GRAND RAPIDS — The winter of 2023 — well, it’s been a whopper.
Wildlife manager Mark Spoden of the Minnesota Department of Natural Resources said this winter ranks as particularly severe — and this is an objective take, based on the winter severity index. Spoden appeared on the Tuesday, March 28, KAXE Morning Show to explain how the index measures the intensity of seasonal pressures on deer populations with a point system.
Snow depths of 15 inches or greater hinder their movements, and sustained cold temperatures sap their energy reserves. This winter has been particularly severe.
“If your grandpa says, ‘Oh, the snow was a lot deeper when I was a kid, as I walked to school uphill both ways,’ you could say, ‘Well, Grandpa, that might not be true. We’re setting snow depth records now,’” Spoden said.
How winter severity is measured
The winter severity index is comprised of two major components: temperature and snow depth.
“We start it on November 1 and it runs until May 31,” Spoden explained. “We record one point each day that the temperature is 0 degrees or lower, and then we record one point each day when snow depths are 15 inches or greater.”
These points accumulate throughout the winter. Winters with less than 50 accumulated points are considered mild, while years with 51-119 points are more typical. A year with 120 or more accumulated points is considered severe.
So far, this year’s total is 135 at the Forest Service’s North Central Research station, which qualifies 2023 as a severe winter. Of those 135 accumulated points, 98 come from snow depth and 37 are attributed to temperature. KAXE phenology expert John Latimer’s severity index, which is based on his phenological observations, also suggest this is a particularly severe winter.
Listen to the full KAXE Morning Show segment above!
Effect on deer populations
Due to the difficulty in navigating deep snows, deer congregate in deer yards or snow shelter areas. Spoden has tracked deer traveling up to 30 miles to reach these locations, where large numbers of deer gather to wait out the winter. In mild winters, deer can move about more easily and spread across much larger areas.
Temperatures also play a large role in their survival. Warm days and cold nights cause a thick crust to form on the snow, enabling the deer to access a wider area. A thin crust, however, is dangerous for deer: a wolf may be able to stay on top of a thin crust, while the deer punches through.
Delayed emergence of spring vegetation places considerable strains on pregnant does. If they lose their fawn, however, it comes with a silver lining: without a child to nurse and raise, the doe is able to expend that energy foraging and putting on fat in preparation for the next winter. Spoden’s research indicates these childless does fare considerably better than their counterparts during the subsequent winter.
To learn more, visit:
dnr.state.mn.us/mammals/deer/management/wsi.html
