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likely because the factors that may cause such differentiation—soil type, historical vegetational
succession, and moisture availability—are less important than temperature and day length in
generating selection pressures leading to morphological or physiological differentiation (Nixon and
McMillan 1964; Casler et  al. 2007a). Nevertheless, there is some differentiation between upland
switchgrass populations deriving from the tallgrass prairie of the Central Great Plains and the
historical Eastern Forest ecosystems (McMillan 1959; Casler et al. 2007a).
The balancing effects of natural selection, creating differentiation among populations and
uniformity within populations, and self-incompatibility, which favors homogenization of populations
and promotes diversity within populations, suggests that switchgrass germplasm can be classified
into relatively few functional gene pools to represent the eastern two-thirds of the United States
(Casler et al. 2007a, b). Eight gene pools are proposed to represent four groups of hardiness zones
within each of the Great Plains region and Eastern Forest biomes east of the Mississippi River
(Figure 22.2). The choice of eight gene pools was somewhat arbitrary and the boundaries between
gene pools are fluid to promote flexibility. Some hardiness zones have been combined because it is
impractical and unnecessarily reductionist to develop and/or recommend switchgrass germplasm for
one hardiness zone. The east-west discrimination of the historical Great Plains and Eastern Forest
biomes is based largely on precipitation and humidity, recognizing that eastern germplasm may
have reduced drought tolerance relative to western germplasm whereas western germplasm may not
have the disease resistance required in the more humid eastern region. For example, biomass yields
of Cave-in-Rock tend to be reduced relative to that of other cultivars as this cultivar is moved west
of its origin, particularly as moisture becomes limiting (Hopkins et al. 1995a; Casler and Boe 2003;
Berdahl et al. 2005). Conversely, Sunburst, originating in the northern Great Plains, tends to have
60°
PP/GPS
HZ3/4
LMF
HZ3/4
EBF
HZ4/5
PP/GPS
HZ4/5
PP/GPS
HZ6/7
PP/GPS
HZ7/8
EBF
HZ6/7
EBF
HZ8/9
15°
FIGure 22.2 (see color insert) Proposed gene pools for deployment of regionally adapted switchgrass
germplasm and cultivars for use in breeding programs or in conservation and restoration projects. PP, prairie
parkland; GPS, Great Plains steppe; LMF, Laurentian mixed forest; EBF, eastern broadleaf forest. (From
Bailey, R.G., Ecoregions: The Ecosystem Geography of the Oceans and Continents, Springer-Verlag Inc.,
New York, 1998; Bailey, R. G., Ecosystem Geography, 2nd ed, Springer-Verlag Inc., New York, 2009.) HZ,
USDA hardiness zone. (From Cathey, H.M., USDA plant hardiness zone map, USDA Misc Pub No 1475. U.S.
Department of Agriculture, Washington, DC, 1998; U.S. National Arboretum, 1990. Available at www.usna.
usda.gov/Hardzone/ushzmap.html)
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