About > The Origins of Oaks

By Professor Richard Jensen,Saint Mary's College, Notre Dame, Indiana, USA

 

The oaks (Family Fagaceae, genus Quercus) belong to an old lineage of trees and shrubs that dates back to at least the Late Cretaceous (about 85 million years ago). The oldest fossils are most closely aligned with Fagus, the beeches, but there are also suggestions of chestnuts (Castanea) in some of these ancient materials. These fossils are known, to date, only from Georgia, in the south eastern United States. When it comes to the origin of the oaks (Quercus spp.), there is much better evidence. The fossil record reveals that trees similar to oaks first appear about 32-35 million years ago, and trees related to extant species appear by about 25 million years ago. By about 23 million years ago, trees representative of most major groups of oaks have appeared.

Oaks are characterized by their fruits – acorns. In botanical terms, the acorn consists of two components: a nut and a cupule. The nut is derived from the ovary of a female flower. The cupule is derived from a series of scales, or bracts, that are found at the base of the female flower. Both the cupule and the nut have distinctive features that allow us to readily identify trees that belong to Quercus. However, there is another group of plants in Fagaceae that also produce acorns, or acorn-like fruits. The stone oaks (genus Lithocarpus) are native to south east Asia, with one allied species, Notholithocarpus densiflorus (the tan oak) occurring in the western United States. Interestingly, the acorn of the stone oaks appears to be analogous to, not homologous with, the acorn of Quercus. That is, despite their superficial similarities, these acorns have separate evolutionary origins.

Taxonomists have dealt with the morphological diversity of acorn-bearing plants in a variety of ways. Historically, all acorn-bearing plants were classified in the genus Quercus, the classical Latin name for oak. This practice was followed, more or less uniformly, until the late 1800s. By that time, several botanists had recognized that the stone oaks, usually identified as a subgenus or section of Quercus, comprised a separate genus, Lithocarpus (= Pasania). Despite the acknowledgement that the stone oaks represent a separate genus, the so called 'true oaks' remain a quite diverse assemblage of plants. Most of us recognize the more-or-less typical lobed leaves of English oak (Q. robur), white oak (Q. alba), northern red oak (Q. rubra) and black oak (Q. velutina). However, for many of us the toothed leaves of cork oak (Q. suber), chestnut oak (Q. montana) or sawtooth oak (Q. acutissima) seem a little strange and the entire leaves of shingle oak (Q. imbricaria), willow oak (Q. phellos) and interior live oak (Q. wislizeni) are simply odd for an oak.

Because of the great diversity of leaf form, as well as clear differences in cupule features, the oaks have been divided into many taxonomic groups. As a prelude to this, the reader needs to know that, in the taxonomic hierarchy, Quercus is a genus. The species within a genus may be divided into additional groups based on a variety of characteristics. Typically, a genus may be divided into two or more subgenera and a subgenus may be divided into groups referred to as sections. If there are distinctly different groups among the species of a section, these may be organized into smaller sets called series. The oaks in Quercus have been organized into all of these taxonomic categories by the various botanists who have studied them and tried to classify them. Thus, species that are very similar may be grouped into a series, similar series will belong to the same section, and two or more sections may belong to a single subgenus.

A classic example of such a classification may be seen in William Trelease's taxonomy of the American oaks published in the mid 1920s. Trelease recognized three subgenera of Quercus in the New World, Leucobalanus (the white and chestnut oaks), Erythrobalanus (the red and black oaks) and Protobalanus (the intermediate oaks), as well as three additional subgenera for the oaks of the Old World: Cerris, Cyclobalanopsis and Heterobalanus. Although he did not recognize sections within each subgenus, Trelease did divide the American oaks into over 150 series, most consisting of only one or two morphologically quite similar species.

Some of the groups Trelease recognized were a reflection of earlier classifications. One of the more important was that of Anders Oersted, a Danish botanist who recognized five subgenera in Quercus. In addition, Oersted also created a new genus, Cyclobalanopsis, for those cycle-cup oaks (the cup scales, instead of overlapping as do roof shingles, form concentric rings around the central axis) that were most closely related to the true oaks. The French botanist Aimée Camus was the last person to try to classify the oaks on a world-wide basis. In her monograph (published during the 1930s), she recognized two subgenera of oaks, Cyclobalanopsis and Euquercus, the latter being divided into six sections: Cerris, Mesobalanus, Lepidobalanus (= Trelease's Leucobalanus in part), Macrobalanus, Protobalanus and Erythrobalanus.

More recently, Kevin Nixon (Cornell University) conducted careful studies to evaluate the status of these taxonomic groups as well as the correct taxonomic names for each. Nixon concluded that Quercus consists of two subgenera, Quercus and Cyclobalanopsis. Further, Quercus subgenus Quercus was divided into four sections: Quercus, Lobatae, Protobalanus and Cerris. Nixon's classification has been accepted by most students of Quercus. However, in their recent taxonomic treatments of Fagaceae in the Flora of China and the Flora Reipublicae Popularis Sinicae, C.C.Chang and colleagues have followed Oersted's classification by treating Cyclobalanopsis as a genus.

The advent of modern molecular biology has provided valuable insights into plant relationships and Paul Manos (a student of Nixon) and his students have drawn some very important conclusions from their studies of DNA patterns among the Fagaceae. These studies (Paul Manos, Alice Stanford, Charles Cannon and Sang-Hun Oh) have revealed that, in an evolutionary sense, the stone oaks and the true oaks are, indeed, separate groups and their classification into different genera is well-justified. What may be more interesting for those of us familiar with the true oaks is that the two primary groups of oaks recognized by Camus and Nixon (although given different names, Camus's subgenus Euquercus and Nixon's subgenus Quercus include the same species of Quercus) turn out not to be good taxonomic groups. Specifically, Manos and his students have shown that section Cerris should not be included in subgenus Quercus. Rather, the true oaks are divided into two primary groups: one group consists of the traditional subgenus Cyclobalanopsis together with section Cerris; the other group comprising the remaining species. Thus, this molecular work suggests that Cyclobalanopsis is not a separate genus and that Quercus subgenus Quercus is not a monophyletic group.

This latest finding has yet to be reflected in nomenclatural changes. However, it shows that the evolution of the true oaks is a fascinating and complex story. As we continue to learn about these most important trees, there will certainly be additional changes to the current taxonomic classification. Do not be surprised if our view of what is really an oak undergoes some significant changes in the near future.