Flora of North America species comparison

Quercus robur is one of the oaks most commonly cultivated in temperate and subtropical parts of the world. In North America it is most commonly seen in the eastern and northwestern parts of the United States and and in southeastern and southwestern Canada, where it tolerates a wide array of conditions and is extremely hardy. In Washington, New Brunswick, and Nova Scotia, apparently reproducing populations persist in the wild. Elsewhere, although actual naturalization appears to be rare, Q. robur should be expected to persist around old homesites and other places of cultivation.

Quercus robur most closely resembles our native species Q. alba in leaf form. In contrast with Q. alba, which has relatively long petioles (longer than 10 mm), acute leaf bases, and subsessile fruit (rarely on peduncles to 25 mm), Q. robur is easily distinguished by its shorter petioles (less than 10 mm), cordate, almost clasping, leaf bases, and fruit on long (more than 35 mm), thin peduncles.

Quercus robur is one of the oaks most widely celebrated in literature; it has wood of exceptionally high quality for the manufacture of furniture, and it previously was the most important wood used in the manufacture of wooden sailing vessels in Europe.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Some reports of Quercus prinoides growing in calcareous soils are probably referable to shrub forms of Q. muhlenbergii.

The debate over whether Quercus prinoides is distinct from Q. muhlenbergii has continued for most of the last century. Little doubt can exist that strong genetic differences, as expressed by characteristics of habit, leaf form, and habitat preference, separate the two taxa; the question is merely whether they are best treated as subspecies or varieties or as separate species. Over most of the eastern United States, the two taxa occur sympatrically over broad areas with little immediate contact (syntopy), because Q. muhlenbergii is found on calcareous soils and Q. prinoides occurs on sands (often acidic) and dry shales. Seedlings of Q. prinoides can flower and produce acorns in as few as 3-5 years from planting, when only 20-50 cm, and maintain their dwarf, clonal habit in cultivation. Quercus muhlenbergii begins fruiting as a small tree of 3 m or more in height. Because of these differences, and interdigitating geographic distributions, the two taxa seem to be similar in pattern of variation and interaction to other closely related oak species of eastern North America, and dissimilar in pattern to infraspecific taxa such as we see in Q. sinuata var. sinuata and Q. sinuata var. breviloba. The populations that are difficult to determine are usually small scrubby trees, probably mostly Q. muhlenbergii, stunted because of less than favorable moisture conditions, with or without indications of introgression from Q. prinoides.

Material of Quercus prinoides from Long Island, coastal Massachusetts, Nantucket Island, and Martha's Vineyard has been segregated as Q. prinoides Willdenow var. rufescens Rehder on the basis of vestiture and minor differences in leaf form. These populations have appressed-stellate leaf pubescence abaxially, as throughout the range of the species; in addition they have reddish, erect, fasiculate hairs similar to those found in Q. michauxii. The hairs make the abaxial leaf surface somewhat felty to the touch. Variability in this characteristic and lack of other consistently correlated features preclude taxonomic recognition of Q. prinoides var. rufescens, but this problem is worthy of further investigation.

(Discussion copyrighted by Flora of North America; reprinted with permission.)