Hybrid aspen performance and genetic gains.

The aspen hybrid breeding program has been active in the Lake States since 1955. This study summarizes long-term performance of aspen hybrids in six locations in Wisconsin and Upper Michigan. Interspecific hybrids of native bigtooth aspen and quaking aspen with other poplar species show improved growth when compared with native aspen species across a range of site quality. Quaking aspen hybrids with Populus tremula (XT-Ta) were the most promising based on growth, wood quality, and disease susceptibility. Individual tree volume growth for the XT-Ta hybrids was mote than double that of progeny of pure quaking aspen parents (XT). Other a,•7•en hybrids with P. davidiana, P. alba, and P. canescens parentage exhibited special growth potential on relatively poor and dry-sites, but were susceptible to a bronze leaf disease. HybrMization was more effective in improving aspen growth than breeding within pure species. Hybrid superiority was consistent across a range of site indexes, and showed the g reatest potential on high-quality sites. Genetic gains for the XTTa hybrids, based on ! 5-yr height measurements, ranged from 29 to 34% across sites. Gain in volume growth is expected to be over 100%. Short-rotation (20-yr) commercial plantings with aspen hybrids are recommended on lowto medium quality northern hardwood sites. North. J. Appl. For. 10(3): 117-122. A cooperative aspen improvement program w s initiated in the Lake States in 1955 at the Institute of Paper Chemistry with support from the pulp and paper industry. Early work in aspen improvement emphasized selection, hybridization, and polyploidy (Einspahr and Winton 1976). Considerable progress has been made in improving aspen growth rate, disease resistance, and wood/fiber quality (Einspahr et al. 1970, Einspahr et al. 1979, Einspahr 1984). This program is currently located at the University of Minnesota and is supported by 17 members representing major forest industry and governmental gencies from six states and one province in Canada. As aspen utilization continues to increase and the Lake States and Northeast forestland base continues to decline, breeding fast-growing aspen hybrids becomes more important as an alternative to help alleviate anticipated future aspen shortfalls. A new initiative was recently made to accelerate the current aspen breeding program and develop anew breeding strategy for longterm aspen improvement for the aspen cooperative breeding program at the University of Minnesota. To develop an effective breeding strategy, we need to summarize what we know about past hybrid aspen performance. Information about hybrid performance on different quality sites as compared to intensive breeding within native species is critical in determining the best strategy for long-term aspen improvement. Such information is also critical in guiding planting programs to maximize genetic gains on different sites. Over the aspen breeding program' shistory, two major North American native aspen species, quaking aspen and bigtooth aspen, were crossed with other aspen species. Early successful crossing and rapid growth resulted in extensive breeding and testing work. Field trials of these hybrid combinations have been established on different sites in the Lake States since 1955. In this paper we summarize the field performance data to answer the following questions: What is the growth difference between native aspen species and hybrids (why hybridization)? Which are the most promising aspen hybrid combinations? Can intensive breeding within native aspen species result in gains similar to those from interspecific hybridization? What is the expected genetic gain for hybrid aspen over unimproved native aspen species on sites of difl•rent quality? Material and Methods Sixteen field trials with at least 15-yr measurement data were used in the analysis. Field trials were located on different quality sites in six locations in Wisconsin and Upper Michigan (Figure 1 ). Soil texture for these test sites ranged from sand to clay, with aspen site indexes of 52 to 72 ft (Table 1). The genetic materials in field tests included open-pollinated progeny of phenotypically selected native quaking aspen trees