Lee Frelich, director of the Center for Forest Ecology at the University of Minnesota, inspected a doomed green ash in Loring Park. (Susan Du, Star Tribune/The Minnesota Star Tribune)

A 300-year-old bur oak known as the "Queen of the Hill" split and crashed down in the Loring Park neighborhood of Minneapolis earlier this summer. Because it had been enormous, growing in a place of distinction atop a hill and surrounded by a circle of benches, its death left a gaping vacancy.

"The tree's strength, perseverance, beauty, majesty got me through COVID and long winters," said Loring Park resident Marty Jones. "It sheltered homeless folk sitting or lying on the benches ... Of course, many squirrels and birds made their home in her."

Soon after, a second bur oak in the park died. The neighborhood association's board chair, Lee Frelich, counted the rings and estimated it could have been 200 years old.

Frelich is also director of the Center for Forest Ecology at the University of Minnesota, and when a third tree — a mature green ash — began to die as well, he implicated the Twin Cities' sustained drought. Enduring for two summers, the drought was elevated Thursday from "severe" to "extreme" by the U.S. Drought Monitor.

In dry summers, the city asks property owners to help water newly planted saplings. The death of mature trees in prolonged drought is less talked about because there's no easy way to water the old ones, whose crowns measure 50 feet and roots stretch twice that distance in every direction, said Frelich.

Walking through Loring Park on Thursday evening, the ecologist pointed out several more centenarian trees of drought-resistant species that were exhibiting ailing brown crowns — a troubling departure from the norm in autumn, when leaves are supposed to turn yellow and fall.

Their symptoms don't look like bur oak blight or oak wilt, Frelich said. "And because the ones that are dying are all on south- and west-facing slopes and it's the worst drought we've had in a lifetime, that's got to be the cause."

The Park Board didn't conduct any kind of autopsy on the Loring Park trees to determine an exact cause of death, said Forestry Director Ralph Sievert. He agrees it wasn't fungal disease, but said it's hard to pin the blame on a single factor.

Park trees suffer from compaction of the soil beneath the tread of visitors who flock to events. Development disturbs roots. The urban heat island, created by impermeable surfaces that absorb sunlight, is another source of stress. All these things in addition to drought could weaken old trees which, like people, decline in vitality with age.

"It's actually unusual to get a tree to live hundreds of years in an urban area. That's almost like an exception," Sievert said.

State Climatologists Pete Boulay and Kenny Blumenfeld were together in their office Tuesday, bouncing theories on the puzzle of the dying trees.

The Twin Cities experienced sustained droughts coupled with high temperatures during the Dust Bowl years of the early 1930s, and again in the late 1980s. Mature trees died then as well.

"The basic bottom line is we had a record dry September. This is the time of the year we should start seeing soil moisture get replenished," Boulay said. "If you look at a stream hydrograph across the state, they're normally starting to go up this time of year. This is the time we have the full recharge set in, when you have plants not drawing as much moisture so more water goes into the ground, goes into streams. And so far, we're missing that."

Blumenfeld added that while last summer registered the highest-recorded daily mean temperature, both 2021 and 2022 summers had particularly high daily minimum temperatures, meaning warm nights. With records dating back to 1873, eight of the 10 summers with the warmest nights in the Twin Cities have occurred since the year 2000.

Extreme fluctuation in precipitation could be another factor. The 2010s were the Twin Cities' wettest decade on record. The aggressively dry start to this decade, coming off a time of excessive rainfall, might pose a shock to trees, Boulay speculated.

Val Cervenka, forest health program consultant with the DNR, said drought conditions in rural Minnesota for two years in a row have coincided with an uptick in opportunistic pests like the twolined chestnut borer, which lives peacefully with oaks until the trees become stressed from either not enough or too much water. "Then watch out!" she said.

"Once the trees' defenses are down, twolined chestnut borer populations build up inside the tree and kill it within a relatively short amount of time."

Another pest, the eastern larch beetle, has dispatched hundreds of thousands of acres of mature tamarack over the past 22 years, Cervenka said. The beetles have generated once per year for thousands of years, but are now able to complete two generations per year as a result of longer periods of warm weather, thanks to climate change.

Bur oak is one of Minnesota's most flexible species, able to tolerate a wider range of wet and dry conditions, added DNR forest health specialist Brian Schwingle. Yet "historically, notable oak death would occur the year after the first severe drought, and drastically increase if there was a significant drought the subsequent year."

At the northern edge of Loring Park, abutting a statue of Norwegian violinist Ole Bull, is a sentinel tree with the gnarled bark and byzantine branches that can only be found on very old bur oaks.

"You'd have to wait 200 years to get another tree with branch patterns like that," said Frelich.