1. Research Fellow, Geophysical Institute, Univ. of Alaska Fairbanks, 903 Koyukuk Dr., Fairbanks, AK 99775-7320 (corresponding author).
2. Professor, Dept. of Bioproducts and Biosystems Engineering, Univ. of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108-6005.