Oak trees have long been appreciated for their size, their age and the fine quality of their wood. And that appreciation is clear in the Triangle where oaks tower over residential streets and spread wide on campuses and in parks.
Raleigh is the “City of Oaks,” one of its historic neighborhoods is Oakwood, breezes sway the mosaic of a giant oak tree on the Raleigh Convention Center’s shimmer wall and the capital city marks the new year by having a crane drop a giant acorn.
But for all their ubiquity, oak trees hold within their mighty limbs an element of mystery. Why are they so enduring, so plentiful and so diverse? How did they emerge over millions of years to proliferate across the Southeastern United States? And now a new question: Will climate change threaten their long run of biological success?
Paul Manos, a Duke professor whose research includes a focus on the evolution of oaks, said science is beginning to answer these questions thanks to the power of sequencing parts of the tree’s genome. The technique allows scientists to peer deep into the origins of plants and trace the branches of their evolution.
Manos joined with two of his longtime collaborators, plant biologists Andrew L. Hipp of the Morton Arboretum in Illinois and Jeannine Cavender-Bares of the University of Minnesota, to write a Scientific American article last August with the title: ”Ascent of the oaks: How they evolved to rule the forests of the Northern Hemisphere.”
By combining the information unwound from oak DNA with evidence from fossilized oak pollen, the authors wrote that scientists have been able trace the history of the trees “going back to the root of the oak tree of life.”
“The story has been enhanced,” Manos said, “by knowing where oaks were in the past and looking at how their distribution has changed as the globe has changed in the last 60 million years. It’s like a puzzle, and if you can get enough of these pieces – and the pieces coming from fossils are usually the hardest pieces to get – some of those pieces have really come together.”
As Manos tells it, the oaks’ story began some 56 million years ago in high northern latitudes when North Carolina still had a tropical climate. As the climate grew colder, oaks moved south in two basic types - red oaks and white oaks – taking over land as tropical trees retreated toward the equator. Oaks can be found as far south as Columbia, South America and they proliferate in the higher elevations of Mexico, but the Carolinas’ Piedmont region and coastal plain has the North American continent’s second highest concentration.
The reason oaks thrive here, Manos said, is their ability to adapt to climate conditions and to their particular settings. The reaching ancient live oaks that define Wilmington or Charleston can be nearly as small as bushes when they grow in windswept dunes. And the willow oaks that are common along streets in Raleigh, Durham and Chapel Hill actually evolved in swampy land but “are happy to be stressed out in an urban landscape,” Manos said.
Oaks, as we are about to find out, are wind pollinated. Spring gusts sweep pollen from male oak flowers to female oak flowers that lead to a rain of acorns in the fall. In all there are 265 oak species in the Americas and 435 worldwide. The most common in the Triangle are varieties of red oak – willow oaks, water oaks, darlington oaks and a white oak newcomer, overcup oaks, known by their acorn cap, which covers most of the nut.
Oaks at times pollinate between species, a hybridization that has helped them adapt and survive climate change. With that, they can also survive time. Oaks can live more than 400 years – some in Europe have reached 900 years – though Manos said most of the fast-growing red oaks here in the Triangle start to wear out after a century or so.
Warmer winters and a reduced die-off of insects and fungi could threaten oaks and all plants, Manos said. But the oak’s genetic malleability may help it survive. “The race out there in biology is species are trying to stay one step ahead of pathogens,” Manos said. He added, “Oaks are incredibly diverse in their genome. Maybe that’s giving them a bit of an edge.”
Barnett: 919-829-4512, firstname.lastname@example.org