For years, wildlife managers have assumed that warmer winters brought on by climate change would be beneficial for whitetail deer here in northern Minnesota. Long and tough winters have traditionally …
For years, wildlife managers have assumed that warmer winters brought on by climate change would be beneficial for whitetail deer here in northern Minnesota. Long and tough winters have traditionally impacted the survival of deer in the region, so the possibility that winters might be shorter and warmer seemed naturally to benefit a species that has long been on the margins of its range in the region.
But, while the climate is clearly changing, not all of those changes are necessarily helpful to whitetails.
A recent analysis of climate trends by a U.S. Forest Service researcher confirms that winters have warmed significantly in recent decades, which is helpful to whitetail deer. Yet, snowfall has been on the rise as well, at least in the past decade, as milder air is able to hold more water vapor and helps lead to deeper snowfalls than was the norm in the recent past. While the data in the Forest Service analysis is for the Grand Rapids area, similar trends have been playing out across the region, and this winter has proven to be yet another example. All of which may be one reason that the whitetail deer herd has struggled to recover from losses suffered in some recent winters, despite warming temperatures.
For years, the Minnesota Department of Natural Resources has used the winter severity index to gauge whitetail survival. The index is calculated on two factors— the daily low temperature and the daily snow depth. Wildlife managers add a point to the index for each day with a sub-zero overnight low and a point for each day with 15 inches of snow, or more, on the ground.
While both temperature and snow depth are factors in deer survival, not all factors are created equal. “Snow is definitely the biggest factor for deer,” said Tower DNR Area Wildlife Manager Tom Rusch. “Not just depth but duration of deep snow is the biggest factor. And that’s especially toughest on fawns.”
While the average winter in the region is now about three degrees warmer than it was through most of the 20th century, precipitation also appears to be on the rise, which is consistent with long-term forecasts on the likely effects of climate change in northern Minnesota. While the decade of the 2000s was modestly wetter than the long-term average, the 2010s ended as both the wettest and the snowiest decade in the past century, at least according to the Grand Rapids data.
“The years 2013, 2016, 2018, and 2019 all fall within the top 15 years for total annual snowfall,” according to Daniel Roman, with the Forest Service, who helped compile the long-term Grand Rapids weather data. “The record for total annual snowfall is 2013, with 109.5 inches.” Despite the warming temperatures, northern Minnesota remains sufficiently cold to maintain, rather than melt, snow for several months at a time. As more snow falls, the duration of deep snow has been increasing and that appears to be limiting deer survival in the winter.
Rusch said a growing scarcity of winter cover also appears to be playing a role. Winter cover can be especially critical in heavy snow years, since snow depth is frequently much less under a dense canopy of conifers, making it easier for deer to move around. Wildlife managers have been warning for several years that changes in timber management policy in Minnesota, which were designed to shorten rotation ages, will further reduce the available winter cover and put more stress on the whitetail deer population. If the trend toward more snowfall continues, wildlife managers say maintaining adequate winter cover on the landscape becomes all the more critical.
And the debate isn’t simply theoretical. Long-term weather records demonstrate a close correlation with the annual deer harvest. During the warmer and only slightly wetter conditions in the late 1990s and most of the 2000s, northern Minnesota’s deer population soared and the annual harvest peaked in 2003 at 289,421 registrations. By 2014, after a series of snowy winters, the deer harvest had fallen by more than half, to 139,442. While harvest numbers recovered somewhat, hunters have yet to reach the DNR’s annual harvest goal of 200,000 deer since 2010.
Preliminary figures from the 2019 hunt indicate that hunters harvested 181,549 deer this past season. That’s the ninth straight year of harvests below 200,000. By contrast, from 2000-2008, hunters topped 200,000 deer registrations every year and averaged 245,000 over the nine year period.
Since then, increasing amounts of snow have clearly made a difference. Rusch said the combination of large snow events and frequent long durations of deep snow in recent years have provided some of the toughest conditions for deer since the 1960s and 70s, when deer numbers and harvest levels fell sharply.
Back then, an overabundance of old forest, and limited browse, played a contributing role in the decline of the deer herd. The 1980s boom in Minnesota’s wood products industry reversed that trend to the point that wildlife managers say the situation is now reversed. “We’ve tipped the balance,” said Rusch. “Now, we have plenty of browse, but not enough winter cover.”
And that’s coming at a time when the region is experiencing longer periods of deep snow than in the past. Without a change, Rusch predicts any recovery in the whitetail deer population could be painfully slow.