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Plant Focus

Quercus skinneri
Quercus skinneri is a Central American oak, distinguished by the large size of its fruit.

Ascent of the Oaks

Scientific American Cover

An article by IOS members Andrew Hipp, Paul Manos, and Jeannine Cavender-Bares made the cover of Scientific American this month. It recounts how advances in genomics have allowed researchers to reconstruct the evolutionary history of oaks and show how they came to dominate the forests of North America. These findings may help predict how oaks will fare in the face of climate change, and thus they will have implications for managing oaks to ensure their survival as the planet warms.

In clear, fluid prose, easily accessible to the layperson, the authors guide the reader through the latest discoveries regarding the history of oaks’ diversification and spread through the Northern Hemisphere. Classification of oaks is complicated by variation within species—and hybridization between them. While previously researchers were able to discern the overall branching structure of the oaks’ tree of life, using only sequences of DNA from chloroplasts and a few nuclear genes, since 2008 new molecular techniques have allowed them to infer oak evolutionary history. Restriction-site associated DNA sequencing is being used to read short regions of DNA, and then statistical methods are employed to reconstruct the order in which species have branched from common ancestors.

It is not certain when or where oaks arose, but the first evidence of their existence are fossilized pollen grains dated to about 56 million years ago, found near what is now Salzburg, Austria. This would have been part of the northern section of the landmass comprising North America and Eurasia, which in the early Eocene were joined by land bridges spanning the Atlantic and Pacific Oceans. Soon the genus separated into two major branches. Then, as global temperatures started to descend, oaks moved south from the land bridges and the geographical division between the two clades became defined: subgenus Cerris would be limited to Eurasia and North Africa, and subgenus Quercus would be limited largely to the Americas (but for two branches that also show up in Eurasia).

In North America, ecological opportunities arose as the tropical forest and broad-leaved (as opposed to needle-leaved) evergreen forests that dominated the continent 56 million years ago were pushed south or driven to extinction by decreasing temperatures. Red and White Oaks moved south into the spaces that opened up, splitting into two lineages either side of the Rocky Mountains. They would in time form the oaks of California and the Pacific Northwest in the west, and in the east the oaks of Eastern North America, which in turn would split into three main branches: a northeastern lineage, a southeastern lineage, and a primarily Texan lineage. Between 10 and 20 million years ago, probably via the Texas lineage, the oaks moved into Mexico.

March of the Oaks

These oaks were cold-adapted, so as they migrated south to Mexico they climbed to higher elevations, encountering topographic variation and different levels of water availability. Populations became ecologically distinct, limiting movement of genes to within populations rather than between them. As mutations and genetic rearrangements accumulated and distinguished populations, species were born. Speciation rate was high as the Mexican oaks moved into the mountains; this had not been possible in the Rockies as they were not able to survive the combination of short growing seasons and cold winters. The increased diversification of Mexican oaks was duplicated by the fact that Red and White Oaks had separated before moving south, so the two lineages evolved in parallel. This may explain the species richness and abundance of oaks in the Americas.

As Red and White Oaks spread south, they repeatedly solved similar ecological challenges. As a result, we often find them growing together in the same habitats. But closely related oaks within the same lineage are not found together: species transition broadly from one to the next as you move through different ecological spaces—for example, as you hike uphill. In the Chiricahua Mountains of Arizona, drought adaptation separates close relatives along an elevation gradient, while in flatter Florida, soil moisture availability and fire intensity structure oak communities. Across the United States, trees with convergent traits from the Red and White Oak lineages tend to grow together.

Oaks form what is called a syngameon, in which distinctive species persist in spite of ongoing gene flow caused by hybridization. Genes migrating between species might help oaks adapt to novel environments. Understanding when, where, and how oaks became so diverse is crucial to understanding how oaks will resist and adapt to rapidly changing environments. Hybridization was key to the rapid response of oaks when they migrated rapidly as continental glaciers receded starting around 20,000 years ago. Oaks may face threats from fungal and insect diseases if they do not evolve quickly enough to resist them. The challenge now is to figure out how differentiation and the movement of genes between species will influence oak evolution. Perhaps this understanding will help predict what our forests will look like a century from now, and perhaps it can guide our planet to manage longer-term survival of oaks.

You can access the full article, of which of the above is a brief summary, on the Scientific American website. It is behind a paywall, but the article can be purchased individually for a modest fee. Click on the link below:

"How Oak Trees Evolved to Rule the Forests of the Northern Hemisphere", Scientific American, August 2020