It’s no coincidence that oaks appear so prominently in premodern European mythology. Thousands of species are known to live among their branches and in their bark; their nuts are foundational to forest food webs. A centuries-old oak is practically an ecosystem unto itself, and though people no longer worship beneath their boughs, oaks remain appreciated and even beloved.
Across the world, however, this magnificent genus is afflicted by a combination of pests, pathogens, and climate change. The precise causes of oak declines are often unknown, but the challenge is clear: to nourish oaks and the nonhumans who rely upon them through an uncertain future.
“I grew up without elms in southern England due to Dutch elm disease but accepted it as ‘normal,’” says ecologist Ruth Mitchell. “The current generation are likely to grow up without many ash trees and will accept this as ‘normal.’ Will future generations grow up without the oak and accept this as normal?”
Mitchell works at the James Hutton Institute, located in the United Kingdom—a nation that prides itself on having more ancient oaks than the rest of Europe combined. In some parts of the U.K., up to 80 percent of oaks now show signs of decline. Not all will die, but future forests could be radically different and diminished.
In a study published in Biological Conservation, Mitchell and colleagues help prepare for that future by assembling a database of plant and animal species associated with Quercus robur and Quercus petraea, the two oaks native to the United Kingdom. They tabulated 2300 species altogether—including 716 lichens, 108 fungi, 229 bryophytes, 1089 invertebrates, 31 birds, and 38 mammals—of which some 326 live only on oaks.
The rest may use other kinds of trees, but the researchers found that no single tree species supports more than a small fraction of oak-associated species. Preventing oak decline from triggering a biodiversity collapse may thus require people to manage woodlands so that oaks are replaced by a diverse mix of trees—alders and birch, rowan and holly, common ash and wych elm, and on and on.
That will require a hands-on effort. “Due to historical management many of our woods are artificially poor in the diversity of tree species found,” says Mitchell. And while research is being conducted on disease-resistant oaks, the challenge is more complicated than simply finding hardy varieties.
Diseases killing British oaks are often symptomatic of deeper, still hazily-understood problems. Climate change adds yet more complications. “When we are unsure of why the trees are declining in health or if it is due to climate change, resilient trees may not be the answer,” Mitchell says. “We may need to think about a more diverse woodland with a wider selection of tree species.”
In addition to managing woodlands for diversity, people can also encourage the next generation of oaks to grow. For reasons unrelated to disease and climate, that’s not now happening—something that’s often attributed to browsing by deer and other herbivores, but may better be blamed on the loss of historical management practices that created openings in mature canopies.
People could also pay more attention to the importance of dead and decaying wood. Of the 2300 species found on oak, nearly 1000 use deadwood. Yet “there is a risk that dead trees, particularly if they are perceived to be a health and safety risk, will be felled and the dead wood removed,” says Mitchell. “It is important to leave dead wood. In the short-term, tree diseases will make new habitat for a range of species.”
Apart from scientific insights, protecting what oaks remain and mitigating the consequences of their loss will require time and resources. Last year British conservation organizations and government agencies founded the Action Oak Partnership to spur this commitment—and, perhaps, set an example for conserving other trees imperiled at a time of fast-spreading pathogens and fast-changing weather.
“If we can’t raise public awareness of oak, a much-loved, iconic tree species,” Mitchell says, “then we can’t do it for other tree species.”
Source: Mitchell et al. “Collapsing foundations: The ecology of the British oak, implications of its decline and mitigation options.” Biological Conservation, 2019.
Image: London Road
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.