February 10, 2022
By Dr. James C. Kroll
I am still amazed how much interest in white-tailed deer management has grown since I began my career in 1973! Yet, I am even more amazed how, after thousands of outdoor articles and programs, there remains a general lack of understanding (even among some professionals) as to just how complex deer management really is.
While every property is different, and thus there is no universal management strategy, I learned long ago the management communality is that you must find out what the limiting factor is on your property. Note, that’s singular, as in the No. 1 limiting factor.Perhaps one of the most common, and least understood is winter thermal cover; and that is what I wish to discuss in this column.
Wildlife management developed as an offshoot of agriculture. That is why most of the wildlife management education programs developed in state land grant colleges and universities. In 1840, the Germans were among the most advanced agriculturists, foresters and wildlife managers in the world.
A scientist named Carl Sprengel came up with a theory, later popularized by Justus von Liebig, who said: “Of all the environmental factors affecting the health of the population, the one in least supply was THE limiting factor!” Hence, the job of the agricultural (wildlife) scientist is to determine what the limiting factors are, and which one is most limiting at a specific site!
Early agriculturists discovered the importance of nutrients like nitrogen, phosphorus and potassium in plant growth. Later, it was discovered that you can supply all the nutrients a plant needs, and yet still not see a positive response.
The problem may turn out to be lack or scarcity of a micronutrient such as boron or zinc. Liebig came up with a clever way to illustrate the point. He used a drawing of a wooden barrel, with some staves shorter than others. Some cause the barrel to lose water, but only one (the shortest stave) causes the most damage!
Is Winter Thermal Cover Lacking?
Every study we have conducted in almost five decades, plus those of many other scientists, point to energy being one of the most limiting factors for whitetails. Satisfying energy demands involves two activities: finding foods with high digestible energy, then conserving energy use through times of scarcity. In North American Whitetail’s next issue, I will discuss the importance of digestible energy, but let’s look first at how deer conserve energy.
They do so in two ways: reducing activities that unnecessarily burn up stored fat, and reducing exposure to adverse climatic conditions. That is where cover often becomes critical. There are two types of cover: thermal and escape. And there are two types of thermal cover: summer and winter. Summer cover mostly involves areas with a light overstory of tree canopies, and low understory vegetation allowing the wind to blow through.
Winter thermal cover, on the other hand, is made up of areas with a dense overstory canopy and higher understory vegetation. The tighter canopy reduces the amount of rain or snow reaching the ground during colder times; and the higher understory prevents the cold wind from passing through. Conifers provide the greatest winter thermal cover benefits, while hardwoods do not.
I have worked with whitetails in almost every state where they reside! More often than not, the limiting factor has been winter thermal cover, sometimes even in the South. Yet, I admit the deficiency is much greater in northern states where winters often are harsh. The Great Lakes states once were dominated by vast forests of conifers, until logging and the “Great Fire” completely changed the forest from conifers to hardwoods.
Surprisingly, most residents of these areas think the massive stands of oaks, maple, poplar and hickory are “natural,” but they are not. Yes, there are still some residual white cedar wetlands, but many of these have disappeared under modern forest management.
The change in forest composition led to deer migrating to conifer cover serving as “yarding habitat.” Most Midwest hunters and some biologists view this migration as being the way deer always have been. However, they are wrong.
A few years ago, we undertook a project at the Grand Rack Hunting Club, located in the northern part of the Lower Peninsula of Michigan. The deer herd was suffering from high winter mortality, both from weather and hunting. Working with wildlife manager Adam Osmun and our colleague, Wayne Sitton, we devised a plan to stop (or at least reduce) the migration from the club in mid-winter.
When we began the management program at Grand Rack, we knew exactly how many deer we had on the Club in late December — zero! This annual migration increased mortality, especially in bucks, and it exposed them to increased harvest mortality from hunters using bait.
Upon analysis, we found two things causing the migration: lack of high energy foods and lack of winter cover. We devised a management program that used plantings of clover, chicory and corn in a strategic pattern, each connected by conifer corridors and cover that we strategically planted.
We used faster-growing pines, mixed with slower-growing spruce, to develop new corridors. But we also removed overstory hardwoods where shade-tolerant conifers were the dominant understory. In only four years, we reduced the migration to a trickle, and we significantly increased survival of bucks and fawn recruitment.
One local professional, on learning of the plan, confidently proclaimed that “… the deer would quit migrating when geese flew north for the winter!” Well, I don’t know about the geese, but recruitment has more than doubled!
Give it a Try
This is just one example of improving the productivity and health of a deer herd. So, how can you put this to work on your hunting territory?
First, obtain a recent aerial image of your land. This cannot be any easier today. In general, there are two types of aerial images available: leaf on and leaf off. These terms mean the image was either taken when the leaves were on the trees or when they were off in the winter. With the leaves gone, you can see clearly how much conifer habitat you actually have.
Note, if you have less land, do the same thing and see what the neighbors have to offer the shared deer herd.
Also, look for obvious travel corridors that may be enhanced to connect thermal cover to food sources. The further south you go, the faster conifer plantings grow, making return on your investment in time and money faster than in the North. In northern properties, you may search for young stands of understory conifers you can use to landscape for whitetails.
My rule-of-thumb is for there to be about 20 percent winter thermal cover. Good deer management involves analyzing each property for the things it provides and those it does not. Then you must decide which one is the most limiting and use a little common sense to figure out how to reduce the impact.