Distinguishing different species of oak in the forests of eastern North America can be challenging. For one thing, there are simply a lot of different species to sort out. A recent reference (Stein et al. 2003), describes 50 species in the genus Quercusoccurring naturally east of the 100th meridian, and 90 species are distinguished for all of North America north of Mexico (Nixon 1997). With so many species to parse, confident identification requires careful study of leaves, stem and leaf hairiness, and fully mature acorns with their caps. But care is not always enough, because in addition to the identification challenges inherent in any species rich group, many oaks hybridize with closely related species, and hybrids, by their nature, blur what should be clear-cut distinctions between species.
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Hayden, W. John. "Hybrid Oaks: Full of Vexation and Wonder." Bulletin of the Virginia Native Plant Society 30, no. 1 (March 2011): 1, 6-7.