The Roots of Western European Cork Trees Can Be Found in East Asia

In a recent study published in the Annals of Botany, Thomas Denk and colleagues decipher the evolutionary and biogeographical history of cork oaks, known scientifically as Quercus section Cerris. This fascinating group of trees comprises 15 species found across Eurasia and is known for its diverse leaf morphologies. Their study uncovers the journey of these trees from their origins in East Asia, their migration to Western Eurasia, and how their various adaptations took shape over millions of years. Understanding the lineage of these trees not only gives us a glimpse into the past but also equips us with vital knowledge about our ecosystem and its biodiversity.

Dr Denk’s team employed cutting-edge techniques to unravel the history of cork oaks. They utilized restriction site-associated DNA sequencing (RAD-seq) to map out a dated phylogeny of cork and holly oaks. This procedure, combined with D-statistics, allowed the researchers to explore hypotheses related to gene flow. They estimated the divergence times using a model based on fossilized birth–death and calibrated it using 47 fossils. Furthermore, they analyzed modern species’ climatic and biotic niches using parameters such as Köppen profiles, bioclimatic parameters, and forest biomes.

The researchers found that cork oaks initially diverged in the Eocene, around 40 to 35 million years ago, with East Asian and Western Eurasian cork oaks branching apart. Following this, four Western Eurasian lineages evolved during the Oligocene and Miocene epochs. Notably, the evolution of leaf size, form, and texture was partially correlated with multiple transitions from humid temperate climates to Mediterranean, arid, and continental climates. This resulted in ecologically similar species converging on similar leaf traits despite being distantly related. 

The researchers found that the cork oaks, originating from temperate biomes in the Eocene to Oligocene periods, had their ranges restricted to higher latitudes, from Siberia to the north of the Paratethys Sea. Meanwhile, the holly oaks, which also originated in temperate biomes, migrated southwards and southwestwards into then subtropical southern China and southeastern Tibet during the Eocene, then moved westwards along existing pre-Himalayan mountain ranges.

Building on fossil records and phylogenetic data, the team’s work provides substantial evidence of the East Asian origins of these important trees and their subsequent westward migration. It paints a vivid picture of how the cork oaks adapted to new climates and environments, evolving a diverse range of leaf traits in the process.

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Denk, T., Grimm, G.W., Hipp, A.L., Bouchal, J.M., Schulze, E.-D. and Simeone, M.C. (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.