Flora of North America species comparison

Quercus engelmannii is closely related to and possibly conspecific with Q. oblongifolia. The cups of Q. engelmannii are larger, deeper, and generally more tuberculate than those of Q. oblongifolia, and the scales are usually larger. Based on available samples, the nuts of Q. engelmannii are consistently larger than those of Q. oblongifolia, apparently with little, if any, overlap in diameter. Considerably more variation occurs within Q. engelmannii in leaf form, possibly reflecting introgression from other white oak species such as Q. cornelius-mulleri, Q. dumosa, and Q. durata (see treatment).

On Catalina Island, Quercus engelmannii is known only from a small grove of trees. Putative hybrids between Q. engelmannii and Q. cornelius-mulleri are common in areas of contact between the two species in Riverside and San Diego counties in southern California. Such a population was the basis for Q. acutidens Torrey [Q. dumosa var. acutidens (Torrey) Wenzig]. Other names applied to those populations are Q. macdonaldii var. elegantula Greene and Q. dumosa var. elegantula (Greene) Jepson. Variable in leaf form and stature, those intermediates form extensive populations and are probably best disposed of under the name Q. ×acutidens.

(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.)