The mighty oak tree is an invader from the North, like the sometimes fearsome Vikings commemorated in Led Zeppelin’s ‘Immigrant Song’. North American oaks trees evolved first in the temperate zone, diversified into red and white oak groups, and then moved south into Mexico. There, oak species diversified further, with more than 150 extant species now in that region.
This may be news to many working out just how trees can move at all. They don’t uproot and walk around, but spread through seed dispersal and then environmental changes can force populations to move around. an ice age with walls of ice moving south force living things to move south. And as populations move and environments change, DNA can change, geographies of populations can separate, and new species arise.
A team of scientists used genome sequencing technology plus herbaria databases of oak species from all over North America to investigate the natural history of oak trees (genus Quercus).
The genus Quercus
Oaks are both ecologically and economically important species. They also can have iconic status culturally (old oak trees, Sherwood forest, people can be described as “solid as an oak”, etc). Oak trees presently dominate forests in the East and they also exist in the west and Mexico. They define ecosystems, and are more dominant in the East after the chestnut blight destroyed nearly all American chestnut trees (the chestnut is in a different branch of the same family as oaks).
Oaks exist across the Northern land masses of Earth. From their origin in Eastern Asia, they spread. Evolving around 100 million years ago (give or take a few tens of millions of years), in the middle of the Cretaceous period, relatively soon after the first flowering plants evolved (around 130 million years ago). Oaks are flowering plants (369,000 species strong, the most of any type of plant), largely wind pollinated and feature the seed pod known as an acorn. Likely all of these features helped the genus spread and diversify globally.
With the evolution of small mammals like squirrels and birds, oak trees had their seeds dispersed. Squirrels cache a lot of seeds and remember their caches to save and eat later. But they forget or never need a lot of their seed cache locations, thus allowing oaks to start the next generation. And being long lived, oaks can produce a lot of acorns in their potential centuries of life, meaning that at least some of an individuals seeds will survive and thrive.
In the New Phytologist survey of North American oak trees ($) from Andrew Hipp et al., the researchers sequenced parts of the genomes of several hundred oak tree samples throughout the US and Mexico. They relied on herbaria specimens collected in specific locations of sampled species, underscoring what a great resource herbaria are for plant scientists to know what was growing where and when. After all, life is tied to geography, as this research shows.
A non-intuitive center of diversity
Nikolai Vavilov, the Russian scientists who hypothesized that finding the centers of diversity of plant species as an indication of where their evolutionary origins lay may well have concluded that oak trees originated in what is now modern-day Mexico and spread world-wide from there (He used his found centers of diversity to find desirable crop traits in natural populations, like disease resistance or heat tolerance).
DNA sequences, something Vavilov could not have known about, however, tells a different story. The 2nd most diverse hotspot of oak trees is in China, hinting at the true Quercus origin. From there, taking advantage of the continents being connected at the time, diversified and migrated to North America, with a population slowly diversifying with each leap into a new range.
Eventually, colder weather in the North came and the oaks living too far north would have perished, leaving the new frontier of present day southern Canada and The United States as American Oak territory, with a broken link to their past. The ice age erased the crossing of the land-bridge. Cold and ice are not conducive to plant life.
The researchers found their DNA sequencing data supported a split of oaks into red and white happening in temperate North America. Those groups then migrated and diversified into the West and to Mexico via the present-day southeast US. Once in Mexico, diversification along climactic gradients happened, with a lot of new places to occupy. Quercus has one oak species living as far south as Panama that has a giant acorn written about by In Defense of Plants. The tropics tend to be biodiversity hotspots on Earth. And it is where a lot of species richness originates. The story of the American oak is just not one of them.
References
Hipp, A. L., Manos, P. S., González-Rodríguez, A., Hahn, M., Kaproth, M., McVay, J. D., Avalos, S. V. and Cavender-Bares, J. (2017), Sympatric parallel diversification of major oak clades in the Americas and the origins of Mexican species diversity. New Phytol. doi: 10.1111/nph.14773
The Quiet Branches 