Populus L.

Poplar Hybrids

Salicaceae -- Willow family

Maurice E. Demeritt, Jr.

Poplar hybrids (Populus spp.) are the result of natural and manmade crosses among poplar species. The genus is further divided into five sections of which four are represented in North America: Leuce (aspen type), to which P. grandidentata and P. tremuloides (bigtooth aspen and quaking aspen) belong; Aigeiros (cottonwood or poplar type), to which P. deltoides, P. sargentii, P. fremontii, and P. wislizeni (eastern, plains, Fremont, and Rio Grande cottonwood) belong; Tacamahaca (balsam poplar type), to which P. balsamifera, R trichocarpa, and P. angustifolia (balsam poplar, black, and narrowleaf cottonwood) belong, and Leucoides (swamp poplar type), to which P. heterophylla belongs. P. balsamifera subsp. trichocarpa has been reported as the correct status of P. trichocarpa (44).

Habitat

Native Range

Poplar hybrids occur naturally throughout the U.S. and Canada wherever compatible species come into close proximity (table 1). Most poplar hybrids, however, result from artificial hybridization and subsequent planting. The first large-scale hybridization project with poplars in the United States was begun in 1925 (41,42). An unknown number of hybrids also form between native species and introduced clones, cultivars, and species. The Jackii poplar is a natural hybrid between P. balsamifera females and P. deltoides males. Populus x Smithii is a natural hybrid between P. tremuloides and P. grandidentata. Hybridization between P. balsamifera and P. trichocarpa occurs in the interior of southeastern Alaska and in the Cook Inlet region. Also, the trihybrid among P. deltoides, P. balsamifera, and P. angustifolia has been reported in southern Alberta.

Climate

In general, poplar hybrids grow best on humid and microthermal areas with adequate moisture during all seasons of the year. They are rarely found on sites that have temperatures of -46° C (-50° F) or on sites that have summer temperatures over 38° C (100° F) for more than a week.

Soils and Topography

Poplar hybrids grow best where soils are at least 1 m (3.3 ft) in depth to interrupted bedrock. The water table and porous or gravel layers should also be at least 1 m (3.3 ft) below the soil surface. Optimum pH ranges from 6.0 to 7.0, though some hybrids tolerate high or low pH conditions. Hybrids grow well on upland and bottom-land soils if the soils have good moisture-holding capacities and are of medium texture. Hybrids show extreme variation in tolerance of adverse site conditions. They grow best on soils of the orders Entisols, Inceptisols, Mollisols, Spodosols, and Ultisols.

Associated Forest Cover

The two natural hybrids, P. x smithii and P. x jackii, are associated with the parental species in the same stand, and the parental species dominate in the stands. The majority of hybrid poplars are planted in pure stands and all competing vegetation is controlled the first few years after planting. Poplars are very intolerant of shade and herbicides and also when young cannot tolerate competition from grass, weeds, and shrubs in their immediate area.

Life History

Reproduction and Early Growth

Flowering and Fruiting- Poplar hybrids are dioecious and first flower at about 8 years of age. The flowers are borne in catkins (or aments) in the spring before leafing. Male and female catkins, when fully developed, are 10 to 15 cm (3.9 to 5.9 in) long. In the female flower, the number of stigmas varies from two to four and are either cap- or y-shaped. In the male flower, the number of stamens varies from 30 to 80. The bract has 20 to 30 digits, depending on the cultivar. The central areas of the bracts are light in color and the digits are dark brown, sometimes tinged with black (34). Many poplar hybrids have never produced flowers and thus are thought to be sterile. Poplars flower between February and May and disperse seed between April and June of the same year. Intersection Leuce hybrids disperse seed a few weeks before intersection Aigeiros and Tacamahaca or intersection Aigeiros-Tacamahaca hybrids. Fruits are one-celled capsules borne in long pendulous clusters (catkins), and each capsule is surrounded by tufts of long, white, silky hairs attached to the short stalks of the seeds, promoting wind dispersion over great distances (36).

Seed Production and Dissemination- Poplar hybrids generally are prolific annual seed producers Individual trees of Populus tremula, an exotic, may yield from 8,000 to 54,000,000 seeds per year; hybrid poplar seed yields are thought to be the same. In Populus deltoides var. uirginiana 35 liters (1 bushel) of fresh fruit yields about 0.9 kg (2 lb) of seeds Populus seeds range in weight from 310,900 to 16,650,000 seeds/kg (141,000 to 7,550,000 seeds/lb) depending on the species, location, and tree (36). The seeds are disseminated some distance from the mother tree by the wind. Natural seed dispersal begins at the same time as seed dispersal of the associated pure species in the area. In northern New England, hybrids of inter- and intersectional crosses involving Aigeiros and Tacamahaca sections disperse seeds about June 1. Hybrids growing south of northern New England disperse seeds somewhat earlier.

Seedling Development- Germination is epigeal. Seedlings develop best on moist mineral soils where competing vegetation is minimal for 1 or 2 years after establishment. Seed germination capacity is retained only a few days under natural conditions. Seeds of Populus deltoides can be stored for at least 6 years at -20° C (-4° F) and 6 to 10 percent moisture without substantial loss in viability (43).

Vegetative Reproduction- Poplar hybrids reproduce vegetatively by natural and artificial means. Leuce-type hybrids root best from root sections, though some selections root adequately from dormant stem cuttings. Aigeiros-type hybrids reproduce well from either greenwood or dormant stem cuttings, although most hybrids are reproduced from dormant cuttings. Dormant cuttings are produced predominately from hybrid stool beds established for that purpose. The cuttings are usually harvested in January to February, and stored in a cold chamber, or frozen as whips or cuttings, until planting. The whips or cuttings should not be allowed to desiccate. Cuttings should be soaked in water for at least 24 hours before planting. When hybrid poplars are planted in open fields or in areas with competing vegetation, good control of weeds, grasses, and vegetation is necessary for the poplars to survive. To establish poplar plantings by vegetative means the following steps should be taken: 1) plow or rototill the area during the summer before planting; 2) disk, cultivate, or rototill the area several times during the summer and fall so weeds grass, and vegetation will not gain a foothold; 3) disk: cultivate, or rototill in the spring before planting; 4) plant the poplars at the desired spacing; 5) cultivate frequently the first 2 years to eliminate competition. As early as 1945, it was established that hybrid poplars performed best in a sod- and weed-free field (37). More recent studies have shown cultivation to be advantageous (4,15,16,17,45). If step 5 is omitted, heavy sod and weed cover will reduce tree survival and growth.

Weed control can be accomplished by use of chemicals, but these can be very injurious to hybrid poplars. Chemicals are not recommended unless great care is taken in their handling and use.

Sapling and Pole Stages to Maturity

Growth and Yield- Poplars grow best on fertile soils, and early height growth can average 1.2 to 1.8 m (4 to 6 ft) per year (7,8).

Superior clones of poplar hybrids established with dormant cuttings spaced 1.2 by 1.2 m (4 by 4 ft) on two upland sites in Williamsburg, MA produced 2.5 to 15.3 in³ (1 to 6 cords) at 4 years, 7.6 to 35.7 m³ (3 to 14 cords) at 9 years, and 56.1 to 117.2 m³ (22 to 46 cords) at 15 years (38). Poplar hybrids growing on a reclaimed strip-mine site in Pennsylvania have maintained an average growth of 1.2 in (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 in (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 10-year-old tree growing in sandy soil, 20 m (66 ft) for an old tree, and 18 in (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. Disease organisms that cause stem canker are hypoxylon canker (Hypoxylon mammatum) that infects aspens and their hybrids in low stocked stands; cytospera canker (Cytospera chrysosperma) that infects poplar hybrids and is promoted by moisture stress; dothichiza canker (Dothichiza populae) that causes decline in Lombardy poplar; septoria leaf spot (Septoria musiva) that causes severe stem infections in densely stocked stands; and pecan feeder root necrosis (Fusarium solani) that may develop in stems under 2 years old.

Foliage diseases of varying severity are melampsora leaf rust (Melampsora medusae), marssonina leaf spot (Marssonina brunnea), oak leaf fungus (Septotinia podophyllina), shepherd's crook shoot blight (Venturia populina) on Tacamahaca poplars, V. macularis on Leuce and Aigeiros poplars), and a leaf spot (Phyllosticta spp.).

The most serious defoliator of poplar hybrids, especially young trees, is the cottonwood leaf beetle (Crysomela scripta). The forest tent caterpillar (Malacosoma disstria), the poplar tentmaker (Ichthyura inclusa), the spiny elm caterpillar or mourning cloak butterfly (Nymphalis antiopa), and the large aspen tortrix (Choristoneura conflictana) can also cause complete defoliation. Leaf damage may also be inflicted by a leaf beetle (Zeugophora scutellaris), the aspen blotch miner (Phyllonorycter tremuloidiella), and the aspen leafminer (Phyllocnistis populiella).

One of the most destructive poplar pests is the cottonwood twig borer (Gypsonoma haimbachiana). It kills buds and up to 25 cm (10 in) of shoot tips. Other borers that do damage are the poplar-and-willow-borer (Cryptorhynchus lapathi), the poplar borer (Saperda calcarata), the cottonwood borer (Plectrodera scalator), the bronze poplar borer (Agrilus liragus), and a flatheaded borer (A. horni).

Infestations of the poplar gall midge (Prodiplosis morrisi) and various aphids and plant lice may reduce the growth of individual trees (9).

Special Uses

Hybrid poplars were initially developed for conventional 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 (4,21,27,28,29). However, at this time, investment rates of return are not attractive for large scale conversions to short rotation intensive culture systems (4,14).

There are many estimates of poplar hybrid biomass yields in the literature, but the following values are averages from intensively managed plantations on many sites in the northeastern United States (9): First-year height growth is 0.9 to 2.4 rn (3 to 8 ft); mean annual height growth after 10 to 20 years is 0.9 to 1.4 m (3 to 4.5 ft); mean annual diameter growth after 10 to 20 years is 1.0 to 1.5 cm (0.4 to 0.6 in); mean annual volume increment after 10 to 20 years is 7.0 to 24.5 m³/ha (100 to 350 ft³/acre); and mean annual biomass increment after 5 to 20 years is 4.5 to 20.2 t/ha (2 to 9 tons/acre).

Growth and yield vary appreciably depending on location, site quality, clone or cultivar used, and silvicultural conditions. These values given are only generally representative. Diameter growth of individual trees depends heavily on stocking density. Wide spacings or frequent thinnings promote rapid diameter growth.

Biomass consists of ovendry, leafless stems and branches. Attainment of maximum mean annual increment occurs only if stands are heavily fertilized and irrigated and occurs much sooner at tree spacings of 2 m (6.6 ft) or less.

In the northeastern United States, moose and deer often browse on poplar hybrids in recently planted plantations. Poplar buds are a choice food supply for ruffed grouse and several kinds of songbirds. Grouse and pheasant also eat the catkins.

In urban areas, poplar hybrids are useful where fast-growing trees are needed for shade, landscaping, and screening around industrial buildings, apartment complexes, recreational playing areas, parking lots, and landfills. These trees live less than 100 years so more tolerant species should be interplanted with them.

Poplar hybrids are used to stabilize soils on hillsides, along streams and rivers, landfills, and borrow pits. They are also planted as fence rows to reduce air speed in agricultural areas where soil is transported by the wind.

Hybrid poplars have been extensively used as test organism for research studies because of their ease of propagation, fast growth, and the variety of clonal parentages. They have been used to study the effect of air pollutants (2,3,10,13,18,19,22,23,24,25,26, 30,31,32,33,46) and wood compartmentalization (11,39,40), to name just two.

Genetics

Approximately 30 species of poplars are available for hybridization as listed below; however, not all possible crosses have been successful or seem feasible at this time (fig. 2).

-Summary of interspecific breeding in the genus Populus (7).

An example of the genetics of one cultivar follows. One of the widely grown hybrid cottonwood cultivars is 'Robusta,' reportedly Populus angulata x P. plantierensis. Populus angulata is a clone of eastern cottonwood (P. deltoides var. deltoides); P. plantierensis is a hybrid of a western European black poplar (P. nigra) and Lombardy poplar (P. 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 P. nigra var. plantierensis and transferred to the 'Robusta' clones. Good apical dominance, numerous side branches, and very narrow branch angle are traits inherited as a single dominant gene. Lombardy poplar is homozygous with respect to this gene, while P nigra var. plantierensis is heterozygous with respect to this gene (20).

Literature Cited

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, Rome. 328 p.
2. Biggs, A. R., and D. D. Davis. 1981. Effect Of S02 on growth and sulfur content of hybrid poplar. Canadian Journal of Forest Research 11(4):830-833.
3. Biggs, A. R., and D. D. Davis. 1982. Effects of sulfur dioxide on water relations of hybrid poplar foliage and bark. Canadian Journal of Forest Research 12(3):612-616.
4. Bowersox, T. W., and W. W. Ward. 1976. Economic analysis of a short-rotation fiber production system for hybrid poplar. Journal of Forestry 74(11):750-753.
5. Brayshaw, T. C. 1965. Native poplars of southern Alberta and their hybrids. Canadian Department of Forestry Publication 1109.
6. Davidson, Walter H. 1979. Hybrid poplar pulpwood and lumber from a reclaimed strip mine. USDA Forest Service, Research Note NE-232. Northeastern Forest Experiment Station, Broomall, PA. 2 p.
7. Demeritt, Maurice E., Jr. 1979. Evaluation of early growth among hybrid poplar clonal tests in the northeastern United States. In Proceedings, Twenty-sixth Northeastern Forest Tree Improvement Conference. p. 133-139.
8. Demeritt, Maurice E., Jr. 1981. Growth of hybrid poplars in Pennsylvania and Maryland clonal tests. USDA Forest Service, Research Note NE-302. Northeastern Forest Experiment Station, Bromall, PA. 2 p.
9. Dickmann, Donald I., and Katherine W. Stuart. 1983. Culture of hybrid poplars in northeastern North America. Michigan State University, Department of Forestry, East Lansing. 168 p.
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11. Eckstein, D., and W. Liese. 1979. Relationship of wood structure to compartmentalization of discolored wood in hybrid poplar. Canadian Journal of Forest Research 9(12):205-210.
12. Ek, Alan R., and David H. Dawson. 1976. Actual and projected growth and yields of Populus 'tristis-l', under intensive culture. Canadian Journal of Forest Research 6(2):132-144.
13. Evans, L. S., N. F. Gmur, and F. DaCosta. 1978. Foliar response of six clones of hybrid poplar to simulated acid rain. Phytopathology 68(6):847-856.
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33. Reich, P. B., J. P. Lassoie, and R. G. Amundson. 1984. Reduction in growth of hybrid poplar following field exposure to low levels Of 03 and (or) S02. Canadian Journal of Botany 62(12):2835-2841.
34. Roller, K. J., D. H. Thibault, and V. Hildahl. 1972. Guide to the identification of poplar cultivars on the prairies. Department of the Environment, Canadian Forestry Service Publication 1311. Ottawa, ON. 55 p.
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40. Shortle, W. C. 1979. Compartmentalization of decay in red maple and hybrid poplar trees. Phytopathology 69(4):410-413.
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