Natural hybridization between Populusalba L. and the native aspens in southeastern Michigan
The objective of the study was to determine the abundance, pattern of occurrence, and ecology of natural hybrids between the introduced European white poplar, Populusalba L., and the native North American aspens, Populusgrandidentata Michx. and P. tremuloides Michx., in southeastern Michigan. A systematic search was made for P. alba and its hybrids in parts of Washtenaw, Livingston, and Jackson counties. Twenty-seven P. alba clones were located; all were female. Forty-one hybrids between P. alba and P. grandidentata and two hybrids between P. alba and P. tremuloides were located. The mean density of all hybrids was one per 898 ha. Eighty-one percent of the hybrids occurred in groups of two or more and 91% were located within 1.6 km of a P. alba tree. Hybrids occurred exclusively on disturbed sites and across a wide range of moisture regimes. Most hybrids became established between 1930 and 1945, a time period when many farms were abandoned in southeastern Michigan. No effective phenological barrier to continued hybridization or backcrossing was found. The absence of hybrid establishment in the last 30 years was attributed to reduction of suitable seed germination sites in the vicinities of P. alba trees due to reduced human activity and progressing plant succession.