Friday, February 14, 2020

C-D-L: An Acronym for Successfully Regenerating Hardwood Forests

By Dave Jackson, Forest Resources Educator, Penn State Extension

BELLEFONTE, PA--Many factors affect our ability to successfully regenerate and sustain forests. Competing vegetation, high deer impact, and light reaching the forest floor, referred to as C-D-L. Successful forest regeneration depends on addressing these three main factors. Joe Harding, the Penn State Forest Land Management Director, considers each factor when examining a forest stand. His prescription for treatment depends upon what he wants to accomplish and the problems he sees. The prescription then guides the management of the area. Joe explains, “If woodland owners follow this simple formula, C-D-L, they can be successful in managing their forests.”
Competing vegetation interferes with the establishment and growth of desirable regeneration - seedlings and sprouts. Common problem plants are beech, striped maple, eastern hophornbeam, hayscented fern, and numerous invasive plants; however, there are many others that can be problematic. The abundance of these undesirable plants has increased over time for a couple of reasons. First, they are low on the deer browse preference list. Where deer impact is high, these less-preferred browse species can dominate forest understories. Second, many of these species are tolerant of shade and grow well in shady understory conditions. They are often well established in mature forests.
Like weeding a garden, controlling interfering plants is imperative to successfully regenerate hardwood forests. Control measures can include several options. Competing trees and shrubs can simply be cut; however, this often results in the plant re-sprouting. Successful control is most often achieved using herbicides labeled for brush control in forests. Researchers have studied different active ingredients, rates, and time of year to develop safe and effective application prescriptions to control competing and invasive plant problems. Certified applicators are available to make herbicide applications for woodland owners.  
Deer browsing impacts forest regeneration in several ways. When deer densities exceed habitat carrying capacity, deer impact the ability of forests to regenerate desirable tree species. Selective deer browsing reduces seedling numbers, surviving seedlings are smaller, and the species composition is shifted to less preferred species, i.e., species deer don’t like to eat. Unfortunately, desirable timber species such as maple, oak, hickory, and yellow poplar are high on the food preference list and can be completely browsed out of forest understories when deer impact is high.
It is essential to control deer populations to maintain a balance with habitat conditions. Until that balance is reached it may be necessary to exclude deer from areas, using deer exclusion fences, for years until desired regeneration is above the deer’s reach. For example, erecting an eight-foot woven wire fence around a cutting unit may be the best option to control high deer impact. In addition to fencing, landowners may consider using the Deer Management Assistance Program or DMAP. DMAP allows landowners to harvest additional antlerless deer on their property during regular hunting seasons.
Lastly, it is necessary to understand the light requirements of desired regeneration. Most desirable timber species such as black cherry, white ash, yellow poplar, hickory, and black walnut are intolerant of shade, meaning they grow best in full sunlight. All oak species are intermediate in shade tolerance; they grow well in the middle ranges of light availability. Sugar maple, basswood, and hemlock are shade tolerant; they can compete well in fully shaded conditions. 
The tree species you are managing for dictate the type of regeneration harvests recommended. When managing for shade intolerant trees, species with high light requirements, practices that let large amounts of sunlight to the forest floor are preferred. These practices include clearcuts (ONLY if advance regeneration is present or for species like aspen that regenerate from root sprouts), shelterwood harvests, and seed tree cuts. Selectively harvesting individual mature trees from the forest canopy allows only small amounts of light to reach the forest floor and will likely result in the regeneration of shade tolerant species.

C-D-L certainly involves investments - planning, money, and time. Failing to address all three components, competing vegetation, deer, and light, can lead to inadequate desirable regeneration and unsustainable conditions. In summary, if competing vegetation is controlled, deer impact is kept low, and the light tolerances of the desired tree species are taken into consideration, you will likely be successful in establishing and sustaining new forests.    

For more on C-D-L practices, see the Penn State Extension Forest Science Fact Sheet entitled: Regenerating Hardwood Forests: Managing Competing Plants, Deer, and Light. It can be found online at:

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