Winter Thermal Cover
Thankfully, we're finally coming to the end of winter! But, it has been a long and harsh one, and it's undoubtedly taken a toll on our deer herds. Whitetail deer need to maintain a toasty 104 degree (F) body temperature. Being highly adaptive critters, they have no serious difficulty regulating that temperature, under “normal” conditions. But, when winter conditions turn from normal to extreme, the chore of keeping their internal boiler stoked becomes even more physically taxing. For many hunters, it’s a frustrating reality that their hunting property doesn’t have the habitat elements deer need when those extreme conditions hit. So, when late season comes and the red falls out of the bottom of the thermometer, their whitetail paradise is deserted for sake of survival, leaving those hunters scratching their heads and eating their tags. You’ve heard their desperate cries, before. “Where did the deer go?” The answer is - Thermal Cover.
This south-facing slope, covered with tall grasses and dotted with spruces, provides all the necessary elements of quality thermal cover, on a small western IL property.
What’s all the hype about?
Thermal cover is a habitat type, or component, specifically sought out by deer (and other wildlife) to gain relief from temperature extremes, so they can regulate body temperature with greater energy efficiency. In this case, our focus is on whitetails and the thermal cover they utilize to reduce the stress they endure during times of sub-zero degree temperature. There’s more than one type of winter thermal cover, and deer use them each under slightly different conditions, where they are present. Here’s an overview of the two most common types where I live, in the Midwest.
Cedar, pine, hemlock and spruce are the most common conifers throughout the whitetails range. In Illinois, Iowa and northern Missouri, eastern red cedars are by far the most common naturally occurring. White pines and blue, Colorado blue and Norway spruce are, also, commonly found in cultivated stands. You don’t have to have a PhD in thermodynamics to understand how this all works, so let’s keep this analysis simple. What makes a conifer stand inherently better at providing thermal relief is the canopy of dark-colored needles, which readily absorbs infrared energy from the sun, while an open winter-time canopy of hard or softwood deciduous timber allows the sun’s rays to reflect back up into the atmosphere; especially, when there’s snow cover.
Side-by-side satellite imagery of a client’s property in eastern IA. (Left) An infrared photo illustrates the prominent heat signature (deep red patches) of eastern red cedar and white pine stands, which are notably warmer during winter. (Right) Notice in this fall, true color photo how difficult it is to identify those late season bedding hot-spots.
Air currents, slowed by a dense conifer canopy, are reduced from sharply biting gusts with polar wind chills, to more moderate and endurable breezes. The warmth collected by the canopy from a daily dose of solar radiation is trapped beneath it and released, slowly, through the overnight hours, offering deer the benefits of convection.
So, how big does a stand need to be to do the job? This question is no different than asking, “What’s the right size for a food plot?”. The answer depends upon what else is available, in the surrounding area, and how many deer per square mile you have. In Minnesota and northern Wisconsin, deer often migrate 2-10 miles to get to good winter cover, and large numbers congregate in big sections of pine forest. In the heart land, thermal cover is not typically available in such size, but the conditions can be just as harsh, at times. So, small stands are highly attractive in these areas and will help you hold and protect many local deer, on your property, when established in strategic locations.
This 3 acre stand of 10 year old conifers is located on a south-facing slope, on a client’s farm in IL. With moderate deer density, four strategically located stands this size offer sufficient thermal cover for every deer on this 640 acre section, during extreme winter weather.
The second habitat type that serves as excellent thermal cover in the midwest is a stand of native warm season grasses. “Prairie grasses”, as they’re often called, offer slightly different benefits than conifers, but serve the same general purpose. They offer quality security cover, and can help deer conserve valuable energy. Because the sun rides a little lower in the sky in the winter, topography plays a slightly more important role in selecting optimal sites for establishment of prairie stands, than it does with conifers. That’s because the biggest benefit of grasses is that they offer outstanding bedding, with full exposure to the sun’s rays, during daytime hours. No other cover type does that. But, the benefits aren’t exclusive to daytime, if the stand is positioned on a south-facing slope, because the ground is warmed by the solar energy and will continue to offer a thermal advantage over colder ground, in the timber, throughout the night.
A thick, tall stand of warm season grasses is best for thermal protection. If that’s the type of cover you want to establish, stick to the more robust species, such as: switch grass, Indian grass, and big blue stem. Some of the ‘experts’ advocate switch grass monocultures, while others say the diversity of a mixed stand is better. I believe both have their place. If you have limited open acreage to work with, and need optimal security/thermal cover, a stand of pure switch grass is king. Alamo, Kanlow, and Cave In Rock are known as three top biomass producing varieties, in Illinois. But, managers should research which varieties are best suited for their soils and climate, for optimal results. If you have more acreage to work with, a mixed stand may provide greater year-round benefit. Both provide necessary thermal cover.
On the Burns' home farm, this mixed stand of switch grass, Indian grass, and big blue stem, located on a sunny hillside near a large late-season destination plot, is littered with deer beds, each winter.
In the northern half of the US and in Canada, winter survival for whitetails depends largely upon two things: caloric rationing, and evading predators. A lot of calories are required to maintain adequate body temp, when the air temp is below zero and night time wind chills are -20 or more. The difference of even a couple of degrees can equate to a lot of calories, over the course of the winter. Every degree of warmth matters and finding refuge in quality thermal cover can mean the difference between maintaining enough fat reserve to survive, or becoming too weak to escape predation.
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