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Little Oaks Help Answer Big Questions

In a faraway field behind the scenes at The Morton Arboretum is a patch of tiny bur oak trees, few of them shoulder high. They don’t look much like the stalwart, craggy-barked old bur oaks that can be seen at the Arboretum and in natural areas, parks, and streets around the Chicago area, but these little trees have a big purpose: to help scientists figure out how oak trees’ genetic variation influences how they adapt to different circumstances.

“This project informs our understanding of how oaks and forests will adapt to climate change,” said Andrew Hipp, who leads the project at the Arboretum. He is director of the Arboretum’s Herbarium and its senior scientist in plant systematics.

This oak patch is part of a common garden–a type of experiment in which scientists plant a variety of populations or species collected from the wild into different environments to see how well they grow. Common garden experiments, in conjunction with genetic analysis, can reveal how adaptable plant species are.

Since bur oaks are native from the Gulf of Mexico north to Manitoba, they already grow in a wide range of climates. Is this possible because every bur oak has a wide range of genetic possibilities? Or is it because trees in each particular location  have evolved to suit that particular location?

Do we need to move bur oak acorns that are native to one place to another place to help the species adapt to a changing climate, Hipp wonders. “Or will they adapt rapidly enough on their own?”

The patch of little oaks at the Arboretum is one of three fields of trees in which the research team is asking these questions. Partners at the University of Minnesota in St. Paul and the University of Oklahoma in Stillwater have planted their own plots as parts of the bur oak common garden. Each plot includes about 1,000 trees, grown from acorns collected from across the three regions, so that oaks from Minnesota and Oklahoma are growing at the Arboretum and vice versa. Systematics Lab staff members Mira Garner and Lindsey Worcester led staff and volunteers to do the plantings in 2021 and the spring of 2023.

The trees got a bit of coddling (in the form of water) for their first two years. Now they’re on their own, and Hipp and his team are watching.

How fast do the trees grow? How many branches and leaves do they develop? How efficiently do they use water? How well do they withstand cold temperatures or weeks of drought? Do trees that are closely related grow more similarly than trees that are more genetically different? Will the trees from the open expanses of Oklahoma, with a climate that has blazing, 100-degree-plus summers, bitter winter blizzards, and tornadoes, be tougher than the trees from a relatively more moderate Chicago? Or are all bur oaks just tough?

An important feature of this experiment is that it will investigate the trees’ relationships with the fungi that live on their roots, as well as the insects and other animals that feed on them. “I see this as a tool for understanding why we have the oaks that we do, and for understanding how it is that oaks disproportionately shape the forest community,” Hipp says.

As with all tree questions, these will take time to answer, because trees grow slowly. Although the project’s initial funding is for five years, he hopes to see these trees grow for a much longer time so they can help answer many more questions over a span of years.

The Morton Arboretum is particularly well adapted to do long-term tree research, Hipp says, because it has time and space to invest. “We have an institutional commitment to research on trees and plants, which allows our work to extend far beyond a short-term grant cycle,” he says. Arboretum research is supported not just by the staff of the Center for Tree Science, but by the entire institution with its greenhouses and expert horticultural staff, and by dedicated donors who believe in its scientific mission.

Oaks are a major focus of Arboretum research because they are a keystone species for ecosystems in the Midwest and many other places around the world. They support many other plant and animal species and tend to dominate forests where they grow. Bur oaks were chosen for this project, Hipp says, because they have such a wide geographic and ecological range.

The common garden is part of a much larger investigation into many aspects of genetic variation in oaks, funded by a five-year grant from the National Science Foundation and the National Natural Science Foundation of China. The Arboretum is leading the larger project, with collaborators from the University of Minnesota, the University of Oklahoma, Duke University, the U.S. Geological Survey’s Fort Collins Science Center, and the U.S. National Phenology Network. Partners in Asia include the Institute of Botany and the South China Botanical Garden of the Chinese Academy of Sciences and the College of Forestry of Beijing Forest University.

The larger investigation is pursuing such questions as how oaks share genes between species by hybridizing; how their gene-swapping affects the wildlife and other plants that depend on them, including their roots’ relationships with fungi in the soil; and whether the flow of genes between species helps oaks adapt to climate extremes.

Oaks in general are known to have a high level of genetic diversity, meaning that there are substantial genetic differences between individual trees of the same species as well as between species. More diversity generally means more opportunity to adapt. That gives Hipp hope for their future.

“If we throw unexpected situations at oaks, we do have good reason to believe they might be able to survive,” he said. The little trees that are opening their leaves to the sun in the Arboretum’s baby bur oak patch are working to provide information that may help people make sure of it.