Growth and Yield Differences in Pure- Versus Mixed-Family Stands: Six-Year Results From Slash and Loblolly Pine Trials Christina L. Staudhammer, Brian E. Roth, and Luis F. Osorio 1 Sustainable forest management involves using practices that maintain both biodiversity and productivity, while ensuring that future ecological and economic functions are also fulfilled. To create sustained economic yield over a smaller landbase, forest managers in the South deploy elite fast-growing southern pine genotypes. Intensive silvicultural treatments that include site preparation, competition control, fertilization and thinnings accompany these germplasm deployment decisions. Families that show high growth potential and disease resistance in single- tree plot greenhouse and field trials are selected with the belief that final yields of wood fiber per unit area when deployed in pure stands will be representative. Pure plantings of elite single families and clones of southern pines are becoming more common, and represent the focus of much current research. For example, questions related to differences in yield potential, genetic x silvicultural treatment (environment) interactions, and the susceptibility of these “new” generation stands to abiotic and biotic damaging agents are relevant and require designed experiments to answer them. Moreover, decisions about which families to plant in pure stands are often made based on single-tree plot mixed-family planting trials, yet there is not sufficient evidence that mixed-progeny trials correlate well with pure-genotype plantations. In order for trees to produce superior yields, they must efficiently exploit the available site resources while not competing adversely with neighboring trees; however, families chosen for pure plantings are usually those that successfully compete in the mixed-progeny trials. The specific objectives of this project were to investigate the following questions: 1) Do elite families of southern pine yield more when grown in pure or mixed family plantings?, 2) Can estimates of stand level attributes, i.e., diameter at breast height (DBH), basal area (ba), trees per hectare (tph), and diameter distributions in pure family plantings be predicted from single tree mixed family plots?, 3) Does spacing, silvicultural intensity, or genotype influence relationships between pure and mixed plantings?, and 4) How long can stand-level estimates of pure plantings be made from single-tree mixed plots (prior to the onset of inter-genotypic competition? MATERIALS AND METHODS In 2000, the Forest Biology Research Cooperative (FBRC) at the University of Florida installed trials to test the growth and yield effects of mixed versus pure-family plantings. Replicated experimental trials were installed at five sites in the Southeast, controlling for planting densities and levels of silvicultural treatment intensity. Pure- and mixed-stands were planted using six elite families of loblolly and slash pine, resulting in more than 100 plots at each of five locations. Each plot contains 48 measured trees, surrounded by a treated buffer strip. Differences between pure and mixed-family plots were investigated using several measures of tree yield as response variables. These trials were installed as a replicated randomized complete block design, with treatments applied in a manner that approximated a split-plot. Thus, analyses 1 Asst. Professor, Biol. Scientist, and Sr. Biol. Scientist, respectively, School of Forest Res. and Conservation, Univ. of Florida.