March 31, 2023
By Dr. James C. Kroll
There are two questions professional deer biologists hate to hear: How many deer are there on this property? And; What kind of acorn crop will we have this year?
First, I have learned that accurately estimating the number of deer on any property (large or small) is the “La Brea Tar Pits” of deer management! It is not practically possible, and no one would believe you if you could do it! Also, even if you conduct exhaustive acorn production surveys in a specific geographic area, whatever you find out will be challenged by the guy who has one highly productive tree on his property.
So, when asked about the acorn crop, I have learned to answer, “It will be spotty!” Still, however, acorns remain a highly important food item for deer during at least 180 days of the year. Understanding what acorns represent nutritionally to deer, what affects their production and how to improve acorn production on a specific property will improve both hunting and management success.
Distribution Of Oaks
Although there are lots of places where deer exist that do not have oaks, it is safe to say that most of the whitetails in the United States live where there are at least some species of oaks. However, most of the oaks occur in the eastern half of the country! There are a total of about 90 species of oaks within the Continental U.S.
Taxonomically, oaks belong to the family Fagaceae, which in Latin means “to eat.” The family includes beeches, chestnuts and oaks, comprising eight genera and about 927 species worldwide.
Oaks in the U.S. are broken down into two groups, white oaks and black oaks (including the red oaks). These two groups are distinguished by soft, rounded edges on white oak leaves, and pointy, armed leaves on red oak leaves. White oaks are larger (80 feet) at maturity than red oaks (70 feet).
The greatest difference in these two groups lies in their reproductive behavior. White oaks flower and mature fruit in only one year, while red oaks take about 18 months; flowering one year and maturing fruit the next. This means that an event, such as a late freeze or drought, in one year will immediately affect white oak acorn production, and a delayed effect the second year for red oaks. This has a profound effect on management strategies to produce large crops of acorns.
Why Are Acorns So Important?
Our research at the Institute for White-tailed Deer Management & Research, and other scientists, clearly show that digestible energy is the most important factor in whitetail nutrition. Acorns and other nuts represent a significant source of digestible energy, particularly to support fat storage prior to harsh winter conditions.
A study my colleague Phil Goodrum conducted back in the 1950s and 1960s showed that, in an area where deer density is about 20 acres per deer, optimum acorn production to support a single deer for 180 days is 22.5 pounds per acre. This estimate took into account other foods available during this period.
The two most important nut crops for deer when western humans arrived on the continent were acorns and chestnuts. This all sounds good for the deer, but oaks have a little surprise for them: their leaves and fruits are very high in tannins, polyphenolic compounds that potentially can affect taste and performance of microorganisms in the rumen.
Anyone who has ever tried to eat a fresh acorn has learned very well that it is not a pleasant experience! Tannin content of acorns can reach as high as 7 percent, but generally range from 3-4.5 percent. Chestnuts, although still containing tannins, are less bitter and more digestible. Yet, deer have fought back by producing tannin-binding salivary proteins that inactivate tannins before they enter the rumen.
Factors Affecting Acorn Production
Now that we have established why acorns are important to deer and how they have adapted physiologically for handling the defenses of oaks against deer browsing their leaves and eating their fruit, lets discuss the factors involved in management of acorn production.
First of all, there are many misconceptions about oak forests; and, no where are there more misconceptions than in the Lake States, where thousands of hunters grew up hunting large stands of old oaks.
The greatest misconception is that these oak stands are natural; when the truth is much of the area was covered by coniferous forests! During the “Paul Bunyan” days, pine stands were quickly cleared for lumber and farming, leaving behind a landscape pretty much devoid of trees. Then followed the “Great Fires,” which consumed the debris and remnant trees left because they had no value.
Whole towns were destroyed in these fires, and there was much loss of life during those days. Yet, these are precisely the conditions oaks “love” for establishment. Modern day foresters often are poorly trained to manage oaks on a sustainable basis, and lack of regeneration has been due to a host of factors; not just deer browsing.
The beautiful oak stands of the late 20th century were pretty much even-aged. There was very little forage beneath them due to the dense canopies that prevented light from reaching the forest floor.
It is indeed a fact that oaks have to be fairly old before they reach maximum acorn production. Goodrum and his research colleagues conducted many studies on factors that determine acorn production; and, age certainly was one of them. In most species, acorn production does not begin until trees are 20-30 years of age. Using Goodrum’s data, I constructed some graphs that illustrate the principles of acorn production.
I noted earlier that there basically are two kinds of oaks (red and white), and each reproduces differently. Since oaks live very long lives, they have a corresponding long time to successfully reproduce. If there is a drought year or a hard late freeze, the oaks just wait for a better opportunity. Since acorns are in great demand by deer and other animals (including at one time, humans), there is an obvious need to produce large numbers of nuts, so that a few germinate and survive. It is kind of like the salmon that swim upstream and lay hundreds of eggs to produce just a few offspring.
If we look at the amount of acorns and probability of producing them each year, there is little predictability! In fact, Goodrum’s research shows there were few years when both types were in synchrony for production.
There were some years when almost 80 percent of trees produced acorns; however, this also points out that not all oak trees, regardless of age, produce reliably. There are genetic differences between trees in the same forest in regard to fruiting habit. That is why we have been involved for the last 16 years in selective breeding of live oaks for both reliability and early acorn production and high yields year after year. The earlier your trees produce acorns, the faster they will maximize benefit to your deer.
Wild Tree Nursery offers a line of live oaks that are producing acorns as early as three years. Exotic species such as sawtooth oaks generally begin production in 4-5 years.
Managing For Maximum Acorn Production
If each species of oak has a specific timing of first production, and if there are genetic differences between individual trees in a stand, does it not seem obvious that you can improve oak stands by selectively adding and removing trees?
There is a type of forest management called Wildlife Stand Improvement (WSI), in which a stand is thinned or harvested to maximize species composition and vigor. The sad truth is that most of the hardwood forests we visit have extremely high densities of trees, with very small crowns produced by the competition.
Young stands often have as many as 600-800 seedlings per acre; some even more! The normal process is for these young trees to compete vigorously for sunlight and nutrients, with many dying during the process. Sometimes, the result is a good stand spacing and composition, but in most cases this never happens.
However, we can go into such stands, census the species present and the numbers of each, then develop a WSI operation that consciously improves the composition. We use non-soil active herbicides either by injection or by basal spraying to kill the trees we don’t want and leave the ones we do want. We develop a species list for the existing trees, then kill trees to adjust species composition.
An old adage of ecology is that diversity breeds stability. A stand almost entirely of red oaks will have sporadic acorn crops due to the lag time between flowering and fruiting. One made up of several species and subspecies of both red oaks and white oaks will be infinitely more stable and predictable for acorn production.
Yet we still have to reduce competition, even when the species diversity is acceptable. We generally like to set a target per acre density of no more than 30 trees by time of peak acorn production. We use a D+ thinning to accomplish this task.
The “D” stands for tree diameter and the “+” represents a constant for certain sized trees. In stands where the average diameter is less than 10 inches, we use a D+10 thinning. We walk into the stand with a herbicide sprayer, select a tree we want to keep, then measure its diameter. If it is 8 inches, we add 10 to that (8+10), then inscribe an 18 feet radius around the tree, killing all other trees in the ring.
Then we move outside the treated area to find another tree, preferably of a different species, and repeat the process. Afterward, you will have a more diverse stand with appropriate spacing to reduce competition. As the trees get larger, the spacing between trees increases. Under what we call intensive management, we keep track of the acorn production of specific trees, by walking the stand in August with binoculars and classifying productive and non-productive trees.
Trees that consistently underperform then are removed by cutting or during a thinning. In cases where there are few oak species, we may resort to creating openings, then planting trees of needed species in these openings. If you do this, you will have to use tree guards around the trees to prevent bucks from rubbing them literally to death.
Whatever the methodology, you can manage your way to a more productive property in regard to acorn production. It takes time, but with hard work you will be surprised how quickly you can see a difference. Using improved varieties and cultivars. Then again, I have said many times, I want to meet the 80-year-old man who is planting walnut seedlings! Now, that man is an optimist!