Improving acorn production - Part I: Factors affecting acorn production of oaks

By Matt Tarr, Whitetail Stewards, Inc.

(This article was originally published in the June 2004 issue of Quality Whitetails, the journal of the Quality Deer Management Association)

Acorns an important Autumn deer food

Biologists and hunters alike recognize that acorns are one of the most important foods for white-tailed deer in regions where deer and oaks occur.  This importance is due to both the nutritional value of acorns and the time of year they are available to deer.  Acorns are easily digestible and high in fat and carbohydrates (Table 1).  In autumn, freshly fallen acorns provide deer with a high-energy food necessary for accumulating winter fat reserves.  In winter, acorns allow bucks to replenish energy depleted during the rut, and they allow deer to reduce fat loss. In early spring, acorns uncovered by melting snow can sustain deer until herbaceous vegetation begins growing.  

Acorn production unpredictable

Although deer use of acorns is predictable, the amount of acorns that will be available to deer each year is not.  Many factors determine whether oaks will produce a large crop of acorns, or whether a mast crop failure will increase the importance of other autumn foods to deer.  Although acorn production can be incredibly variable, hunters can manage their oak stands to reduce some of that variability and to assure maximum acorn production when conditions are favorable.

Factors affecting acorn production

A number of factors affect acorn production in oaks.  Those most important to the deer manager include:

When combined, these factors make acorn production highly variable from year to year, between the different oak species, between trees of the same species, and from one property to another.  Let's take a look at each of these factors and determine which ones you can work with to encourage consistent, abundant acorn crops from your oaks.  

The masting cycle

Much of the variability in acorn production is the result of a natural cycle in oaks called "masting".  In this cycle, oaks produce low or moderate acorn crops most years, and an abundant acorn crop once every two to five years.  Acorn production during an abundant crop year may be 80 percent higher than in a low production year; the difference to deer can be hundreds of pounds of acorns per acre.  Although the exact mechanisms that control masting are not fully understood, biologists believe that oak species, weather, and genetics are important factors that determine how often oaks produce abundant crops.  

Oak species and weather

About 70 oak species occur within the United States.  These species are classified in groups as either "White Oaks" or "Red Oaks", based on their flowering biology and the time required to produce mature acorns.  Common species in the "White Oak" group include: white oak ( Quercus alba ), swamp white oak ( Q. bicolor ), burr oak ( Q. macrocarpa ), overcup oak ( Q. lyrata ), post oak ( Q. stellata ), chestnut oak ( Q. prinus ), and chinkapin oak ( Q. muehlenbergii ).  Common species in the "Red Oak" group include: northern red oak ( Q. rubra ), southern red oak ( Q. falcata ), black oak ( Q. velutina ), pin oak ( Q. palustris ), scarlet oak ( Q. coccinea ), and blackjack oak ( Q. marilandica ).  

Oak species vary in the age at which they produce their first acorn crop, the age and diameter at which they produce their largest crops, and how often they produce abundant crops (Table 2).  In general, most oak species produce their first crop of acorns when the trees are 20 to 25 years old.  They produce their largest crops when they are between 50 and 200 years old and when they are greater than 20 inches in diameter.  On average, most oak species produce an abundant acorn crop once every three to five years.      

Important difference between white oaks and red oaks

The difference in flowering biology between "White Oaks" and "Red Oaks"  is important, because it explains some of the year-to-year variation we observe in acorn production.  All oaks produce flowers sometime between February and May. In " White Oaks," it takes only three months for the flowers to develop into mature acorns.  However, in " Red Oaks," acorns require a full 15 months (two growing seasons) to mature.  So, if spring weather conditions in your hunting area caused oak flowers to go unfertilized this year,  your "White Oaks" will fail to produce acorns this autumn, but your "Red Oaks" will fail to produce mature acorns next year.      

Weather conditions that result in poor acorn production     

Weather conditions that result in poor acorn production include extended periods of rain and/or high humidity during the flowering period; these conditions reduce a tree's ability to pollinate.  Additionally, late spring frosts and summer drought can kill pollinating insects and cause damage to flowers and young acorns.  Both conditions reduce the number of acorns that will develop to maturity.

Genetics

Most of the variability we observe in acorn production is due to the combined effects of weather and tree genetics.  The genetic program of a tree is important; it determines how often that tree will produce abundant acorn crops and how large its average crop will be.  Some trees are born with the genetics to be excellent acorn producers; these trees produce good acorn crops nearly every year.  Other trees are genetically poor producers and on average, produce the smallest acorn crops in your stand.  If a tree is born a poor producer, it will always be a poor producer, regardless of what you do to the tree.  In a typical, unmanaged oak stand more than 50 percent of the trees can be poor acorn producers.  

Tree dominance

The position of a tree's crown within the stand determines its dominance (Fig. 1). Dominant trees have crowns that stand above all surrounding trees.  Codominant trees have crowns that are within the upper portions of the forest canopy, but they may be partly shaded by other trees.  Intermediate and suppressed trees have crowns that are overtopped and shaded by other trees in the stand.  Trees with the best potential for good acorn production are dominant and codominant trees because they have room to grow large crowns and they receive the sunlight (energy) needed to produce large acorn crops.  Generally, oaks with well-rounded crowns that are exposed to full sunlight will produce more acorns than trees that are totally or partly shaded.

The above are some general guidelines for determining which trees in your stand have the best potential to produce large acorn crops on your property. Unfortunately, there really are no physical characteristics of oaks that can be used to identify excellent acorn producers from poor producers consistently. This means that even healthy, dominant trees with full crowns can be the poorest acorn producers within a stand.  

Acorn predators

During most years, less than half of your acorn crop will reach the ground in sound condition.  There are more than 100 wildlife species besides deer that eat acorns, and many insects lay their eggs in developing seeds.  The combined impact of wildlife and insects can be significant, and during years of poor acorn production, your entire acorn crop can be damaged before the acorns are available to your deer.  Since there is no effective method to control damage from acorn predators, your best defense is to manage your oak stands to produce abundant acorn crops to offset their impact.     

How many acorns are enough to support deer?

So how many acorns must your stands produce to meet the needs of deer?  Well, on average, a deer eats about 2.2 lbs. of acorns per day.  Between autumn and early winter, this equates to about 200 lbs. of acorns/deer.  Some of the scientific literature suggests  that 100 lbs. of acorns/acre are needed to meet the requirements of a variety of wildlife.  However,  I suggest a better goal is at least twice this amount, especially in areas where deer densities are greater than 20 deer per square mile.  This level of acorn production can be accomplished in carefully managed oak stands.  

Applying what you know

With all these factors controlling acorn production, it may seem like the odds are against us if we want our oaks to produce consistent, abundant acorn crops.    Although we can't control the weather, we can apply what we know about the other factors when we manage our oak stands for deer.  Specifically, we can encourage desirable oak species to grow to the proper age and size required for maximum acorn production.  Additionally, we can identify which trees are genetically superior acorn producers and, through forest management, provide those trees with the room they need to grow large, healthy crowns.           

In the next article, I tell you how you can identify the oaks that are the best acorn producers on your property.  

Click to read the next article, Part II: Identifying oaks with superior acorn production - Whitetail Stewards, Inc.

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Selected References

Greenberg, C. H. 2000. Individual variation in acorn production by five species of southern Appalachian oaks. Forest Ecology and Management. 132:199-210.

Greenberg, C. H., and B. R. Parresol. 2000. Acorn production characteristics of southern Appalachian oaks: a simple method to predict within-year acorn crop size. Res. Pap. SRS-20. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 14p.

Healy, W. M. 1997. Thinning New England oak stands to enhance acorn production. N. J. of Applied Forestry. 14:152-156.

Healy, W. M., A. M. Lewis, and E. F. Boose. 1999. Variation of red oak acorn production. Forest Ecology and management. 116: 1-11.

Johnson, P. S. 1994. How to manage oak forests for acorn production. TB-NC-1. USDA Forest Service. North Central Forest Experiment Station 4 pp.

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Pekins, P. J., and W. M. Mautz. 1987. Acorn usage by deer: significance of oak management. N. J. Applied Forestry. 4:124-128.

Perry, R. W. 1999. Estimating mast production: an evaluation of visual surveys and comparison with seed traps using white oaks. Southern J. Applied Forestry. 23:164-169.

Sharp, W. M. 1958. Evaluating mast yields in the oaks. Bull. 635. University Park, PA: Pennsylvania State University, College of Agriculture, Agriculture Experiment Station. 22pp.

Seeds of Woody Plants in the United States. 1974. C. S. Schopmeyer Tech. Coordinator. Agricultural Handbook No. 450. USDA. Washington, D.C. 883 pp.