Abstract
Tree hybrids are commonly reported and described but little is known about their actual abundance compared with that of their parents. The hybrid between bigtooth aspen (Populus grandidentata Michaux) and trembling aspen (Populus tremuloides Michaux), Populus χsmithii Boivin, is one of the most common spontaneous woody plant hybrids. The purpose of the study was to determine the relative abundance of the hybrid and parents in three geographic areas of Michigan. In southeastern Michigan, 1.5% of all aspen clones were hybrids, whereas in northern Lower Michigan and in western Upper Michigan the proportions were only 0.04 and 0.05%, respectively. Bigtooth aspen is most common in northern Lower Michigan, trembling aspen is more abundant in the Upper Peninsula than in northern Lower Michigan, and both parents are relatively infrequent in southeastern Michigan. Hybrids are typically disease prone and short-lived, and both parents and hybrids are being replaced in southeastern Michigan by longer-lived species. Thus, the opportunity for gene flow between the parent species continues to decline.