Black maple Acer nigrum Michx.f. and sugar maple Acer saccharum Marsh. largely overlap in distribution, have similar floral morphology, and are known to hybridize freely. They are most often distinguished by their leaf morphology, yet the literature has relied heavily on qualitative rather than quantitative measures of leaf shape and pubescence. This study uses quantitative morphometric techniques to explore morphological variation in leaf shape, pubescence, buds, and stipules in the two taxa and their putative hybrids. 870 individual trees were sampled in allopatric A. nigrum populations in Iowa, allopatric A. saccharum populations in Ontario, and mixed, sympatric populations in Ontario. Leaf outlines were captured using Morphosys and analyzed using three separate measurement suites, on their own, and in combination with pubescence counts, stipule length and bud characteristics. The results of Principle Components Analysis and Discriminant Functions Analysis show that the measurement suite devised by Anderson and Hubricht (1938) is unable to provide clear separation of the taxa. The other measurement suites (e.g. truss measurements, and combinations of area, perimeter, and selected measurements) successfully sort the individuals into four groups: Acer nigrum, Acer saccharum and two classes of intermediates. We conclude that despite the occurrence of hybridization A. nigrum is maintaining itself as a distinct entity even in sympatry with A. saccharum. We note that in addition to the morphometric contrasts between the two species there are also a number of diagnostic field marks for each one that will prove useful in identification.

Key words: Acer nigrum, Acer saccharum, Aceraceae, hybridization, morphometrics