Morphological variation in white and black spruce: investigation of natural hybridization between Picea glauca and P. mariana

Abstract

To investigate the possibility that white spruce (Picea glauca (Moench) Voss) and black spruce (P. mariana (Mill.) B.S.P.) produce natural hybrids, a morphological analysis of cone and twig characters was performed on 41 white and black spruce trees from three sites in northwestern Ontario and on the Rosendahl spruce from Cromwell, Minnesota. For each tree 13 quantitative and 10 qualitative characters were determined, and the diagnostic ability of all characters was tested by frequency histograms. Scatter diagrams and two hybrid indices were constructed; one index was based only on quantitative data left in continuous form, while the second index was based on all diagnostic characters. Scatter diagrams consistently separated the two taxa, and both hybrid indices clearly showed a bimodal distribution of trees coinciding with species boundaries and not correlating with site or environmental factors. Some spruce trees were intermediate in certain respects, but no tree exhibited a pattern of consistent intermediacy. These results strongly suggest that(1) no hybrids or introgressed spruces were sampled from northwestern Ontario and (2) the Rosendahl spruce is not an F1 hybrid between white and black spruce, contrary to several reports, but rather is a white spruce. Earlier conclusions that white and black spruce successfully hybridize in nature apparently have resulted from the use of nondiagnostic characters, underestimation of the variation present in both species, and a priori weighting inherent in the methods used to calculate conventional hybrid indices.