Breeding• A brief history
Populus species (poplars, cottonwoods, and aspens) were among the first trees domesticated worldwide. In fact, the first artificial forest regeneration program in the United States involved the establishment of a black cottonwood (P. trichocarpa) plantation in 1911 to supply wood fiber to Willamette Paper’s pulp mill in West Linn, Oregon. Then in 1925-1927, a large poplar-breeding project was undertaken to develop improved hybrids for a plantation supporting the Oxford Paper Company’s mill in Rumford, Maine.1• Commercial use
Populus domestication in North America was originally promoted as a fiber resource for the pulp and paper industry. It is now viewed as the premier woody perennial feedstock candidate for the fast-growing cellulosic ethanol industry. The environmental amelioration industry is also actively developing poplar for phytoremediation of contaminated sites, the land application of municipal and industrial effluent, and for agricultural streamside filter plantings. Regionally, poplar is also being developed for various non-structural, specialty solid wood markets.
• Conventional improvement
Growth rate, stem form, adventitious rooting, pest resistance, climatic adaptability, and wood specific gravity are the main characteristics in which improvements are sought.2 First-generation hybridization and backcross breeding are the most common improvement strategies. Many programs function to produce new varieties as substitutions for the lowest-ranking ones in commercial deployment pools and as replacements for those culled as losses of disease resistance occur over time. Long-term improvement efforts involving reciprocal recurrent breeding programs for the multiplicity of parental species participating in first-generation hybridization programs are often too costly to pursue in a meaningful way.
• Species
Worldwide, poplar cultivation relies nearly exclusively on select inter-specific hybrid clonal varieties.3 Eight of the 29 species in the genus – P. alba, P. ciliata, P. deltoides, P. nigra, P. suaveolens, P. tremula, P. tremuloides, and P. trichocarpa – are used in breeding the major hybrid taxa: P. x canescens, P. deltoides x P. ciliata, P. x canadensis, P. x generosa, and P. tremuloides x P. tremula. Between ~ 34° and ~ 23° of latitude, select varieties of southern sources of P. deltoides are often used in place of hybrids.
Eight species of Populus, P. angustifolia, P. balsamifera, P. deltoides, P. fremontii, P. grandidentata, P. heterophylla, P. tremuloides, and P. trichocarpa, are native to the United States. Work with native and exotic species occurs nearly exclusively in three regions featuring the following taxa:
1.) Lower Mississippi River Valley – P. deltoides
2.) Central and North Central – P. deltoides, P. x canadensis, P. nigra x P. maximowiczii
3.) Pacific Northwest – P. x generosa, P. x canadensis, P. deltoides x P. maximowiczii.
• Commercial poplar plantations
Today, there are 86,000 plantation acres in the United States and 35,000 acres in Canada. A much larger concentration of poplar plantations is found in Europe and Asia. There are approximately 2,800,000 acres throughout Europe where France and Italy account for nearly 40 percent of the total. In China, about 1,900,000 acres have been planted mainly in Jiangsu, Anhwei, Hunan, Hubei, Shandong, Shanxi, and Liaoning Provinces. Chile has recently embarked on a research program to diversify their radiata pine industry with a poplar program that ultimately may reach a scale of 1,000,000 or more plantation acres.
The mean annual increment of poplar stands varies between 230 and 486 cubic feet per acre per year with rotations spanning 8 to 25 years dependent upon markets and site.
1 Stout, A. B. and Schreiner, E. J. 1933. Results of a project in hybridizing poplars. Journal of Heredity 24: 216-229.
2 Riemenschneider, D. E., Stanton, B. J., Vallee, G., and Perinet, P. 2000. Poplar breeding strategies. Chapter 2. In Poplar Culture in North America. Edited by D. I. Dickmann, J. G. Isebrands, J. E. Eckenwalder, and J. Richardson. NRC Research Press, National Research Council of Canada, Ottawa, ON, Canada. Pp. 43-76.
3Stanton, B. J. 2003. Poplars. Encyclopedia of Forest Sciences. Edited by J. Burley, J. Evans, and J. A. Younquist Elsvier Ltd. Oxford, U. K. pp 1441-1449.
