Phylogeny

The following cladogram summarizes the relationships that Denk et al. used to draw up their 2017 classification:^[1]

Section Lobatae

Quercus sect. Lobatae was established by John Claudius Loudon in 1830.^[3] The section, or part of it, has also been treated under names including Quercus sect. Rubrae Loudon and Quercus sect. Erythrobalanus Spach. It has also been treated as the subgenus Erythrobalanus and as the full genus Erythrobalanus (Spach) O.Schwarz. Its members may be called red oaks.^[1]

The perianth of the pistillate flowers has a characteristic flange-like shape. The staminate flowers have up to six stamens. The stalk connecting the perianth to the ovary is cone-shaped and often has rings. The acorns mature in two years, rarely in one year. The 'cup' (cupule) of the acorn is fused with its stalk (peduncule) forming a connective piece. Both the connective piece and the cup are covered with small triangular scales, mostly thin and membranous with broadly angled tips. The leaves typically have teeth with bristle-like extensions, or just bristles in leaves without teeth.^[1]

The section contains about 125 species native to Northern America (including Mexico), Central America, and Colombia in South America.^[1] The red oaks of Mexico are one of the groups of oaks that have most rapidly diversified into different species. Molecular evidence suggests that there are significant numbers of undescribed Quercus species in Mexico, so the number of known species in the section is likely to be an underestimate of the total diversity.^[4]

• 
  Quercus castanea leaves and acorns
• 
  Quercus coccinea
• 
  Leaves of Quercus ellipsoidalis
• 
  Quercus myrtifolia growing as a shrub
• 
  Quercus rubra in autumn (in cultivation)

Section Protobalanus

Quercus sect. Protobalanus was first established as a subgenus by William Trelease in 1922 and then later treated as a section by Otto Karl Anton Schwarz in 1936 and Aimée Antoinette Camus in 1938.^{[1][note 1]}

The staminate flowers have eight to ten stamens. The pollen ornamentation has only small wrinkles or folds (verrucae). The acorns mature after two years. The cup (cupule) at the base of the acorn has triangular scales that are fused at the base and have sharp angled tips. The scales are thick and compressed into rings, often forming small bumps, that may be obscured by glandular hairs. The leaf teeth end in spines.^[1]

The section contains only five species, native to southwestern North America and northwestern Mexico.^[1]

• 
  Quercus cedrosensis in habitat in Mexico
• 
  Quercus chrysolepis leaves showing spines
• 
  Quercus chrysolepis acorns
• 
  Quercus tomentella in habitat on Santa Rosa Island, California
• 
  Quercus tomentella acorn

Section Ponticae

Quercus sect. Ponticae was first established by Boris Stefanoff in 1930. It has also been treated as a subsection and a series, including under the name Q. ser. Sadlerianae Trelease.^[1]

Species are shrubs or small trees, with rhizomes. The staminate catkins are up to 10 cm long. The cup (cupule) at the base of the acorn has scales with sharp angled ends. The leaves are either evergreen or deciduous, with simple or compound teeth. The leaf buds are large, enclosed in loosely attached scales.^[1]

There are only two species, Quercus pontica and Quercus sadleriana.^[5] They have disjoint distributions. Quercus pontica is native to mountainous areas of north-eastern Turkey and western Georgia. Quercus sadleriana is native to northern-most California and southern-most Oregon in the United States.^[1]

• 
  Quercus pontica in habitat
• 
  Quercus sadleriana in cultivation

Section Virentes

Quercus sect. Virentes was first established by John Claudius Loudon in 1838. It has also been treated as a series.^[1] Members of the section may be called live oaks.^[6]

Species are trees or rhizomatous shrubs. They are evergreen or brevideciduous. The acorns mature in a year. The cup at the base of the acorn has narrowly triangular scales, with thin keels, at most small bumps (tubercules), and sharp angled ends. The leaves are evergreen or almost so. A distinctive feature of the section is that the germinating seed has fused seed leaves (cotyledons) and an elongated stem above the cotyledons (the epicotyl) that forms a tube, while the stem below the cotyledons (the hypocotyl) is tuberous.^[1]

The section contains seven species, native to south-eastern Northern America, Mexico, the West Indies (Cuba), and Central America.^[1]

Section Quercus

Quercus sect. Quercus has been known, either in whole or part, by a variety of names in the past, including Quercus sect. Albae, Quercus sect. Macrocarpae and Quercus sect. Mesobalanus. Members of the section may be called white oaks. The section includes all white oaks from North America (treated by Trelease as subgenus Leucobalanus).^[1]

The staminate flowers have seven or more stamens. The acorns mature in one year. The seed leaves (cotyledons) are either free or fused together. The cup at the base of the acorn has thickened triangular scales that are either free or fused at the base and have sharp angled tips. The scales have keels and are often covered with small bumps (tuberculate). The leaf teeth typically do not have either bristle-like or spiny tips.^[1]

There are about 150 species, native to Northern America, Mexico, Central America, western Eurasia, East Asia, and North Africa.^[1]

• 
  Large Quercus alba growing in New Jersey
• 
  Quercus alba catkins (staminate or 'male' flowers)
• 
  Quercus berberidifolia in habitat in California
• 
  Illustration of Quercus lusitanica showing staminate (left) and pistillate flowers (top right)
• 
  Quercus montana in Pennsylvania
• 
  Quercus petraea in England, about 300 years old
• 
  New leaves and pistillate ('female') flowers of Quercus robur
• 
  Quercus robur leaves and acorns