effect of repeated harvesting on biomass production and sprouting of betula pubescens

Biomass and Bioenergy 20 (2001) 237–245

E�ect of repeated harvesting on biomass productionand sprouting of Betula pubescens

Jyrki Hyt$onena ; ∗, Jorma Issakainenb

aFinnish Forest Research Institute, Kannus Research Station, P.O. Box 44, FIN-69101 Kannus, FinlandbFinnish Forest Research Institute, Muhos Research Station, Kirkkosaarentie, FIN-91500 Muhos, Finland

Received 31 May 2000; received in revised form 27 November 2000; accepted 29 November 2000

Abstract

The e�ect of repeated coppicing and rotation length on the biomass production and sprouting of downy birch (Betulapubescens Ehrh.) was studied in two-1eld experiments established in central and northern Finland. The harvesting cyclesstudied were 1; 2; 4 and 8 years in central Finland and 1; 2; 4; 8; 12 and 16 years in north Finland. The randomized blockdesign with four replications was used. Biomass was harvested, weighed and subsampled for determining the moisturecontent. Consecutive 1- and 2-year rotations led to a decrease in the dominant height of birches. Only 13% of the originalbirches were able to sprout after four 1-year rotations and 22% after two 2-year rotations. Some birches were, however, ableto sprout for even more than 10 years. Annual harvests led to a rapid decrease in biomass production. Already the second1-year-rotation yielded three times less biomass than the 1rst and subsequent production decreased further. 4-year rotations,even though yielding much less biomass than longer rotations, did not lead to a decrease of the coppicing capacity or biomassproduction during consecutive rotations. The mean annual dry-mass production was very low when 1- or 2-year rotationswere applied. 8-year rotations in central Finland yielded 7–8 times more and in northern Finland 9–13 times more than1- and 2-year rotations. The longer the rotation, the higher the mean annual biomass production. The highest mean annualincrement (MAI) was achieved with the longest (i.e. 16-year) rotation. Compared with the 1-year rotation, the MAI for the16-year rotation was 20 times higher. The results show that downy birch is not suitable for biomass production when usingshort rotations. Three annual or biannual cuttings reduce the sprouting capacity and growth of birch considerably and helpto control coppicing in softwood plantations. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Betula; Coppicing; Cutting cycle; Rotation length

1. Introduction

Birches are the single most important deciduous treespecies in most of the Nordic countries. In Finland,birches (silver birch Betula pendula Roth. and downy

∗ Corresponding author. Tel.: +358-6-873211; fax: +358-6-873-201.

E-mail address: [email protected] (J. Hyt$onen).

birch B. pubescens Ehrh.) are also the most commondeciduous tree species. There are almost 1.2 millionha of forest with a predominance of downy birch andover 0.2 million ha with a predominance of silver birch[1–3]. Over half of these forests are on peatlands.Downy birch is a biologically vigorous pioneer speciesthriving on its main growing sites, i.e. moderately fer-tile and fertile drained peatlands. Downy birch forestsare characterized by the small size of the trees. The

0961-9534/01/$ - see front matter c© 2001 Elsevier Science Ltd. All rights reserved.PII: S0961 -9534(00)00083 -0

238 J. Hyt,onen, J. Issakainen / Biomass and Bioenergy 20 (2001) 237–245

suitability of downy birch stands for plywood produc-tion when growing on its main sites is generally low[4].Birch forests are easily reproduced naturally with

the aid of seedlings or via stump sprouts. When cut,birches produce stump sprouts from dormant basalbuds [5]. Most of the buds (70–95%) are located un-derground [6–8]. The origin, structure, development,spatial distribution, number and bursting dynamics areimportant aspects a�ecting coppicing [9]. Many ex-ternal factors have been shown to have only a minore�ect on the number of sprouts. However, the cuttingseason, the size and age of the trees, stump height, andsite quality can signi1cantly a�ect the early develop-ment of birch sprouts [10–15].Existing thickets of birch, especially on peatlands,

could also be grown by applying the principles ofshort-rotation management. In the late 1970s andearly 1980s, experiments were launched to study thesuitability of naturally regenerated birch thickets forbiomass energy production using coppice regenera-tion [13,16]. Birch coppices also o�er the attractiveoption of thinning the stand and growing it to producepulpwood. Planting and regeneration costs could beavoided as compared to willow plantations whoseestablishment costs are quite high [17]. The earlydevelopment of birch sprouts is more vigorous thanthat of seedlings [18]. At the age of 3 years, birchesof sprout origin can be 1–1.5 m taller than birches ofseedling origin of the same age, but the height growthof the latter soon reaches that of coppice stands [18].Already at the age of 15–20 years birches of sproutorigin can fall behind trees of seed origin [13]. Birchpossesses many characteristics that make it highlysuited for short rotation cultivation. The mean annualleaIess biomass production of young, naturally regen-erated birch thickets has been 2–5 ton=ha [13,16,19].In conventional silviculture, vigorously sprouting

downy birch is often considered in a negative light,especially when regenerating intermixed with soft-woods. The competition o�ered by fast-growing de-ciduous trees of sprout origin often impairs the out-come of forest regeneration. The actual 1nal tree cropruns the risk of su�ering from the shading and me-chanical injury inIicted by birch to such an extent thatthe sapling stand is ultimately lost and deciduous treestake over the site. Since control of sprouts accountsfor considerable amount of the costs of young stand

management its optimization is an important part offorest regeneration.The optimum cutting cycle and e�ect of suc-

cessive harvests on stump mortality and biomassproduction in coppice management are quite com-plicated but important aspects. Resprouting afterharvest and maintaining productivity over multiplecutting cycles is thus fundamental to short-rotationcoppice forestry. There might be problems in theshoot-to-stump-to-stool-to-root system when severalrotations are used in coppicing and sometimes evenearlier [20]. The silviculturalist may, on the otherhand, be interested in knowing whether or not succes-sive coppicing cycles decrease the coppicing abilityof undesirable hardwood species.The objective of the present study was to investigate

the e�ects of repeated coppicing on the sprouting andbiomass production of downy birch.

2. Material and methods

Two-1eld experiments were established in 1980 inthickets dominated by downy birch and growing ondrained peatlands (Fig. 1). A 1eld experiment wasestablished in Kannus, Antinoja (63◦54′N, 24◦12′E,75m asl, mean temperature sum (¿ + 5◦C) during1980–1987 1051◦C) in a 20-year-old birch thicket(stand density 30,000–36,000 stems=ha) growing on a40–50 cm thick peat substrate. The stand was clear-cutin the autumn of 1980 leaving 10 cm long stumps. Atotal of 16 plots (each 50 m2 in size) were laid outthe following spring. The treatments consisting of 1-,2-, 4- and 8-year-long rotations were replicated fourtimes in a randomized block design. The 1-year rota-tion treatment was fenced to prevent moose and haredamage. Since no browsing damage occurred in theother treatments fencing of only 1-year rotation treat-ment did not cause bias in the results. The sproutswere assessed (species, height, vitality: 1 = Living,2=Dead) annually on all the subsample plots (10 m2)in the treatment plots. Dominant height was calcu-lated as the mean height of 1ve tallest birches on theplots.Another 1eld experiment was established in

Rovaniemi mlk., Kivalo (66◦27′N, 26◦40E, 180 m asl,mean temperate sum (¿ + 5◦C) during 1980–1997881◦C), in a 20-year-old thicket dominated by downy

J. Hyt,onen, J. Issakainen / Biomass and Bioenergy 20 (2001) 237–245 239

Fig. 1. Location of the study areas.

birch 3–4m in height (over 30,000 birches=ha). Thisstand, growing on a substrate of peat some 10–40 cm thick, was clear-cut in October 1980. A total of28 plots (each sized 310–340m2) were laid out thefollowing spring. The treatments consisting of 1-, 2-,4-, 8-, 12- and 16-year-long rotations were replicatedfour times in a randomized block design. The studyarea was enclosed with a 2m high fence to protectthe sprouts from browsing by animals. The sproutswere not measured in this experiment.The biomass on the plots was measured using the

harvesting method, where all materials within a givenunit area is harvested and subsequently weighed [21].Subsample plots were used in Kannus for the biomassmeasurements. In the case of the Rovaniemi experi-ment, the sprouts were harvested during 1982–1988from square-shaped subsample plots sized 100 m2.Due to the high amount of harvested material foursmaller circular subsample plots (total size 36–50m2

were used in 1989–1997. The trees were cut to ap-proximately 10 cm long stumps using secateurs, brushsaws or chainsaws (older birches) and the woody leaf-less biomass weighed. Harvesting was done during thedormant season (late autumn or early spring) so asnot to have a negative e�ect on the coppicing capac-ity [15]. Harvesting on the Rovaniemi experiment wasdone during the 1rst days of June before the leaveshad burst. However, in 1982 and in 1990 harvestingwas done just before mid June and in 1993 on the 8thof June when the leaves were already half of their 1-nal size and were included in the weighed biomass.One sample plot (treatment 4-year rotation) had to beomitted in the last measurement (spring 1998) becausethe fence had broken and birches on this plot had beenbrowsed to a considerable extent. For moisture con-tent determination a subsample from the biomass ineach harvested plot was taken. The fresh mass of thesesamples was determined and the samples were driedat 105◦C for 2–4 days and weighed to obtain the drymass.The sprouts in treatments 2-, 4- and 8-year rota-

tions in Rovaniemi were harvested 1 year older thanplanned in 1989. In the calculation of the results thishas been corrected by deducting the mean incrementof 1 year from the measured biomass. Two-way anal-yses of variance was used in the statistical analyses ofthe results. The homogeneity of variances was testedwith Levene’s test. Transformations to stabilize vari-ances were needed when testing the e�ects of rotationlength on biomass production.

3. Results

3.1. Height

The mean height in Kannus experiment was muchless than dominant height since most of the sprouts,especially in the 1-year rotation treatment, were verysmall. The dominant height of the sprouts in the secondrotation was much lower than in the 1rst, but decreasedin later rotations less (Fig. 2). The dominant heightof birch decreased only in the third and fourth 2-yearlong rotation. The two consecutive 4-year rotationsproduced sprouts of the same height. In all of the casesthe 1rst year’s growth after coppicing was the highestand growth declined thereafter.


Fig. 2. The dominant height of birch sprouts in di�erent rotation treatments in the Kannus experiment. Annual height growth indicatedwhen the rotations applied were longer than 1 year. The means that do not di�er from each other at the 0.05 signi1cance level by Tukey’stest are marked with the same letters.

3.2. Sprouting

The number of sprouting stumps in Kannus exper-iment declined already after the 1rst cut and 34% ofthe original trees forming the mother stand failed tosprout (Fig. 3). The number of sprouting stumps de-clined in the 1-year rotation every year. After the sec-ond 1-year rotation 57% of the original stumps failedto sprout. The number of non-sprouting stumps rose to73% in the third and to 87% after the fourth 1-year ro-tation. Thereafter, the decline in the number of sprout-ing stumps continued. However, a few stumps weresprouting even after eight 1-year rotations. A simi-lar decline was noticed also when using 2-year longrotations. The number of sprouting stumps declinedafter each harvest. However, the two 4-year harvestsdid not signi1cantly reduce the number of sproutingstumps, even though the number was smaller than inthe original stand. According to observations somebirch stumps in Rovaniemi were still able to sprouteven after ten consecutive 1-year rotations.

3.3. Biomass production

The length of the rotation had a signi1cant ef-fect on the mean annual increment (MAI) of birchstands both in Kannus (F = 86:72, p= 0:000) and inRovaniemi (F = 58:70, p = 0:000). The longer therotation, the higher the MAI (Fig. 4). 1- and 2-yearrotations yielded very little biomass during the studyperiod and there was little di�erence between the twoexperimental areas in this respect. Increasing rota-tion length to 4 years greatly increased the biomassproduction compared with 1- or 2-year rotations. Asimilar increase was obtained by further increasingrotation length to 8 years. In Kannus, the 1-year ro-tations yielded 7.8 and 2-year rotations 7.0 times lessbiomass than the 8-year rotations. The corresponding1gures in Rovaniemi were 13.3 and 9.5. MAI wasshown to increase with increasing rotation length upto 16 years. However, even though the 16-year-longrotation yielded more than the 12-year-long rotation,these treatments did not di�er signi1cantly from each


Fig. 3. The number of birch stumps with live sprouts in the Kannus experiment. M= original stand before the 1rst clear-cut. The meansthat do not di�er from each other at the 0.05 signi1cance level by Tukey’s test are marked with the same letters.

Fig. 4. Mean annual dry-mass increment of downy birch stands in Kannus (A) and in Rovaniemi (B). The number of rotations is indicatedinside the bars. The means that do not di�er from each other at the 0.05 signi1cance level by Tukey’s test are marked with the same letters.

other. TheMAI for the 16-year rotation was 19.9 timeshigher than the MAI for the 1-year-long rotation.One-year rotations caused the biomass production

of birch stands to decrease dramatically already in thesecond harvest cycle (Fig. 5). In Kannus the second1-year rotation yielded 3.0 times and in Rovaniemi3.1 times less than the 1rst 1-year rotation. The yielddecreased further during the following years. The pro-

portion of willows in the plots increased. The propor-tions of willow and birch biomass were measured sep-arately in the 1-year rotation treatment at Rovaniemiin 1991. Then willow biomass amounted to 38.5% ofthe total biomass.Also consecutive 2-year rotations led to a decrease

in production. In Kannus this decline continued overfour rotations. In Rovaniemi the production remained


Fig. 5. The dry-mass production of birch stands cut consecutively while applying di�erent rotation lengths in Kannus and Rovaniemi. Themeans that do not di�er from each other at the 0.05 signi1cance level by Tukey’s test are marked with the same letters.

low but increased after the second rotation. Thiswas probably caused by the increase of willow inthe stands. Consecutive 4-year rotations led only toa small decrease in biomass production in Kannusand to an increase in Rovaniemi. These di�erenceswere not statistically signi1cant. In Rovaniemi itwas possible to compare two 8-year-long rotations.The second yielded somewhat more than the 1rst,but the di�erence was not statistically signi1cant(F = 2:91; p= 0:101).

4. Discussion

Quite high mortality (34%) followed the cuttingof the original stand. After clear cutting, the num-ber of non-sprouting birch stumps has generally beenin the range of 10–40% [12,14,18,15]. The percent-age mortality increased with each successive coppicecutting. With 1-year rotation cycles, four consecutivecuttings led to 87% of the original stumps not pro-ducing sprouts. In Rovaniemi some birch stumps were


able to produce sprouts even after ten annual consecu-tive cuttings. The 8-year rotation cycle did not reducethe number of sprouting stumps. Also, with increas-ing rotation length competition-induced mortality canincrease. The results showed that coppicing apply-ing short intervals of 1–2 years increases considerablythe mortality of downy birch stumps and reduces thegrowth of the sprouts. Short rotations have been ob-served to reduce the sprouting capacity and growth ofseveral other species as well [23–26]. Aspen regener-ation ceased after seven repeated cuts in 1-year rota-tion [26]. Poplars have been observed to survive quitewell even after four 2-year rotations [23], but shortcutting cycles can be detrimental for hybrid poplars aswell [25]. Many willow species are able to withstandseveral consecutive harvests without decrease of theirsprouting capacity [27–29].Second rotation mean annual biomass production in

willow and poplar short-rotation plantations has gen-erally been considerably higher than those obtainedfrom the 1rst harvest [25,28–36]. With downy birch,the yield of successive harvests depended on the ro-tation length. Short, 1- or 2-year rotations lead to arapid decrease in biomass production. In willow plan-tations increased production has been noticed duringat least three consecutive 1-year harvests [28]. Theyield of downy birch did not decrease when subjectedto 4- or 8-year-rotations. In one experiment, the sec-ond 4-year rotation led to a small but not signi1cantdecrease in yield but in the other experiment yield in-creased (but not signi1cantly) during four consecu-tive 4-year rotations. The second 8-year long rotationyielded more than the 1rst. Aspen (Populus tremu-loidesMichx.) has been shown to yield less even whenusing 4- or 8-year rotations [26]. Second and thirdrotations’ birch biomass production may be higherthan that obtained from the 1rst rotation if the rota-tion length is long enough. This could be promoted bythe fact that when birch stump sprouts new dormantbasal buds are formed at the base of the new sprouts.Downy birches 20 years old which originated fromsprouts have been shown to have over 50% more budsin their stumps than trees of seed origin [7]. How-ever, on the basis of these and earlier results [19],it seems that the second birch coppice rotation doesnot yield signi1cantly more biomass than the 1rst atleast when rotation lengths less than 10–15 years areused.

The yield of downy birch decreased as coppicingfrequency increased when using 1- or 2-year rotations.The yield of the 1-year rotation decreased after thesecond rotation to one-third and thereafter only littleto the third or fourth harvest. Also 2-year-rotationsled to a decrease of yield during consecutive rotations,but less sharply. Birch was partially replaced by wil-lows and aspens which are able to tolerate annual har-vests quite well [26–29]. The yield and regenerationof aspen (Populus tremuloides Michx.) on 1-year ro-tations decreased much less after two or three annualcuts than the yield of downy birch in this study [26].Even though carbohydrate levels in roots under

most conditions do not correlate with the numberof sprouts [14,37–40] they can be a signi1cant con-tributor to sprout weight [38]. Successive coppicesat short rotations, decreasing yield and increasingstump mortality seem to be strongly associated withthe depletion of root carbohydrates [9]. During theearly summer root carbohydrate levels are relativelylow [38,41]. In Rovaniemi cutting was done during3 years with the leaves having already reached halfof their full size. This could have had a minor e�ecton the growth of these sprouts [10–12,14,15,22,42]even though root starch concentrations in fall havebeen highest following cuts at the very beginning ofthe growing season [41].The optimum rotation period or harvesting cycle for

biomass production when applying coppice regenera-tion is species speci1c and a�ected by stand density.For dense plantations of bush-like willows the sug-gested coppicing cycles are between 3 and 6 years[28,29,32], for alder and hybrid poplar between 5 and15 years [23,25], and for aspen preferably more than15 years [26]. Downy birch is not able to withstand ro-tations of 1 or 2 years. After two or three consecutiverotations yields greatly decreased and regeneration ca-pacity was very low. However, birches were able towithstand even four 4-year-long rotations without de-creases in yield. Thus, when the aim is to decrease thegrowth of undesirable birch in softwood plantations,cutting the birch sprouts back several times applying1 or 2-year intervals would decrease their growth andsurvival very much. Grey birch (Betula populifolia)has responded to even shorter cutting cycles, multiplecuttings during growing season, with very high mor-tality [41]. When growing downy birch for biomassand energy, it seems that birch is able to withstand


several rotations of 4 or 8 years. However, in bothexperiments, mean annual increment increased withrotation length. In both experiments the longest rota-tions (8 and 16 years) resulted in the highest MAI.Thus, even though birch can withstand coppicing in4-year rotations, it is not suited to very short rotationsbecause it does not then yield enough woody biomass.These results con1rm earlier conclusions that the op-timum rotation length for downy birch biomass pro-duction should be 16 years or even longer [13,19,43].

Acknowledgements

The Kannus experiment was established and de-signed by the late Dr. Ari Ferm. The authors wishto thank the personnel of Kivalo Research Area inRovaniemi for their 17 years of 1eld work. At Kan-nus Research Station, the measurement work mainlydone by Olavi Kohal and Jaakko Miettinen is grate-fully acknowledged. Keijo Polet helped to 1nalise thegraphs. Dr. Anneli Kauppi is acknowledged for herconstructive comments on the manuscript.

References

[1] Kuusela K, Salminen S. Ahvenanmaan maakunnan jamaan yhdeks$an etel$aisimm$an piirimets$akautakunnan alueenmets$avarat 1977–1979. (Summary: Forest resources in theprovince of Ahvenanmaa and the nine southernmost ForestryBoard Districts in Finland 1977–1979.) Folia Forestalia1980;446:90.

[2] Kuusela K, Salminen S. Mets$avarat Etel$a-Suomen kuudenpohjoisimman piirimets$alautakunnan alueella 1979–1982sek$a koko Etel-Suomessa 1977–1982. (Summary: Forestresources in the six northernmost Forestry Board Districtsof South Finland, 1979–1982, and in the whole of southFinland, 1977–1982.) Folia Forestalia 1983;568:79.

[3] Kuusela K, Mattila E, Salminen S. Mets$avarat piirimet-s$alautakunnittain Pohjois-Suomessa 1982–1984. (Forestresources in North Finland by Forestry Board Districts 1982to 1984.) Folia Forestalia 1986;655:86.

[4] Verkasalo E. Hieskoivun laatu vaneripuuna. Mets$antutki-muslaitoksen tiedonantoja 1997;632:481 [in Finnish].

[5] Kauppi A, Paukkonen K, Rinne P. Sprouting ability of aerialand underground dormant basal buds on Betula pendula.Canadian Journal of Forest Research 1991;21:528–33.

[6] Kauppi A, Rinne P, Ferm A. Initiation, structure andsprouting of dormant basal buds in Betula pubescens. Flora1987;179:55–83.

[7] Kauppi A, Rinne P, Ferm A. Sprouting ability and signi1cancefor coppicing of dormant buds on Betula pubescens. Ehrh.

stumps. Scandinavian Journal of Forest Research 1988;3:343–54.

[8] Johansson T. Dormant buds on Betula pubescens andBetula pendula stumps under di�erent 1eld conditions. ForestEcology and Management 1992;47:245–59.

[9] Sennerby-Forsse L, Ferm A, Kauppi A. Coppicing ability andsustainability. In: Mitchell CP, Hinkley T, Sennerby-Forsse,editors. Ecophysiology of short rotation crops. Amsterdam:Elsevier, 1992. p. 146–84.

[10] Mikola P. Koivun vesomisesta ja sen mets$anhoidollisestamerkityksest$a. Referat: $Uber die Ausschlagsbildung bei derBirke und ihre forstliche Bedeutung. Acta Forestalia Fennica1942;50(3):102.

[11] EtholQen K. Kaatoajankohdan vaikutus koivun jahaavan vesomiseen taimiston hoitoaloilla Pohjois–Suomessa.(Summary: The e�ect of felling time on the sprouting ofBetula pubescens and Populus tremula in the seedling standsin the northern Finland.) Folia Forerstalia 1974;213:16.

[12] Ferm A, Issakainen J. Kaatoajankohdan ja kaatotavanvaikutus hieskoivun vesomiseen turvemaalla. Mets$antutki-muslaitoksen tiedonantoja 1981;33:13: [in Finnish].

[13] Ferm A. Coppicing, aboveground woody biomass productionand nutritional aspects of birch with speci1c referenceto Betula pubescens. Mets$antutkimuslaitoksen tiedonantoja1990;348:35.

[14] Johansson T. Sprouting of 2- to 5-year-old birches (Betulapubescens Ehrh. and Betula pendula Roth) in relationto stump height and felling time. Forest Ecology andManagement 1992;53:263–81.

[15] Hyt$onen J. E�ect of cutting season, stump height and harvestdamage on coppicing and biomass production of willow andbirch. Biomass and Bioenergy 1994;6(5):349–57.

[16] Ferm A. Birch production and utilization for energy. Biomassand Bioenergy 1993;4(6):391–404.

[17] Toivonen RM, Tahvanainen LJ. Pro1tability of willowcultivation for energy production in Finland. Biomass andBioenergy 1998;15(1):27–37.

[18] Kauppi A, Kiviniitty M, Ferm A. Growth habits and crownarchitecture of seed and sprout origin Betula pubescens Ehrn.Canadian Journal of Forest Research 1998;18:1603–13.

[19] Hyt$onen J, Kaunisto S. E�ect of fertilization on the biomassproduction of coppiced mixed birch and willow stands on acut-away peatland. Biomass and Bioenergy 1999;17:455–69.

[20] Ferm A, Kauppi A. Coppicing as means for increasinghardwood biomass production. Biomasss 1990;22:107–21.

[21] Hyt$onen J, Lumme I, T$orm$al$a T. Comparison of methodsfor estimating willow biomass. Biomass 1987;14:39–49.

[22] Schierbeck O, Clarke WBM. Sprout control in wire (grey)birch. Silvicultural Research Note 63. Dominion ForestService. 1940. p. 12.

[23] Harrington CA, DeBell DS. E�ects of irrigation, pulp millsludge, and repeated coppicing on growth and yield ofblack cottonwood and red alder. Canadian Journal of ForestResearch 1984;14:844–9.

[24] Steinbeck K, Brown CL. Yield and utilization of hardwood1ber grown on short rotations. Applied Polymer Symposium,Vol. 28. New York: Wiley, 1976. p. 393–401.


[25] Strong T. Rotation length and repeated harvesting inIuencePopulus coppice production. Forest Service North CentralExperimental Station. Research Note NC-350, 1989. 1–4.

[26] Perala DA. Regeneration and productivity of aspen grownon repeated short rotations. USDA Forest Service. ResearchPaper NC-176. 1979. p. 7.

[27] Paukkonen K, Kauppi A, Ferm A. Origin, structure andshoot-formation ability of buds in cutting-origin stools ofSalix ‘Aquatica’. Flora 1992;186:53–65.

[28] Willebrand E, Ledin S, Verwijst T. Willow coppice systems inshort rotation forestry: E�ects of plant spacing, rotation lengthand clonal composition on biomass production. Biomass andBioenergy 1993;5(5):323–31.

[29] Hyt$onen J. Biomass production and nutrition of short-rotationplantations. The Finnish Forest Research Institute, ResearchPapers 1996;586:1–61.

[30] Heilman P, Peabody DV, DeBell DS, Strand RF. A testof close-spaced, short-rotation culture of black cottonwood.Canadian Journal of Forest Research 1972;2:456–9.

[31] Cannell MGR. Productivity of closely-spaced young poplaron agricultural soils in Britain. Forestry 1980;53:1–21.

[32] McElroy GH, Dawson WM. Biomass from short-rotationcoppice willow on marginal land. Biomass 1986;10:225–40.

[33] Bowersox TW, Blankenhorn PR, Strauss CH. Secondrotation growth and yield of a Populus hybrid.Mets$antutkimuslaitoksen tiedonantoja 1988;304:66–73.

[34] Wright LL. Are increased yields in coppice systems a myth?.Mets$antutkimuslaitoksen tiedonantoja 1988;304:51–65.

[35] Heilman PE, Stettler RF. Genetic variation and productivityof Populus tirchocarpa and its hybrids. IV. Performance in

short-rotation coppice. Canadian Journal of Forest Research1990;20:1257–64.

[36] Hyt$onen J. Ten-year biomass production and stand structureof Salix ‘Aqautica’ energy forest plantation in southernFinland. Biomass and Bioenergy 1995;8(2):63–71.

[37] Wenger KF. The sprouting of sweetgum in relation toseason of cutting and carbohydrate content. Plant Physiology1953;28:35–49.

[38] Schier GA, Zasada JC. Role of carbohydrate reserves in thedevelopment of root suckers in Populus tremuloides. CanadianJournal of Forest Research 1973;3:243–50.

[39] Taylor JS, Blake TJ, Pharis RP. The role of plant hormonesand carbohydrates in the growth and survival of coppicedEucalyptus seedlings. Physiologia Plantarum 1982;55:421–30.

[40] Blake TJ. Coppice systems for short-rotation intensiveforestry: the inIuence of cultural, seasonal and plant factors.Australian Forest Research 1983;13:279–91.

[41] Kays JS, Canham CD. E�ects of time and frequency ofcutting on hardwood root reserves and sprout growth. ForestScience 1991;37(2):524–39.

[42] Johansson T. Sprouting of 10- to 50-year-old Betulapubescens in relation to felling time. Forest Ecology andManagement 1992;53:283–96.

[43] Ferm A, Kaunisto S. Luontaisesti syntyneidenkoivumetsik$oiden maanp$a$allinen lehdet$on biomassatuotosentisell$a turpeennostoalueella Kihni$on Aitonevalla. Summary:Above-ground leaIess biomass production of naturallygenerated birch stands in a peat cut-away area at Aitoneva,Kihni$o. Folia Forestalia 1983;558:32.

effect of repeated harvesting on biomass production and sprouting of betula pubescens

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