Populus La Poplar Hybrids Sal icaceae Willow family Maurice E. Demeritt, Jr.
Poplar hybrids (Populus spp.) are the result of Soils and Topography natural and manmade crosses among poplar species. The genus is further divided into five sections of Poplar hybrids grow best where soils are at least which four are represented in North America: Deuce 1 m (3.3 ft) in depth to interrupted bedrock. The (aspen type), to which P grandidentata and P. water table and porous or gravel layers should also tremuloides (bigtooth aspen and quaking aspen) be at least 1 m (3.3 ft) below the soil surface. Op- belong; Aigeiros (cottonwood or poplar type), to which timum pH ranges from 6.0 to 7.0, though some P deltoides, I! sargentii, I? fremontii, and P wislizeni hybrids tolerate high or low pH conditions. Hybrids (eastern, plains, Fremont, and Rio Grande cotton- grow well on upland and bottom-land soils if the soils wood) belong; Tacamahaca (balsam poplar type), to have good moisture-holding capacities and are of which P. balsamifera, P. trichocarpa, and I? angus- medium texture. Hybrids show extreme variation in tifolia (balsam poplar, black, and narrowleaf cotton- tolerance of adverse site conditions. They grow best wood) belong, and Leucoides (swamp poplar type>, to on soils of the orders Entisols, Inceptisols, Mollisols, which I? heterophylla belongs. l? balsamifera subsp. Spodosols, and Ultisols. trichocarpa has been reported as the correct status of l? trichocarpa (44). Associated Forest Cover
The two natural hybrids, P x smithii and P. x Habitat jachii, are associated with the parental species in the same stand, and the parental species dominate in the Native Range stands. The majority of hybrid poplars are planted in Poplar hybrids occur naturally throughout the U.S. pure stands and all competing vegetation is control- and Canada wherever compatible species come into led the first few years after planting. Poplars are close proximity (table 1). Most poplar hybrids, how- very intolerant of shade and herbicides and also ever, result from artificial hybridization and sub- when young cannot tolerate competition from grass, sequent planting. The first large-scale hybridization weeds, and shrubs in their immediate area. project with poplars in the United States was begun in 1925 (41,42). An unknown number of hybrids also Life History form between native species and introduced clones, cultivars, and species. The Jackii poplar is a natural Reproduction and Early Growth hybrid between P balsamifera females and P del- toides males. Populus x Smithii is a natural hybrid Flowering and Fruiting-Poplar hybrids are between P. tremuloides and P. grandidentata. dioecious and first flower at about 8 years of age. The Hybridization between P. balsamifera and l? flowers are borne in catkins (or aments) in the spring trichocarpa occurs in the interior of southeastern before leafing. Male and female catkins, when fully Alaska and in the Cook Inlet region. Also, the developed, are 10 to 15 cm (3.9 to 5.9 in) long. In the trihybrid among l? deltoides, I? balsamifera, and P. female flower, the number of stigmas varies from two angustifolia has been reported in southern Alberta. to four and are either cap- or y-shaped. In the male flower, the number of stamens varies from 30 to 80. Climate The bract has 20 to 30 digits, depending on the cul- tivar. The central areas of the bracts are light in color In general, poplar hybrids grow best on humid and and the digits are dark brown, sometimes tinged microthermal areas with adequate moisture during with black (34). Many poplar hybrids have never all seasons of the year. They are rarely found on sites produced flowers and thus are thought to be sterile. that have temperatures of 46” C (-50” F) or on sites Poplars flower between February and May and dis- that have summer temperatures over 38” C (100’ F) perse seed between April and June of the same year. for more than a week. Intersection Leuce hybrids disperse seed a few weeks before intrasection Aigeiros and Tacamahaca or in- tersection Aigeiros-Tacamahaca hybrids. Fruits are The author is Research Geneticist, Northeastern Forest Experi- one-celled capsules borne in long pendulous clusters ment Station, Radnor, PA. (catkins), and each capsule is surrounded by tufts of
577 Table l-Naturally occuring hybrids among native populus species (5,9,35)
Parentage Hybrid designation Common name I? alba x F? grandidentata I? x roulwauiana Boivin I? alba x I? tremula f? x canescens Sm. I? alba x I? tremuloides I? x heimburgeri Boivin F? angustifolia x /? deltoides f? x acuminara Rydb. Lanceleaf (f! x andrewsii Sarg.) Cottonwood P angustifolia x f? tremuloides F! x sennii Boivin I? balsamifera x f? deltoides P. x jackii Sarg. Jackii poplars I? balsamifera x I? tremuloides P x duti//$ Lepage I? deltoides x P nigra F! x euramericana (Dode) Euramerican Guinier (P x canadensis Moench) poplars I? deltoides x P tremuloides /? x bernardii Boivin Bernard poplars P deltoides x f! trichocarpa I? x generosa Henry Interamerican (and reciprocal) (i? x interamericana Brockh.) poplars P. fremontii x F! trichocarpa P. x parryi Sarg. Parry cottonwood I? grandidentata x I? tremuloides P x smithii Boivin I? laurifolia x P nigra I? x berolinensis Dippel Berlin or (P rasumowskyana Schr. Russian poplars and P x petrowskyana Schr.) I? deltoides x I? balsamifera x I? angustifolia Unnamed Unnamed (natural trihybrid)
long, white, silky hairs attached to the short stalks Vegetative Reproduction-Poplar hybrids of the seeds, promoting wind dispersion over great reproduce vegetatively by natural and artificial distances (36). means. Leuce-type hybrids root best from root sec- tions, though some selections root adequately from Seed Production and Dissemination-Poplar dormant stem cuttings. Aigeiros-type hybrids hybrids generally are prolific annual seed producers. reproduce well from either greenwood or dormant Individual trees of Populus tremula, an exotic, may stem cuttings, although most hybrids are reproduced yield from 8,000 to 54,000,OOO seeds per year; hybrid from dormant cuttings. Dormant cuttings are poplar seed yields are thought to be the same. In produced predominately from hybrid stool beds es- Populus deltoides var. virginiana 35 liters (1 bushel) tablished for that purpose. The cuttings are usually of fresh fruit yields about 0.9 kg (2 lb) of seeds. harvested in January to February, and stored in a Populus seeds range in weight from 310,900 to cold chamber, or frozen as whips or cuttings, until 16,650,OOO seeds/kg (141,000 to 7,550,OOO seeds/lb) planting. The whips or cuttings should not be al- depending on the species, location, and tree (36). The lowed to desiccate. Cuttings should be soaked in seeds are disseminated some distance from the water for at least 24 hours before planting. When mother tree by the wind. Natural seed dispersal hybrid poplars are planted in open fields or in areas begins at the same time as seed dispersal of the with competing vegetation, good control of weeds, associated pure species in the area. In northern New grasses, and vegetation is necessary for the poplars England, hybrids of inter- and intrasectional crosses to survive. To establish poplar plantings by vegeta- involving Aigeiros and Tacamahaca sections disperse tive means the following steps should be taken: 1) seeds about June 1. Hybrids growing south of north- plow or rototill the area during the summer before ern New England disperse seeds somewhat earlier. planting; 2) disk, cultivate, or rototill the area several times during the summer and fall so weeds, Seedling Development-Germination is epigeal. grass, and vegetation will not gain a foothold; 3) disk, Seedlings develop best on moist mineral soils where cultivate, or rototill in the spring before planting; 4) competing vegetation is minimal for 1 or 2 years plant the poplars at the desired spacing; 5) cultivate after establishment. Seed germination capacity is frequently the first 2 years to eliminate competition. retained only a few days under natural conditions. As early as 1945, it was established that hybrid Seeds of Populus deltoides can be stored for at least poplars performed best in a sod- and weed-free field 6 years at -20” C (-4” F) and 6 to 10 percent mois- (37). More recent studies have shown cultivation to ture without substantial loss in viability (43). be advantageous (4,15,16,17,45). If step 5 is omitted,
578 Populus
heavy sod and weed cover will reduce tree survival hypoxylon canker (Hypoxylon mammatum) that in- and growth. fects aspens and their hybrids in low stocked stands; Weed control can be accomplished by use of chemi- cytospera canker (Cytospera chrysosperma) that in- cals, but these can be very injurious to hybrid fects poplar hybrids and is promoted by moisture poplars. Chemicals are not recommended unless stress; dothichiza canker (Dothichiza populae) that great care is taken in their handling and use. causes decline in Lombardy poplar; septoria leaf spot (Septoria mu&a) that causes severe stem infections Sapling and Pole Stages to Maturity in densely stocked stands; and pecan feeder root necrosis (Fusarium sol&) that may develop in Growth and Yield-Poplars grow best on fertile stems under 2 years old. soils, and early height growth can average 1.2 to 1.8 Foliage diseases of varying severity are m (4 to 6 ft) per year (7,s). melampsora leaf rust (Melampsora medusae), Superior clones of poplar hybrids established with marssonina leaf spot (Marssonina brunnea), oak leaf dormant cuttings spaced 1.2 by 1.2 m (4 by 4 ft) on fungus (Septotinia podophyllina), shepherd’s crook two upland sites in Williamsburg, MA (fig. 1) shoot blight Nenturia populina) on Tacamahaca produced 2.5 to 15.3 m3 (1 to 6 cords) at 4 years, 7.6 poplars, I? macularis on Leuce and Aigeiros poplars), to 35.7 m3 (3 to 14 cords) at 9 years, and 56.1 to 117.2 and a leaf spot (Phyllosticta spp.). m3 (22 to 46 cords) at 15 years (38). Poplar hybrids The most serious defoliator of poplar hybrids, espe- growing on a reclaimed strip-mine site in Pennsyl- cially young trees, is the cottonwood leaf beetle vania have maintained an average growth of 1.2 m (4 ft) per year and have reached 19.8 m (65 ft) in height growth after 16 growing seasons, producing an average of 12.6 m3/ha (2 cords/acre) per year (6). Early field tests of closely spaced Populus ‘Tristis’ hybrids at 4 years of age produced 11.2, 12.6, and 7.6 t/ha (5.0, 5.6, and 3.4 dry tons/acre) per year of stems and branches at spacings of 0.23 m (0.75 ft), 0.30 m (1 ft), and 0.61 m (2 ft), respectively (12).
Rooting Habit-Aigeiros-type hybrids have strong horizontal surface roots from which plunging roots develop. Leuce-type hybrids develop plunging roots constituting 40 to 50 percent of their entire root system. In other poplar hybrids, horizontal roots have been measured at 15 m (50 ft) for a lo-year-old tree growing in sandy soil, 20 m (66 ft) for an old tree, and 18 m (60 ft) for an old Populus alba. The development of plunging roots is limited by the level of the water table or by the soil condition (1).
Reaction to Competition-Poplars and poplar hybrids are very intolerant of shade in the forest community, especially in comparison with other more shade- and competition-tolerant species. Poplar hybrids are usually established in pure plantings, using dormant cuttings. With this method, hybrids cannot tolerate weed, grass, and shrub competition during the first 2 years after planting. Space around each tree is also needed during the growth of the stand. If branches of trees overlap, growth and vigor are reduced and recovery of growth rate is slow.
Damaging Agents-In the Northeast, poplar hybrids are susceptible to many diseases and insects. Figure I-Hybrid poplar clonal test planted in 1952 at Disease organisms that cause stem canker are Williamsburg, MA.
579 Poplhs
(Crysomela scripta). The forest tent caterpillar In the northeastern United States, moose and deer (Malacosoma disstria), the poplar tentmaker (Ich- often browse on poplar hybrids in recently planted thyuru inclusu), the spiny elm caterpillar or mourn- plantations. Poplar buds are a choice food supply for ing cloak butterfly (Nymph&is antiopu), and the ruffed grouse and several kinds of songbirds. Grouse large aspen tortrix (Choristoneura conflictana) can and pheasant also eat the catkins. also cause complete defoliation. Leaf damage may In urban areas, poplar hybrids are useful where also be inflicted by a leaf beetle (Zeugophora scutel- fast-growing trees are needed for shade, landscaping, lark), the aspen blotch miner (Phyllonorycter and screening around industrial buildings, apartment tremuloidiella), and the aspen leafminer (Phyllocnis- complexes, recreational playing areas, parking lots, and tis populiella). , landfills. These trees live less than 100 years so more One of the most destructive poplar pests is the tolerant species should be interplanted with them. cottonwood twig borer (Gypsonoma haimbachiana). Poplar hybrids are used to stabilize soils on It kills buds and up to 25 cm (10 in) of shoot tips. hillsides, along streams and rivers, landfills, and bor- Other borers that do damage are the poplar-and-wil- row pits. They are also planted as fence rows to low-borer (Cryptorhynchus lapathi), the poplar borer reduce air speed in agricultural areas where soil is (Saperda calcarata), the cottonwood borer transported by the wind. (Plectrodera scalator), the bronze poplar borer Hybrid poplars have been extensively used as test (Agrilus liragus), and a flatheaded borer (A. horni). organism for research studies because of their ease Infestations of the poplar gall midge (Prodiplosis of propagation, fast growth, and the variety of clonal morrisi) and various aphids and plant lice may parentages. They have been used to study the effect reduce the growth of individual trees (9). of air pollutants (2,3,10,13,18,19,22,23,24,25,26, 30,31,32,33,46) and wood compartmentalization Special Uses (11,39,40), to name just two. Hybrid poplars were initially developed for conven- Genetics tional pulpwood (42). In recent years, more interest has been placed on evaluation of hybrid poplar for short-rotation chip production for pulp and energy uses Approximately 30 species of poplars are available for hybridization as listed below; however, not all (4,21,27,28,29). However, at this time, investment rates of return are not attractive for large scale conversions possible crosses have been successful or seem to short rotation intensive culture systems (4,14). feasible at this time (fig. 2). There are many estimates of poplar hybrid Classification of Populus (47) biomass yields in the literature, but the following Section and Species Geographic distribution values are averages from intensively managed plan- Turanga Bge. tations on many sites in the northeastern United euphrutica Olivier West and Central Asia, States (9): First-year height growth is 0.9 to 2.4 m (3 (syn: pruinosa Schrenk) North Africa to 8 ft); mean annual height growth after 10 to 20 Leuce Duby years is 0.9 to 1.4 m (3 to 4.5 ft); mean annual ao!enopodu Maxim. China diameter growth after 10 to 20 years is 1.0 to 1.5 cm alba L. Europe, Asia, North Africa (0.4 to 0.6 in); mean annual volume increment after duuidiana (Dade) Schneid. Northeast Asia 10 to 20 years is 7.0 to 24.5 m3/ha (100 to 350 grandidentatu Michx. North America sieboldii Miq. Japan, Korea ft3/acre); and mean annual biomass increment after tomentosa Carr. Asia 5 to 20 years is 4.5 to 20.2 t/ha (2 to 9 tons/acre). tremula L. Europe, Asia Growth and yield vary appreciably depending on tremuloides Michx. North America location, site quality, clone or cultivar used, and sil- Leucoides Spach vicultural conditions. These values given are only ciliatu Wall Central Asia generally representative. Diameter growth of in- heterophylla L. Southeastern United States dividual trees depends heavily on stocking density. lasiocarpa Oliv. China Wide spacings or frequent thinnings promote rapid wilsonii Schneid. China diameter growth. Tacamahaca Spach Biomass consists of ovendry, leafless stems and angustifolia James North America balsamifera L. North America branches. Attainment of maximum mean annual in- cuthuyana Rehd. Northeast Asia crement occurs only if stands are heavily fertilized koreana Rehd. Korea and irrigated and occurs much sooner at tree spac- laurifolia Ledeb. Siberia ings of 2 m (6.6 ft) or less. maximowiczii Henry Northeast Asia, Japan
580 Populus
simonii Carr. Asia suaveolens Fisch. Asia szechuanica Schneid. China trichocarpa Torr. & Gray North America yunnannensis Dode China Aigeiros Duby deltoides Bartr. ex Marsh. North America deltoides var. occidentalis Rydb. North America (syn: sargentii Dade) fremontii Wats. North America fremontii var. wislizeni Wats. North America (syn: wislizeni Wats.) nigra L. Europe, Asia, North Africa
An example of the genetics of one cultivar follows. One of the widely grown hybrid cottonwood cultivars is ‘Robusta,’ reportedly Populus angulata x I? plan- tierensis. Populus angulata is a clone of eastern cot- tonwood (l? deltoides var. deltoides); P plantierensis is a hybrid of a western European black poplar (P. nigra) and Lombardy poplar (F! nigra var. italica). Lombardy poplar is a fastigate form of black poplar, native to Iran. Populus nigra var. betulifolia of western Europe is distinguished from the more easterly forms by the slightly hairy petioles and tips of young shoots. These are found in I? nigra var. plantierensis and transferred to the ‘Robusta’ clones. Fertile crosses Good apical dominance, numerous side branches, and ----Difficult crosses very narrow branch angle are traits inherited as a -II- Incompatible single dominant gene. Lombardy poplar is homozygous -?- No information with respect to this gene, while P nigra var. plantierem sis is heterozygous with respect to this gene (20).
Literature Cited Figure Z-Summary of interspecific breeding in the genus Populus (7). 1. International Poplar Commission. 1979. Poplars and willows in wood production and land use. FAO Forestry Series 10. Food and Agriculture Organization of the United Nations, 8. Demeritt, Maurice E., Jr. 1981. Growth of hybrid poplars in Pennsylvania and Maryland clonal tests. USDA Forest Rome. 328 p. Service, Research Note NE-302. Northeastern Forest 2. Biggs, A. R., and D. D. Davis. 1981. Effect of SO2 on growth and sulfur content of hybrid poplar. Canadian Journal of Experiment Station, Bromall, PA. 2 p. Forest Research 11(4):830-833. 9. Dickmann, Donald I., and Katherine W. Stuart. 1983. 3. Biggs, A. R., and D. D. Davis. 1982. Effects of sulfur dioxide Culture of hybrid poplars in northeastern North America. on water relations of hybrid poplar foliage and bark. Michigan State University, Department of Forestry, East Canadian Journal of Forest Research 12(3):612-616. Lansing. 168 p. 4. Bowersox, T. W., and W. W. Ward. 1976. Economic analysis of 10. Dochinger, L. S., and K. F. Jensen. 1975. Effects of chronic a short-rotation fiber production system for hybrid poplar. and acute exposure to sulfur dioxide on the growth of hybrid Journal of Forestry 74( 11):750-753. poplar cuttings. Environmental Pollution 9:219-229. 5. Brayshaw, T. C. 1965. Native poplars of southern Alberta and 11. Eckstein, D., and W. Liese. 1979. Relationship of wood their hybrids. Canadian Department of Forestry Publication structure to compartmentalization of discolored wood in 1109. hybrid poplar. Canadian Journal of Forest Research 6. Davidson, Walter H. 1979. Hybrid poplar pulpwood and 9(2):205-210. lumber from a reclaimed strip mine. USDA Forest Service, 12. Ek, Alan R., and David H. Dawson. 1976. Actual and Research Note NE-232. Northeastern Forest Experiment projected growth and yields of Populus ‘tristis-1’, under Station, Broomall, PA. 2 p. intensive culture. Canadian Journal of Forest Research 7. Demeritt, Maurice E., Jr. 1979. Evaluation of early growth 6(2):132-144. among hybrid poplar clonal tests in the northeastern United 13. Evans, L. S., N. F. Gmur, and F. DaCosta. 1978. Foliar States. In Proceedings, Twenty-sixth Northeastern Forest response of six clones of hybrid poplar to simulated acid rain. Tree Improvement Conference. p. 133-139. Phytopathology 68(6):847-856.
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14. Ferguson, K. D., D. W. Rose, D. C. Lothner, and J. 31. Patton, R. L., and M. 0. Garroway. 1986. Ozone-induced Zavitkovski. 1981. Hybrid poplar plantations in the Lake necrosis and increased peroxidase activity in hybrid poplars States-A financial analysis. Journal of Forestry (Pop&s sp.) leaves. Environmental and Experimental 79( 10):664-667. Botany 26(2):137-141. 15. Hansen, E., D. Netzer, and W. J. Rietveld. 1984. Weed control 32. Reich, P. B., and J. P. Lassoie. 1984. Effects of low level 03 for establishing intensively cultured hybrid poplar exposure on leaf diffusive conductance and water-use plantations (Populus candicans x Populus x berolinensis, efficiency in hybrid poplar. Plant, Cell and Environment Populus nigra x Pop&s laurifolia, Populus tristis X Populus 7:661-668. balsamifera). USDA Forest Service, Research Note NC-317. 33. Reich, P. B., J. P. Lassoie, and R. G. Amundson. 1984. North Central Forest Experiment Station, St. Paul, MN. 6 p. Reduction in growth of hybrid poplar following field exposure 16. Hansen, E. A., D. A. Netzer, and R. F. Woods. 1986. Tillage to low levels of 03 and (or) SOz. Canadian Journal of Botany superior to no-till for establishing hybrid poplar plantations. 62(12):2835-2841. Tree Planters’ Notes 376-10. 34. Roller, K. J., D. H. Thibault, and V. Hildahl. 1972. Guide to 17. Hansen, E., L. Moore, D. Netzer, M. Ostry, H. Phipps, and J. the identification of poplar cultivars on the prairies. Zavitkovski. 1983. Establishing intensively cultured hybrid Department of the Environment, Canadian Forestry Service poplar plantations for fuel and fiber. USDA Forest Service, Publication 1311. Ottawa, ON. 55 p. General Technical Report NC-78. North Central Forest 35. Rood, S. B., J. S. Campbell, and T. Despins. 1986. Natural Experiment Station, St. Paul, MN. 24 p. poplar hybrids from southern Alberta. I. Continuous 18. Harkov, R., and E. Brennan. 1980. The influence of soil variation for foliar characteristics. Canadian Journal of fertility and water stress on the ozone response of hybrid Botany 64:1382-1388. poplar trees. Phytopathology 70(10):991-994. 36. Schopmeyer, C. S., tech. coord. 1974. Seeds of woody plants in 19. Harkov, R., and E. Brennan. 1982. The effect of acute ozone the United States. U.S. Department of Agriculture, exposures on the growth of hybrid poplar. Plant Disease Agriculture Handbook 450. Washington, DC. 883 p. 66(7):587-589. 37. Schreiner, Ernst J. 1945. How sod affects establishment of 20. Heimburger, C. 1979. Genetics of hybrid poplars. North hybrid poplar plantations. Journal of Forestry 43:412-427. American Poplar Council Meeting, Thompsonville, MI. 38. Schreiner, Ernst J. 1971. Application of tree improvement to 21. Holt, D. H., and W. K. Murphey. 1978. Properties of hybrid mini-, midi-, and maxi-rotation management. In Proceedings, poplar juvenile wood affected by silvicultural treatment. Eighteenth Northeastern Forest Tree Improvement Wood Science 10(4):198-203. Conference. p. 39-48. New Haven, CT. Jensen, K. F. 1979. A comparison of height growth and leaf 22. 39. Shigo, A. L., W. Shortle, and P. Garrett. 1977. parameters of hybrid poplar cuttings grown in ozone- Compartmentalization of discolored and decayed wood fumigated atmospheres. USDA Forest Service, Research associated with injection-type wounds in hybrid poplars. Paper NE-446. Northeastern Forest Experiment Station, Journal of Arboriculture 3(6):114-118. Broomall, PA. 3 p. 40. Shortle, W. C. 1979. Compartmentalization of decay in red 23. Jensen, K. F. 1981. Growth analysis of hybrid poplar cuttings fumigated with ozone and sulfur dioxide. Environmental maple and hybrid poplar trees. Phytopathology Pollution (Series A) 26:243-250. 69(4):410-413. 24. Jensen, K. F., and R. D. Noble. 1984. Impact of ozone and 41. Stout, A. B., and E. J. Schreiner. 1933. Results of a project in sulfur dioxide on net photosynthesis of hybrid poplar hybridizing poplars. Journal of Heredity 24:216-229. cuttings. Canadian Journal of Forest Research 14(3):385-388. 42. Stout, A. B., R. H. McKee, and E. J. Schreiner. 1927. The 25. Kohut, R. J., D. D. Davis, and W. Merrill. 1976. Response of breeding of forest trees for pulp wood. Journal of New York hybrid poplar to simultaneous exposure to ozone and pan. Botanical Gardens 28:49-63. Plant Disease Reporter 60(9):777-780. 43. Tauer, Charles G. 1979. Seed tree, vacuum, and temperature 26. Krause, C. R., and K. F. Jensen. 1979. Surface changes on effects on eastern cottonwood seed viability during extended hybrid poplar leaves exposed to ozone and sulfur dioxide. storage. Forest Science 25(1):112-114. Scanning Election Microscopy 3:77-80. 44. Viereck, Leslie A., and Joan M. Foote. 1970. The status of 27. Marton, R., G. R. Stairs, and E. J. Schreiner. 1968. Influence Populus balsamifera and P. trichocarpa in Alaska. The of growth rate and clonal effects on wood anatomy and Canadian Field-Naturalist 84:169-173. pulping properties of hybrid poplars. Tappi 51(5):230-235. 45. von Althen, F. W. 1981. Site preparation and post-planting 28. McGovern, J. N., J. F. Laundrie, and J. G. Berbee. 1973. weed control in hardwood afforestation: white ash, black Assessment of a rapid-growth hybrid poplar for kraft pulping. walnut, basswood, silver maple, hybrid poplar. Canadian University of Wisconsin, Forest Research Notes 180,5 p. Forest Service, Re PO-x-325. Great Lakes Forest Research 29. Murphey, W. K., D. H. Holt, T. W. Bowersox, P. R. Center, Sault Ste. Marie, ONT. 17 p. Blankenhorn, and R. C. Baldwin. 1977. Selected wood 46. Wray, P. H., and J. C. Gordon. 1975. Effects of photoperiod on properties of young hybrid poplar. TAPPI Forest Biology, growth and peroxidase in three hybrid poplars. Canadian Wood Chemistry Conference. p. 231-237. June 20-22, 1977, Journal of Forest Research 5(4):735-738. Madison, WI:
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