Evolution and Subsection Classification of Section Cerris Oaks

A study by Thomas Denk and colleagues published in April 2023 in the Annals of Botany explores the evolutionary and biogeographical history of the oak species in section Cerris. This relatively small section includes 15 species distributed across Eurasia, from the Atlantic coast of Portugal and Morocco to the Pacific Coast of Japan. They are remarkable in that their leaf morphology is highly variable, ranging from deeply lobed leaves of Quercus cerris to the dentate margins of Q. macrolepis and the finely toothed leaves of Q. acutissima. The study traces the migration of these oaks from their origin in northern East Asia across the continent to Western Europe, and describes how their adaptations evolved over millions of years. A greater understanding of the phylogeny of these trees allows us to see into the past but also illuminates our knowledge of our ecosystem and its biodiversity.

The team of researchers employed cutting-edge techniques to decipher the history of section Cerris. To come up with a dated phylogeny of the species in this section and the closely related section Ilex, they used what is known as RAD-seq, or Restriction site-Associated DNA sequencing. In simple terms, this method involves identifying specific locations in an organism’s DNA, called restriction sites, and then sequencing the DNA fragments adjacent to these sites. This technique helps researchers study genetic variations within populations, track evolutionary relationships, and investigate traits related to specific regions of the genome. Dr. Denk and his team combined this procedure with D-statistics, short for allele frequency differentiation statistics, which are measures used in population genetics to understand how genetic variations are distributed among different populations or groups of individuals. This allowed them to explore hypotheses related to gene flow. They also estimated divergence times using a model based on fossilized birth–death, which is used to estimate the rates of speciation (birth of new species) and extinction (death of species) over time. To calibrate the model they used 47 oak fossils. In addition, they analyzed modern species’ climatic and biotic niches using the Köppen Climate Classification System, which classifies climates based on temperature and precipitation patterns.

The researchers determined that section Cerris oaks initially branched apart into East Asian and Western Eurasian lineages around 40 to 35 million years ago, in the geological epoch known as the Eocene (defined as approximately 56 to 33.9 million years ago). Later, four Western Eurasian lineages evolved during the Oligocene (33.9 to 23 million years ago) and Miocene (23 to 5.3 million years ago). The evolution of leaf size, shape, and texture seems to be related in part to the various transitions from humid temperate climates to Mediterranean, arid, and continental climates. As a result, species that shared the same ecology developed similar leaf traits, despite being distantly related.

The study shows that section Cerris oaks, which originated from temperate biomes in the Eocene to Oligocene periods, were restricted to higher (i.e., northern) latitudes, from Siberia to north of the Paratethys Sea, which was a large inland sea that existed in the region that is now southeastern Europe. Meanwhile, the section Ilex oaks, which also originated in temperate biomes, were able to migrate during the Eocene to the south and southwest into what was then subtropical southern China and southeastern Tibet. Later they moved west along the pre-Himalayan mountain ranges that existed at the time.

In their conclusion, the researchers propose that this divergence may be related to the evolutionary legacy of the evergreen section Ilex and the (semi-)deciduous section Cerris, resulting from their differential early range expansions from Northeast Asia. Section Cerris’s shift to deciduous or semi-deciduous leaves would have preadapted the lineage to the dry and cold climates it encountered in its westward expansion. Section Ilex, on the other hand, did not lose its evergreen character and did not need to adopt deciduousness while colonizing winter-dry and -cold habitats in the Himalayas and warm, summer-dry environments in the Mediterranean region. Western Eurasia became the meeting ground for three lineages: section Cerris from Northeast Asia, section Ilex from Southeast Asia, and the eastern North American oaks of section Quercus, which joined the party about 10–20 million years ago. These legacies would explain why species from these sections co-occur in the Mediterranean climate of Western Eurasia and how their distributions follow environmental and climatic gradients in the wider Mediterranean region.

The paper also introduces a new common name for the section Cerris species, which they refer to as “Cork Oaks”, equivalent to “White Oaks” for section Quercus and “Red Oaks” for section Lobatae. It is interesting that they propose the common name of Q. suber to be applied to the section, rather than that of Q. cerris (Turkey oak), from which the section takes its scientific name. On the other hand, “Turkey Oaks” does not seem a plausible name for this group, and it is true that other species in the section also have corky bark, notable Q. variabilis, known as Chinese cork oak. Section Ilex oaks are referred to in this paper as “Holly Oaks”, a term that has been in use for a few years now. Using common names for sections is surely a misguided choice, as it is likely to cause confusion. The majority of oak sections have been given a common name that is the same as one of the species in the section, so it is not clear, for example, if the term “cork oaks” refers to the species in section Cerris or a group of Q. suber trees. (The two exceptions are Ring-cupped Oaks for section Cyclobalanopsis and Golden-cupped Oaks for section Protobalanus.) In IOS publications we try to distinguish the meaning by using upper case when referring to the section and lower case when referring to a species, but surely it would be simpler to stick to scientific names for sections.

The study also puts forward a subsectional classification of the species in section Cerris, which builds on previous classifications proposed by Ørsted (1871), Schwarz (1936), Camus (1936–1954), and Menitsky (1984). The species are grouped into five subsections, four of which are found in Western Eurasia. The three species found in East Asia (Q. acutissima, Q. variabilis, and Q. chenii), are grouped in subsection Campylolepides, a name that was coined by Aimée Camus and refers to the recurved scales on the cupules. It is derived from Ancient Greek καμπύλος (kampúlos, “bent, curved”) + λεπίς (lepís, “scale”). Subsection Suber includes Q. suber and Q. crenata, commonly considered a hybrid between Q. suber and Q. cerris.² Subsection Aegilops comprises the Southeastern European/West Asian species Q. brantii, Q. ithaburensis, and Q. macrolepis. Three other species that had been grouped with this last group in previous classifications are here separated in the new subsection Libani: Q. afares, Q. libani, and Q. trojana. Finally, subsection Cerris consists of Q. cerris, Q, look, Q. castaneifolia, and the recently recognized species Q. euboica (with subsection Libani morphology but genetically closer to subsection Cerris).

For more detail, read the article:

Denk, T., G.W. Grimm, A.L. Hipp, J.M. Bouchal,  E.-D. Schulze, and M.C. Simeone. 2023. “Niche evolution in a northern temperate tree lineage: biogeographical legacies in cork oaks (Quercus section Cerris),” Annals of Botany, 131(5), pp. 769–787. Available at: https://doi.org/10.1093/aob/mcad032.

¹ Regarded by many as a hybrid between Q. cerris and Q. suber, for which the correct name is Q. ×hispanica.

² For which the correct name is Q. ×hispanica.