The North American Betulapopulifolia differs from the European Betulapendula in having a larger number of peltate resin glands on greyish twigs, leaves with long–caudate tips, and a dense pubescence on the adaxial surface of the pistillate scales. Furthermore, B. populifolia is a plant of acid, peaty, or sandy substrates, whereas B. pendula grows in acid to neutral or occasionally slightly alkaline substrates. The bark of B. pendula exfoliates in long thin strands, whereas in B. populifolia it exfoliates as small, very thin rectangular flakes or appears not to exfoliate at all. The basal parts of older trunks of B. pendula often develop vertical fissures, which never develop in B. populifolia. Analysis of variance suggests that the most useful quantitative characters for distinguishing the two species are scale width and the length/width ratio of the terminal one-quarter of the leaf. Several other quantitative characters are useful discriminators but are not entirely reliable by themselves. Discriminant analyses with and without qualitative characters separated a sample of North American B. populifolia from a sample of European B. pendula. The derived discriminant formula placed many members of a variable natural population from near Ottawa in an intermediate position, suggesting hybridization. The use of heavily weighted qualitative characters in the discriminant formula placed much of the hybrid population in the B. populifolia group, whereas the use of quantitative characters alone placed much of the hybrid population in the B. pendula group. As putative hybrids are being widely distributed by nurseries, the identification of the two species is likely to become increasingly problematic. Betulapendula is a widespread and naturally spreading escape from cultivation in southern Ontario, while B. populifolia is a native species confined in Ontario to the eastern portion of the province between the Ottawa and St. Lawrence rivers, with a possible outlier in Elgin County in southwestern Ontario.
Subspace estimation from unbalanced and incomplete data matrices: ℓ2,∞ statistical guarantees
