At the northern edge of Eugene T. Mahoney State Park in eastern Nebraska lies a graveyard of dead trees. Many of the remaining nearby oaks are bare with silver crusty patches wrapping like a grip around trunks and branches.
Noah Sundberg, a Nebraska Game and Parks horticulturist who’s cared for Mahoney Park’s trees for several years, first noticed the dieback among these bur oaks in 2023.
“It brought some concern, but we weren't sure what was going on,” Sundberg said. “We thought the oaks could pull out of it.”
The trees’ quickly declining appearances were out of the norm for Nebraska’s towering bur oaks. They’re some of the most common native oaks in the state, and their centuries of resistance to extreme weather and pests have earned them the nickname “King of the Great Plains native hardwoods.” But the once soaring and wide-canopied trees are dying en masse throughout eastern Nebraska’s deciduous forest.
“We’re not talking square feet here. We’re talking acres,” Nebraska Game and Parks horticulturist Richard Whemeyer said. “I believe Ponca State Park has it the worst. We're seeing 70% loss of bur oak canopy – just large dead swaths of oaks on our ridge lines. We lost elms, we lost hackberries, we lost ash, red oaks. We've lost a lot of species, but the species that happen to be very prominent in these areas were the bur oaks.”
Game and Parks horticulturists sent samples off to its diagnostic lab, hoping to uncover a solution that could help save their historic trees.
The results identified a sneaky, tree-killing culprit that wasn’t one known to fatally strike in Nebraska: hypoxylon canker. It’s a disease that’s becoming a larger threat in the Midwest and Great Plains as the region faces multi-year droughts.
When the lab called Whemeyer back, they said he might find more answers down south.
A familiar foe in the South travels to the Midwest
Clarissa Balbalian, a diagnostician and lab manager with Mississippi State University Extension, wasn’t surprised last summer to receive an uptick in calls from neighbors wondering why their oak trees weren’t looking too well. During the last half of 2023, Mississippi suffered from months of “exceptional drought” – the worst classification of drought on the National Drought Mitigation Center’s scale.
The dry conditions took a toll on a number of the southern state’s trees and ushered in a harmful caterpillar species and fungus to team up and feast on them. In years with regular moisture, neither would have left a significant mark on most healthy trees. But the severe drought dealt the trees “a compromised ability to defend themselves against any kind of trauma, including damage from insects and disease,” according to a news release issued last summer by the extension office.
Both tree irritants weren’t new to Mississippi. The caterpillar even arrives in two different generations to swarm the state’s trees during different points of the year. The fungus causes hypoxylon canker, which is a common oak tree killer in Mississippi. It’s known to take over rapidly if a tree’s defenses are knocked down.
“It's very cyclical, because it's closely linked with severe drought,” Balbalian said. “When those conditions ease, we get back into more favorable conditions for the trees, and we stop seeing the problem. Then people forget about it, because usually we're fine for maybe a decade or so, and then we have another really bad couple of years, and it pops up again.”
Hypoxylon canker can attack a variety of tree species, but finds an especially fitting and familiar home in oak trees. There’s no cure or treatment that can save a tree once the fungus spreads and closes in on it. Eventually, hypoxylon canker will cause the tree’s sapwood vessels to collapse, which cuts off water and nutrient movement.
“It'd be like having a kink in a water hose or a tourniquet on your arm,” Balbalian said. “And when enough of that sapwood is compromised, the tree is going to die just from lack of water, essentially.”
The disease is more common in the South following periods of drought. But now several Midwestern states, such as Kansas and Iowa, have recorded widespread hypoxylon canker cases following extremely dry periods.
In its last report on forest insects and diseases issued in 2023, the U.S. Forest Service said climate change is dialing up the effects that complex disturbances, such as fires, flash floods and drought, have on trees across the country. And in return, pests and pathogens have more opportunity to attack.
Following the 2012 drought, University of Illinois forestry and research specialist Chris Evans saw thousands of oaks die.
“We had whole swaths of acres of trees die at times,” Evans said. “It was just constant calls about, ‘Why is my tree dying? What’s wrong with my tree?’”
Before that, Evans said he hadn’t seen such widespread hypoxylon canker cases in southern Illinois. Both forested and urban areas saw oak decline immediately after the 2012 drought and then about six years later, since it can take some time before trees fully succumb to diseases. Nearby Missouri suffered a similar oak decline following that historic drought.
At the time, Evans told concerned property owners to consider replanting with trees that could survive a future with less water.
“Instead of a pin oak, maybe think about a black oak or a white oak,” Evans said. “Or move over to other species in general. Things like black gums, some of the hickories are more resistant as well.”
Changing climate and changing tree canopy
A similar planting experiment is happening in Nebraska City, known as the “Tree City of the World,” where arborists are trying to rebuild canopy cover amid the drought-induced die-offs.
Standing in the town’s Arbor Lodge State Park, Rob Schreiner said he wants to retain the diverse group of trees that attract visitors from across the state.
“Any tree that you want in the state of Nebraska is probably in this park somewhere,” said Schreiner, who runs the tree crew for the city’s utilities department and chairs Nebraska City’s tree board. “We've got them all.”
Schreiner knows just about every tree that’s sprouted here in his hometown. After seeing a number of diseases, bugs and now drought take out several different kinds of Nebraska City’s trees, including ones in his own yard, he’s hoping the newer species he’s planting can better withstand today’s weather.
“I’m doing a lot of experiments now with the Japanese maples,” Schreiner said. “We’re having really good luck with those. They’ve come out with some new hybrid elms, which so far have been out for quite a while and doing well. They’ve built a better tree, basically. They’ve gotten rid of some of the bad characteristics, and they seem to be doing really well.”
The Nebraska Game and Parks horticulturists have several thoughts about what’s next for one of the state’s most popular parks, but it’s still difficult to picture what the land will look like after more large bur oaks are cleared away this winter. And they admit it’s an unfamiliar feeling to not know what’s in store for this area of Nebraska.
“We are getting to see a pretty significant reset to a climaxing system in our own time,” Whemeyer said. “It’s affecting spaces that we were very comfortable with them having stayed the way they were for a very long period of time. It's an adjustment for everybody, not just the folks working on the land, but the folks that come and visit and enjoy it.”
Sundberg and Whemeyer, who both know these fields like their own backyards, believe these oaks didn’t go down without a hefty fight, and their stories may not be completely over. A few months after clearing 75 trees, Sundberg noticed a few resprouts starting from several stumps – sowing some hope that the mighty native Nebraska oaks might one day return.
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.