Short rotation woody crop trials for energy production in north central U.S.

Tree plantations at several sites have numerous clones with heights greater than 45 feet and diameters of 6+ inches in eight years. The fastest growth rates have been attained in a plantation on a wet site at Milaca, MN, a plantation at Granite Falls, WI, and a plantation at Mondovi, WI, where the largest trees are up to 8 inches DBH at age 8. Mean annual production ranges from 4 to 5+ dry tons per acre in the best clonal blocks, and up to 8.1 tons per acre for the best new hybrids. Reduced growth at some sites was related primarily to insufficient soil water during the growing season, and susceptibility to the disease Septoria musiva. Most tree mortality (36 percent) occurred during the establishment year with only an additional 2 percent mortality over the next 7 years. Leaf tissue nitrogen (N) levels decreased as trees aged and approached the hypothesized 3 percent critical level as trees reached 5- and 6-years old. Fertilization at 75 and 150 lbs/acre N resulted in significant increases in leaf tissue. However, no significant increase in tree growth has been detected. There are significant clonal differences in leaf tissue nitrogen. Hybrid poplar plantations planted on agricultural fields produce significant increases in soil carbon, although there may be carbon loss during the early years of plantation establishment. Septoria musiva is the major pathogen affecting survival and growth of hybrid poplar plantations. A collection of 859 Septoria musiva and Septoria populicola isolates has shown considerably variability in the microorganism. Tissue culture techniques are being used to increase resistance to Septoria in clone NE-308. Over 200 generation 2 plants are ready for field testing in 1995.