The Oak Colonization of North America

Oaks are such an important part of the temperate forest ecosystem that it’s hard to imagine they originally evolved near the arctic circle.  During the Eocene about 45 million years ago the earth was mostly tropical and sea levels were much higher than they are today.  There were no ice caps, and climate at the poles was warm and temperate.  Nevertheless, for almost half the year the sun didn’t rise near the arctic circle, just as today night is nearly 6 months long in places like Alaska.  Seasonal darkness led to the evolution of deciduous trees that saved energy by dropping their leaves during winter when the sun didn’t rise.  This adaptation became a great advantage when worldwide climate cooled.  Deciduous trees pushed south because they were able to survive dormant cool seasons that began to occur during the start of the Oligocene ~33 million years ago.  Deciduous trees, especially oaks, replaced tropical species incapable of coping with winter frosts.  Deciduous trees didn’t waste energy with unnecessary growth during winter.

Evidence of the ancient forests where oaks originated exists near the arctic circle at a site known as Axel Heiberg Forest.  Today, this site is a polar desert, but wind erosion is gradually uncovering the forests that existed here 46 million years ago.  A series of floods, perhaps 1 every 10,000 years, covered these forests in sediment, so there are layers of tree stumps, roots, and fallen logs continuously being revealed, as winds strip the sediment away.  Sediment covered the forests rapidly during these catastrophic floods.  It is not a petrified forest because the geological conditions did not favor fossilization.  So once exposed to air, the ancient wood begins to decay, though the process is slow in cold arid conditions.  Scientists think the environment was a warm seasonal rain forest.  Tree composition consisted of dawn redwood, Chinese cypress, hemlock, pine, spruce, larch, gingko, and extinct species of birch, alder, sycamore, walnut, hickory, and oak.

Location of Axel Heiberg forest–site of the oldest subfossil remains of oaks. Today, it is a polar desert, but during the Eocene it was a temperate seasonal rain forest.

Subfossil wood from Axel Heiberg forest.

Comparison between white oak leaves (top) and red oak leaves (bottom).  White oaks and red oaks ecologically complement each other and colonized North America at the same time.

Oaks are classified into 2 groups–red oaks and white oaks.  Genetic evidence suggests red oaks diverged from white oaks about 33 million years ago when they both began to colonize latitudes south of the arctic circle.  Red oaks produce crops of bitter acorns every other year, while white oaks produce more palatable acorns annually.  The strategic difference in acorn production is an ancient ecological balance, attracting squirrels and other seed distributors equally.  Genetic evidence also shows eastern red and white oaks are sister species to western red and white oaks.  Mexican oaks are sister species to eastern oaks, having diverged between 10-20 million years ago.  Oaks colonized eastern and western North America at the same time, then later eastern oaks invaded Mexico.

Mexico has more species of  oaks than any other region in the world (154 species).  If a region has more species of a genus, it usually is thought to be the region where that genus originated.  Instead, scientists believe Mexico has a greater number of oaks species because of differences in elevation in mountains closer to the equator.  Mexican mountains host many different ecological niches causing frequent speciation among oaks.  This explains why Mexico is home to more species of oaks than any other region in the world, though it is not where they originated.

Reference:

Hipp, Andrew; et. al.

“Sympatric Parallel Diversification of Major Oak Clades in the Americas and the Origin of Mexican Species Diversity”

New Phytologist September 2017

Tags: Axel Heiberg Forest, oak colonization of North America