pruned in the summer, during the 2 growing seson (Fig. 2b). When these first epicormic brnches were removed, trees produced little less new epicormic brnches the following yer (6.4 brnches per tree on verge; 211_new). On the other hnd, if epicormic brnches produced during the first growing seson (2) were left on, trees hd. epicormic brnches on verge fter two growing sesons (211_old; Fig. 2b). However, the number of epicormic shoots gretly vried during the second growing seson, nd the effect of pruning seson ws no longer significnt in 211 (Fig. 2b). Biomss of epicormic brnches ws 2.1 times greter for the 2/3 pruned trees compred to the 1/3 pruned trees (Fig. 1b). Other thn pruning intensity, men biomss of epicormic brnches ws ffected by the interction yer c Pruning seson Yer b Fig. 2 Predictions of the men number of epicormic shoots produced by trees, representing the interctions seson clone () nd yer seson (b), ll pruning intensities combined. Yer 2 corresponds to epicormic brnches produced in 2, 211_new corresponds to epicormic brnches produced in 211 nd 211_old corresponds to epicormic brnches produced in 2 nd 211 nd collected t the end of 211 growing seson. Errors brs re stndrd errors of the men. Brs lbeled with the sme letter re not significntly different (Tukey s test t P<.) b

6 43 A. Desrochers et l. Tble 3 Selected model testing the effect of pruning intensity (1/3, 2/3), seson (fll, spring, summer), nd yer (2, 211_new, nd 211_old) on biomss of epicormic shoots (MASS model) Tested fctors Estimte vlue Stndrd error DF P vlue (Intercept) <.1 yer211_new <.1 yer211_old Intensity2/ <.1 Sesonspring Sesonsummer <.1 yer211_new:sesonspring yer211_old:sesonspring yer211_new:sesonsummer <.1 yer211_old:sesonsummer Significnt P vlues re given in bold. The fctor yer 2 corresponds to epicormic brnches produced in 2, 211_new corresponds to epicormic brnches produced in 211, nd 211_old corresponds to epicormic brnches produced in 2 nd 211 nd collected t the end of 211 growing seson DF degrees of freedom seson (Tbles 1 nd 3), showing tht the biomss of epicormic brnches produced fter one (2) nd two (211_old) growing sesons ws smller for trees pruned in summer compred to trees pruned in fll nd spring, while the biomss of epicormic shoots produced solely during the second growing seson (211_new) ws smller nd independent of pruning seson (Fig. 3). 3.2 Growth responses Pruning 1/3 crown height slightly reduced height of trees fter one nd two growing sesons ( 2 %), while it hd no effect of stem dbh (Tble 4 nd Fig. 4). Pruning 2/3 of crown length Men biomss of epicormic shoots (g) cd d 2 b 211_new 211_old fll spring summer Pruning seson Fig. 3 Predictions for men biomss of epicormic shoots per tree for ech yer nd seson combintion, ll clones nd pruning intensities combined. Yer 2 corresponds to epicormic brnches produced in 2, 211_new corresponds to epicormic brnches produced in 211, nd 211_old corresponds to epicormic brnches produced in 2 nd 211 nd collected t the end of 211 growing seson. Errors brs re stndrd errors of the men. Brs lbeled with the sme letter re not significntly different (Tukey s test t P<.) cd d reduced height by 3 nd % fter one nd two growing sesons, respectively, while bsl dimeter (dbh) of 2/3 pruned trees ws 4 nd 8 % smller thn unpruned trees, fter one nd two growing sesons, respectively (Fig. 4). Production of epicormic shoots did not ffect tree brest height dimeter nd totl height growth since pruned trees with or without epicormic shoots hd similr dbhs nd heights within pruning intensity (Fig. 4). 3.3 TNC Model selection showed tht only pruning intensity ffected root TNC concentrtions, while there ws no effect of clone or pruning seson (Tbles nd 6). Root TNC concentrtions were lower for ll pruning tretments compred to unpruned trees, except for the 2/3 pruned trees with epicormic shoots which hd similr levels (Fig. ). For given pruning intensity, root TNC concentrtions were similr between trees with or without epicormic shoots (P>.). 4 Discussion All four studied hybrid poplr clones responded to pruning by the production of epicormic shoots, which, however, did not ffect dbh or height growth of pruned trees, since there ws no difference in size between trees with or without epicormic shoots fter one or two growing sesons. Pruning intensity nd seson both ffected the number of epicormic shoots produced fter pruning; Trees pruned to 2/3 crown length produced nerly twice s mny epicormic

7 Epicormic shoots in hybrid poplr 431 Tble 4 Models testing the effects of pruning tretment on tree height (HEIGHT nd HEIGHT11) nd dbh (DBH nd DBH11) fter the 2 nd 211 growing sesons, respectively Model Tested fctors Estimte vlue Stndrd error DF P vlue DBH (Intercept) <.1 D <.1 Tretment Tretment Tretment E Tretment E <.1 HEIGHT (Intercept) <.1 H <.1 Tretment Tretment <.1 Tretment E <.1 Tretment E <.1 DBH11 (Intercept) D Tretment Tretment Tretment E Tretment E HEIGHT11 (Intercept) <.1 H <.1 Tretment <.1 Tretment <.1 Tretment E <.1 Tretment E <.1 Sttisticlly significnt vlues (P<.) re given in bold. Tretment corresponds to the combintion between pruning intensity nd epicormic brnches tretment (1=1/3 pruned tree without epicormic shoots, 2=2/3 pruned tree without epicormic shoots, E1=1/3 pruned tree with epicormic shoots, E2=2/3 pruned tree with epicormic shoots) DF degrees of freedom brnches with over twice the biomss of 1/3 pruned trees (Fig. 1), which could be interpreted s mens for trees to restore the blnce between lef re nd non-photosynthetic orgns (Nicolini et l. 21). Hevy pruning hs been shown to be significnt promoter of epicormic sprouting (Gordon et l. 26; Wring nd O Hr 2). The bsence of interctions between pruning intensity nd clone or pruning seson is indictive tht one cn expect similr responses to pruning intensity regrdless of the clone or of when pruning is done. The removl of folige with pruning cused decreses in root TNC concentrtions compred to unpruned trees (Fig. ). This drop in TNC ws either due to the production of new folige nd/or the reduction of photosynthte production by the remining leves of pruned trees (Lovett nd Tobiessen 1993). Although 2/3 pruning pproximtely doubled the mount of folige removed nd the mount of epicormic brnches produced, root TNC reserves were similr between the two pruning intensities. This suggests tht root TNC reserves were not proportionlly solicited to re-estblish photosynthetic cpcity of trees or to supply crbohydrtes for respirtion demnds of trees. The fct tht root TNC concentrtions were similr for trees with or without epicormic brnches within pruning tretment indicte tht the photosynthtes produced by epicormic brnches were not entirely exported to the roots nd were probbly mostly used to support the growth nd mintennce of their lrge biomsses. Our results lso show tht more thn one growing seson post-pruning will be required for trees to reconstitute root TNC concentrtions to similr levels of unpruned trees. Trees pruned in summer produced the lest epicormic shoots fter one growing seson (Fig. 2b), consistent with previous studies such s Wignll et l. (1987) in ok (Quercus robur L.) or Bchelrd (1969) in Euclyptus polynthemos. Sesonofpruningwslsofoundtobe the most importnt fctor ffecting shoot initition nd growth in Prunus persic (Gordon et l. 26). The fewer

8 432 A. Desrochers et l. Fig. 4 Predictions for height nd dbh, one (, b) ndtwo(c, d) growing sesons fter pruning on trees with (1/3epi, 2/3epi) or without ( = unpruned, 1/3, 2/3) epicormic shoots, ll clones combined. Errors brs re stndrd errors of the men. Brs lbeled with the sme letter within grph re not significntly different (Tukey s test t P<.) ) Height (m) d c b b) Dbh (cm) b b b c) Height (m) c b b d) Dbh (cm) b b b 6 4 1/3 1/3 epi 2/3 2/3 epi Pruning tretment 4 1/3 1/3 epi 2/3 2/3 epi Pruning tretment epicormic shoots produced by summer-pruned trees could be explined by the fct tht development of epicormic shoots depends on bud brek, which occurs t the beginning of the lefy period (Hrmer 1988), on light vilbility in erly spring nd on light vilbility throughout Tble Globl model testing the effect of pruning (unpruned, 1/3 nd 2/3 pruning of crown length), seson (fll, spring, nd summer), nd clone (79, 7472, 91319, nd DN2) on root totl non-structurl crbohydrte concentrtions (TNC) of trees with or without epicormic shoots Tested fctors numdf dendf F vlue P vlue (Intercept) <.1 Clone Tretment <.1 Seson Clone:Tretment Clone:Seson Tretment:Seson Clone:Tretment:Seson Significnt P vlues re given in bold. Tretment corresponds to the combintion between pruning intensity nd epicormic brnch tretment (unpruned, 1/3 pruned with or without epicormic shoots nd 2/3 pruned with or without epicormic shoots NumDF numertor of degrees of freedom, dendf denomintor of degrees of freedom the growing seson (Colin et l. 28). Trees pruned in fll or spring hd more vilble light t the time of bud brek nd lso hd more time to develop epicormic buds over the growing seson. Moreover, prdormncy of epicormic buds, dormncy moderted by plnt structures externl of the buds (Lng 1987), could be lrgely controlled by downwrd uxin trnsport from young expnding leves (Aloni et l. 23), which were bsent when trees were pruned in the fll nd spring (before lef flush). It is lso thought tht epicormic buds could be Tble 6 Selected model testing the effect of pruning tretment on root totl non-structurl crbohydrte concentrtions (TNC) Tested fctors Estimte vlue Stndrd error DF P vlue (Intercept) <.1 Tretment Tretment <.1 Tretment E Tretment E Significnt P vlues re given in bold. Tretment corresponds to the combintion between pruning intensity nd epicormic brnches tretment (1=1/3 pruned tree without epicormic shoots, 2=2/3 pruned tree without epicormic shoots, E1=1/3 pruned tree with epicormic shoots, E2=2/3 pruned tree with epicormic shoots). DF degrees of freedom

9 Epicormic shoots in hybrid poplr 433 TNC (% dry mss) 2 1 b entering dormncy erly in the growing seson (Wignll et l. 1987), in this cse due to the physiology of the bud itself, i.e., endodormncy (Lng 1987). When the epicormic brnches produced in 2 were left to grow on trees until the end of 211, i.e., for two growing sesons (211_old), the effect of pruning seson ws no longer significnt nd trees hd similr numbers of epicormic shoots (Fig. 2b). This suggests tht summerpruned trees produced new epicormic shoots in 211 or tht some epicormics from fll- nd spring-pruned trees died during the second growing seson (Tkiy et l. 2; WringndO Hr 2). However, biomss of these epicormic brnches shows tht even though numbers of 2-yer-old brnches (211_old) were similr for ll pruning sesons (Fig. 2b), their biomss ws much smller in summer-pruned trees (Fig. 3). This underlines the importnce of removing epicormic brnches produced on fll- or spring-pruned trees, to void very lrge knots on boles. Epicormic brnches produced in 211 (211_new) were lso in comprble numbers thn 2- yer-old brnches (Fig. 2b) but hd much less biomss (Fig. 3), indicting tht trees produced much smller epicormic brnches during the second growing seson fter pruning. In our study, growth rte did not ffect the number of epicormic shoots produced fter pruning, since clone DN2 (P. deltoides P. nigr), which hd the highest growth rtes, produced similr numbers of epicormic shoots thn clone 7472 (P. blsmifer P. trichocrp), which hd the lowest growth rte. Others hd found tht smllsized or slower growing trees produced more epicormic shoots thn lrge dominnt trees (Nicolini et l. 23; O Hr nd Vlppil 2). In this cse, the effect of tree size nd vigor ws probbly relted to the comprtive strength of crbohydrte sinks between sequentil nd b 1/3 1/3 epi 2/3 2/3 epi Pruning intensty with or without epicormic shoots Fig. Predictions for men totl non-structurl root crbohydrtes for unpruned trees (), 1/3 pruned trees without epicormic shoots (1/3), 1/3 pruned trees with epicormic shoots (1/3 epi), 2/3 pruned trees without epicormic shoots (2/3), nd 2/3 pruned trees with epicormic shoots (2/3 epi), ll clones nd pruning sesons combined. Errors brs re stndrd errors of the men. Brs lbeled with the sme letter re not significntly different (Tukey s testtp<.) epicormic buds. Lrge sequentil buds of expnding shoots in the upper cnopy re strong sinks compred smll epicormic buds locted lower on the stem, while this difference in sink strength is much reduced in suppressed trees, which could led to greter epicormic shoot development (Meier et l. 212). In our study, however, trees were of the sme ge nd of reltively similr sizes nd socil position in the cnopy due to their clonl nture, which could explin why growth rtes were unrelted to epicormic shoot production. 4.1 Silviculturl implictions Bsed on this experiment, we recommend pruning 1/3 crown length in summer to reduce the number nd biomss of epicormic shoots produced from the stress of pruning. However, if pruning needs to be done during the dormnt seson, second pruning to remove epicormic shoots should be plnned fter the next growing seson to prevent them reching very lrge sizes (biomsses). Epicormic shoots did not ffect tree dimeter growth 1 or 2 yers fter tretment. Hence, we conclude tht epicormic brnches cn be removed by plnttion mngers without compromising tree growth, if greter qulity bole is desired. Conclusion In summry, pruning hybrid poplrs t ny intensity or seson resulted in the production of epicormic shoots. Pruning intensity hd strong effect on the number nd biomss of epicormic shoots produced fter one nd two growing sesons, pproximtely doubling between pruning 1/3 nd 2/3 crown length. Pruning trees in summer produced the lest number of epicormic shoots fter one nd two growing sesons. Hormonl control of prdormncy of epicormic buds from young expnding cnopy shoots nd from self-regulted endodormncy of buds during the summer seson most probbly explins this result. An dditionl pruning during the first growing seson (removl of the epicormic brnches) significntly reduced the size of new epicormic shoots produced the next yer for ll pruning tretments. Epicormic shoot production did not restore root crbohydrte reserves one growing seson following pruning nor did it ffect tree growth 1 or 2 yers fter tretment. Hence, we conclude tht epicormic brnches cn be removed by plnttion mngers without compromising tree growth, if greter qulity bole is desired, becuse they reched very lrge size during the second growing seson.

10 434 A. Desrochers et l. Acknowledgments We thnk the Sustinble Center for Forest Reserch, Quebec Ministry of Nturl Resources, Réseu Ligniculture Québec, University of Guelph New Liskerd Agriculturl Reserch Sttion, Northern College, Line Blckburn, nd other technicins for field nd lb work. Funding This work ws supported by the Cndin Ecology Centre Forestry Reserch Prtnership [FRP Project no ], Tembec, the government of Ontrio, Nturl Resources Cnd Cndin Forest Service, nd by Albert-Pcific Forest Industries, Norbord, nd the Nturl Sciences nd Engineering Reserch Council of Cnd through collbortive reserch nd development grnt [CRDPJ to A.D.]. References Aloni R, Schwlm K, Lnghns M, Ullrich C (23) Grdul shifts in sites of free-uxin production during lef-primordium development nd their role in vsculr differentition nd lef morphogenesis in Arbidopsis. Plnt 216: Auchmoody LR (1972) Epicormic brnching: sesonl chnge, influence of fertiliztion, nd frequency of occurrence in uncut stnds. 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