The oak and oak/hickory forests make up about 46% of Iowa's forestland. Approximately, only a third of these woodlands have adequate regeneration for the woodlands of the future. In most locations in Iowa, oak is not the climax species; as a result, much of the natural regeneration that is present is not oak but often includes such species a ironwood, hickories, ashes, sugar maple, or black cherry. If our oak forests in Iowa are to continue to exist, cultural activities will be necessary to obtain suitable oak seedlings.
Requirements for Oak Regeneration
Regeneration is a forestry practice intended to facilitate the establishment of new trees as the old trees become mature and are harvested. Depending on the characteristics of the woodland, techniques to obtain regeneration are done either before, during or after the final harvest. The techniques used will be determined by the condition of the woodland; consideration should be given to soils, topography, species present in the woodland, woodland density, and owner objectives.
A wide variety of oak species are found on a wide range of soils and topography. They are probably most successful on moist well-drained soils on a mid-slope position.
Oaks will begin to bear fruit at about 25 years of age in periods ranging from 2-10 years. White oak (white, bur, swamp white, chinkapin, and overcup) acorns will germinate in the fall after seed fall while red oak (red, black, pin, northern pin, and shingle) acorns will not germinate until the following spring.
Oak is considered somewhat intolerant of shade; this means that oak seedlings can tolerate some shade, but will eventually need full sunlight and release from competition to become fully established.
Oak Regeneration Techniques
Oak regeneration is obtained by either planting or performing cultural techniques to encourage natural regeneration. Planting is done 2-4 years before harvest or immediately after harvest. Planted seedlings must be protected from competition for 3-5 years or until they are 4-6 feet tall.
For natural regeneration, harvest techniques and/or pre-harvest cultural activities are designed to favor oak reproduction over other less desirable species. Because of their requirement for sunlight, two regeneration techniques which appear to work best are the "clearcut" and "shelterwood" methods.
Clearcutting involves the removal of all trees larger than 1-2 inches in diameter. The use of clearcutting as a regeneration procedure requires either adequate seedlings (at least 450 per acre) established prior to harvest or that the harvesting must be timed with a good acorn crop.
Acorn germination is usually enhanced with soil disturbance which will place more acorns in contact with bare mineral soil. It is critical that is using a clearcutting technique, oak regeneration must become established immediately or other less desirable species will take over the site.
In the shelterwood system, a new stand is established under the shelter of a portion of the older trees. This usually involves removing most of the understory competition as well as some of the mature trees to open the area to more sunlight and causing some soil disturbance for the stimulation of greater acorn production and acorn germination.
Once the seedlings are well established, the remaining overstory is removed in the final harvest. Three to seven years may be required between initial harvest and final harvest to ensure that the seedlings are well established.
With all methods of oak regeneration, it may be important to control understory vegetation as the seedlings are becoming established. This may require controlling both herbaceous and woody weeds.
The methods for naturally regenerating oak in Iowa are very site specific and should be tailored for the individual's woodland. Consult your Iowa Department of Natural Resources district forester for advice and assistance in regenerating oak.
For more information about woodland understory regeneration check out the Native Iowa Woodland Understory Regeneration website.