Panicum virgatum L. (Switchgrass) is a warm-season bunchgrass that occupies a broad geographic range from Central Mexico to southern Canada. The genetic diversity of this native species has been extensively studied because of its potential as bioenergy feedstock. Two broad ecotypes predominate in the range of P. virgatum throughout the Midwest and Great Plains: a northern upland ecotype and a more southerly lowland ecotype found in floodplain habitats. In the Great Lakes region, local populations are generally upland and upland ecotypes are most commonly planted in restoration and conservation projects. Although some trials have demonstrated that lowland switchgrass can yield well in some northern regions, it is commonly assumed that because this ecotype can withstand wet soils it would do poorly on the drier marginal soils considered for many conservation and bioenergy plantings.
To test this assumption experimentally, we grew 3 upland and 2 lowland cultivars in large (400-m2) monoculture stands at 12 sites across a range of soil types in Michigan. We seeded plots and estimated establishment rates in 2012, during a significant regional drought. We compared the productivity of the ecotypes in 2014 and 2015, when rainfall was above average.
Contradicting the general assumption that lowland ecotypes of switchgrass would perform poorly on drier soils, we found that both ecotypes performed equally well across all soils. Both ecotypes established equally well during one of the worst droughts in recent years. In sandy soils, both ecotypes responded positively to increased precipitation and more so than on loam. Between 2014 and 2015, both ecotypes saw similar increases (ca. 4.5 Mg/ha) in productivity, with both ecotypes displaying similar gains within sites. In contrast to the 2012 drought, June 2015 was the wettest month on record for much of southern Lower Michigan. Both ecotypes were significantly negatively affected by higher levels of precipitation, in both loam and sandy soils. The strikingly similar responses observed, in light of the wide range of soil and precipitation conditions, raises many interesting questions regarding the future range of this species and the historical divergence of its ecotypes.