1. Department of Agronomy & Horticulture; University of Nebraska-Lincoln; Lincoln NE 68583 U.S.A.
2. U.S. Geological Survey, Nebraska Cooperative Fish and Wildlife Research Unit, School of Natural Resources; University of Nebraska; Lincoln NE 68583 U.S.A.
3. Department of Forestry; University of Missouri; Columbia MO 65411 U.S.A.