An unprecedented outbreak of destructive beetles has killed nearly half the tamarack trees in Minnesota, and foresters say thousands of acres that have succumbed may never recover, endangering the broader ecosystem in northern parts of the state.
As a result, the Minnesota Department of Natural Resources (DNR) is trying to accelerate tamarack timber auctions in hopes that some of the remaining trees can be harvested while they are still standing and the wood is useful. Harvesting affected trees before it’s too late could also protect the fragile ecology of the forests.
Because there is little demand for tamarack wood, the state is expected to sell just a fraction of the timber rights it has made available during its first full year of an expanded harvest, said Kristen Bergstrand, timber utilization and marketing consultant for the DNR.
“Most of it will most likely go unsold,” Bergstrand said.
Nevertheless, state foresters hope to press ahead, because if the mature tamaracks can be cleared before they are overwhelmed by the eastern larch beetle, younger saplings and other tree species can start to grow up to replace them.
Early studies from the DNR and the University of Minnesota have shown that when tamaracks are just left for the beetle and decimated where they stand, young saplings that are left behind are less likely to survive. By one estimate, they may survive only at a rate of 100 or so new trees per acre. That’s low enough to risk deforestation in many of the state’s large bogs and lowland woods, where tamaracks represent up to 90% of the tree cover.
On the other hand, when the trees can be cleared by foresters and reseeded before the beetle begins killing them off, they can replenish with about 800 to 1,000 new saplings per acre, well within the bounds of a healthy young forest, said Paul Dubuque, a DNR silviculturist.
But even if tamarack saplings can take root, it may just be a matter of time before the voracious beetle wipes the species out of much of Minnesota.
The risk: Warming climate
The outbreak of eastern larch beetles began in 2001 and shows no signs of slowing down. Nearly 500,000 acres of tamarack forests — 780 square miles — have been destroyed or affected over the past 18 years, leaving about 500,000 acres intact.
Unlike other destructive tree pests — such as the invasive emerald ash borer, which has the potential to kill every ash tree in Minnesota, and the invasive mountain pine beetle, which has already destroyed hundreds of millions of acres of forests in the western United States and Canada — the eastern larch beetle is native to Minnesota and can be found everywhere that tamaracks grow.
Throughout history, the beetles never posed a grave threat to tamaracks where they feed and lay their eggs, unless the trees were already stressed by floods, storms or disease.
“There is more than a century of data on the eastern larch beetle that basically says the best way to solve an outbreak is to close your eyes or turn around, because by the time you turn back it won’t be a problem anymore,” said Brian Aukema, forest entomologist at the U.
The difference now, Aukema said, is the warming climate.
The beetles have historically emerged from underground in the spring, while a tree’s roots are still frozen, to gnaw into the bark and lay their eggs. The larvae grow and chew their way out of the tree in the late summer, march down the trunk and bury themselves underground to survive the winter and mature. By springtime, the beetles are fully developed and can fly, and gnaw back into the bark to lay their eggs.
But starting in 2001 and 2002, Minnesota’s springs, summers and falls became just warm and long enough to give the young beetles a little more time to develop in the first cycle of their lives. They can now fully mature and start to fly before winter arrives. They emerge from one tree and fly off to attack another in the same season.
Tamaracks aren’t strong enough to survive two generations of beetles in one year.
“Climate change is unlocking some secrets,” Aukema said. “It’s showing us that as things warm, the tamarack doesn’t belong this far south.”
Hard to replace
Tamaracks play a unique role in Minnesota. They’re the only pine tree to lose their needles in the winter, after they turn a bright yellow in the fall. They are one of the cold-hardiest species in North America, surviving as far north as the tundra line near the Arctic. In Minnesota, they typically grow in bogs, swamps and other areas too wet for most trees to survive, making them extremely difficult to replace, Dubuque said.
As tamaracks die off in large numbers, the water table rises in these already damp areas. Scientists fear that their loss will essentially flood out new trees before they can take root, turning acres upon acres of forests into open wet meadows or alder swamps with little tree cover, Dubuque said.
Young trees at risk
While foresters note that saplings are taking root and some regeneration is occurring after the beetle causes large die-offs, it is unclear how long those young tamaracks will survive. The beetles have typically ignored young trees in favor of chewing into larger mature tamaracks.
“The big concern is that once those mature tamaracks are gone, what will happen when the beetles start attacking smaller trees?” Dubuque said. “Will it hit these newly regenerated stands?”
With help from the U and the U.S. Forest Service, the DNR is looking for tree species that could either replace tamaracks or mix with them so there is enough diversity that entire forests won’t fall with the death of one species.
“We just don’t want to lose these sites to swamping,” Dubuque said.