It hit 43 below up north. Here's why that's good for our trees.

It’s been bitterly cold at times in much of Minnesota this winter. And that’s a good thing. If you like our trees.

Temperatures plunged to negative 40 degrees at many locations in northern Minnesota Sunday morning.

Magic number: negative 40

So why are temperatures of negative 40 so important up north? I’ve talked with University of Minnesota forestry biologist Lee Frelich about that several times.

Beetle mortality is complicated and depends on seasonal timing, snow cover and other factors. But as many as 95 percent or more of pine bark beetle larvae are killed at temperatures of negative 40 degrees and colder.

Sunday was not the first time this winter we’ve recorded 40 below zero up north. The multiple occurrences of negative 40 or colder are even more effective at keeping tree-killing pests at bay.

Emerald ash borer

I wrote in 2014 why mortality rates for the destructive emerald ash borer larvae are also high in extreme cold like this winter. We hit negative 17 degrees at the Minneapolis-St. Paul International Airport this winter on Jan. 7. But most suburbs and locations in southern Minnesota have dipped to 20 below zero or colder.

That’s good enough to wipe out about 70 percent of emerald ahs borer larvae, according to a U.S. Forest Service study.

And with several nights of temperatures at or below negative 30 degrees up north this winter, as many as 99 percent of emerald ash borer larvae likely perished.

So the extreme cold this winter has likely wiped out a high percentage of pine bark beetle and emerald ash borer larvae. That’s great news for our trees and forests. It puts far less stress on the trees and reduces the risk of large die-off areas and more severe forest fires.

It’s a silver lining of a Minnesota winter that we can celebrate the extreme cold nights we’ve endured this winter in Minnesota for the benefit of our trees.