Oaks are among the most remarkable plant groups on the face of the planet, and they dominate North American forests. Their global distribution bears the fingerprint of millennia of spread, while their physical and genetic traits offer remarkable insights into both evolution and ecology. Long-lived, abundant and diverse—more than 400 species occupy a spectrum of habitats across five continents—these trees hold remarkable clues to how organisms spread and evolve over time, forming dynamic communities in which the various components both compete with and complement each other.
“Oaks make a good model system for integrating ecology and evolution because they’re ecologically relevant and there’s so much data available about them,” says Jeannine Cavender-Bares, a professor in the Department of Ecology and Evolutionary Biology who has spent much of her professional life studying these ubiquitous trees. With the recent addition of extensive genomic data, she says, “we now have much of what we need to ask questions we couldn’t ask before to understand how whole ecosystems came to be.”
In a review paper published in New Phytologist, Cavender-Bares examines how oaks came to dominate North American forests, and summarizes what we already have learned—and hope to yet learn—from this large and diverse group of organisms by studying them simultaneously through the lenses of ecology and evolutionary biology.
One area in which oaks have yielded important clues is in how different species come together to create forest communities packed with diverse oaks. Originating at high northern latitudes, North American oaks spread south tens of millions of years ago, evolving into several main lines in the process, two of which are highly diverse and extend throughout the continent on both sides of the Rockies, into Mexico and throughout Mesoamerica. As they spread, those two lines converged in form and function, so today white oaks and red oaks are often found in the same habitats right next to each other—with remarkably similar traits that evolved independently. By studying how the two lineages function somewhat differently within the same places, researchers have gained valuable clues into how species live together without stepping on each other’s toes (or roots, as the case may be).
“There aren’t many instances where we can see how a whole biome was assembled to create the ecosystems that we see around us today with such clarity both on the ecological and on the evolutionary side,” Cavender-Bares says.
Oaks also have shed light on how evolution works in long-lived organisms. Rather than evolving to thrive in current conditions, as is the case for bacteria and other creatures that measure their lifespan in months or minutes, oaks experience selection as they sprout, with seedlings surviving (or not) depending on that year’s circumstances. The result, combined with the fact that oaks are able to borrow genes from close relatives and that characteristics such as the drought sensitivity of leaves can vary from year to year depending on conditions, is a forest of adult trees with a diverse range of survival skill sets—diverse enough for at least some of their offspring to take whatever nature throws their way.
The information oaks are yielding, Cavender-Bares says, is useful for better understanding the theoretical underpinnings of how evolution and ecology intertwine to shape real-world communities. It also offers valuable insights for predicting and managing how species and communities are likely to respond to future changes. That in turn can help us help them thrive and even re-assemble communities that have been lost.
“If you want to forecast the future, you have to take the past into account,” she says. “Important insights emerge from considering the evolutionary history that led to how species assembled into communities to create ecosystems.” -Mary Hoff