Genetic Differences in Height Growth and Survival of Cottonwood

--Sixteen full-sib families from crosses between four fast-growing female and four fast-growing male clones of cottonwood (Populus deltoides Bartr.) were clonally evaluated for 1 year in a replicated field test. Seedling size had no effect on clonal performance. Approximately 70 percent of the genetic variance for height and 20 percent of that for survival were additive. The cloned families survived better and grew taller than 12 Stoneville select clones included for comparison, but this difference may have been due to repeated propagation of the select clones. Additional keyWOrds: Populus deltoides, genetic variances, clonal propagation. The experiment described here was designed to learn how the genetic superiority of fast-growing clones of Populus deltoides Bartr. can be explo,ited through breeding and selection. Four tast-growing female and four f~st~growing male clones were crossed. The resulting 16 full-sib tamilies plus 12 clones selected in the Stoneville cottonwood improvement program were evaluated in a replicated clonal test. Age-l survival and height, two important characters in plantation establishment, were studied. Additive and dominance components of variance were estimated. The relative sizes of these components indicate the probable effectiveness of selecting and breeding for genetic improvement. Effects of seedling size on clonal performance were determined, and performances of the full-sib tamilies and of the Stoneville select clones were compared. MATERIALS AND METHODS The parents were four female and four male clones, including the five clones with most rapid diameter growth through age 6 from a study by Mohn and Randall (1971) and three of the 14 Stoneville select clones described by Mohn et ale (1970). All originated from within 50 miles of Stoneville, Mississippi. The parents were crossed in all possible combinations to produce 16 full-sib families. Seeds were planted in small peat pots in June 1970 and seedlings were transferred to the nursery at 12by 40-inch spacing after 1 month. They were cut back to near ground level during February 1971 and allowed to regrow for 1 more year. II Plant Geneticist, Institute of Forest Genetics, Southern Forest Experiment Station, USDA Forest Service, stationed at Stoneville, Mississippi. Y Associate Silviculturist, Southern Hardwoods Laboratory, which is maintained by the Southern Forest Experiment Station in cooperation with the Mississippi Agricultural and Forestry Experiment Station and the Southern Hardwood Forest Res~arch Group.