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How Dense Can You Get?
Article #230, January 2016
By Bill Cook

Forest density is a critical measure that strongly affects forest health, plant composition, stand structure, and ecological function.  Forest density is easily measured and can be managed.   

     In many forest stands, the amount of light is the most limiting ecological factor, especially for regenerating trees and other plants.  The forest canopy determines how much light reaches each tree and how much light eventually reaches the forest floor.  Canopy characteristics are often highly variable.  Forest management manipulates canopy characteristics and light penetration, among other things, to work towards specific goals. 
     Light is required to drive the process of photosynthesis.  Photosynthesis produces sugar, which stores chemical energy, converted from light energy.  The energy is consumed in respiration so that plants and animals can live, grow, and reproduce.  These chemical reactions fuel life as we know it. 
     All plants, including trees, have different requirements for light.  Some species prefer full sunlight.  Other species do better in partial shade.  Strategies to obtain light vary greatly.  Next time that you’re in a forest, take some time to see which tree species are regenerating, if any.  Also, pay attention to light hitting the forest floor, and then monitor a few of those places during the year to see what happens. 
     Spring flora run most of their life cycle in full sunlight between the winter thaw and leaf-out among the trees.  After leaf-out, the shorter plants must adapt to what’s left, go dormant for years, or die-out.  This long-term struggle for light in the forest influences key ecological processes such as succession and disturbance. 
     Tree rings offer insight to past light conditions.  Wide rings suggest more light and more growth.  Narrowing rings show less growth, and possibly less light.  The declining growth causes tree stress and increased vulnerability to insects and pathogens. 
     Foresters express forest stocking with a measure called “basal area” or BA.  By definition, BA is the cross-sectional area of all trees at 4.5 feet from the ground (often trees at least five inches in diameter), on a given acre.  BA is measured in the number of square feet per acre (there are metric equivalents, of course).  Optimal BA for a forest will depend on the type of forest, but typically ranges run from 70 to 150 square feet per acre for forest types in the upper Great Lakes. 
     Because forests can be rather complex, different stand density guidelines exist for a variety of conditions, such as species mix, age, average diameter, etc.  Northern hardwoods (sugar maple, basswood, yellow birch, hemlock) require the highest level of skill and experience to manage.  Northern hardwoods are the most common forest types in the northwoods and are growing more common as the decades roll by. 
     Alternatively, aspen types are comparatively simple.  In fact, aspen types regenerate from a “catastrophic disturbance” such as fire, blowdown, or clearcutting.  Aspen trees are quite intolerant of shade.  Seldom is thinning cost-effective in aspen types.  The life strategies of aspen and northern hardwoods are polar opposites. 
     Red pine offers another set of peculiarities in response to light.  Whether naturally regenerated or planted, red pine “close the canopy” at some point in time.  Growth begins to slow.  Lower branches die and eventually fall-off.  Once trees near their genetic height maximum, shrinking crowns present a problem.  While healthy red pine stands respond well to thinning, the trick with is to thin these stands before the crown size is reduced too far. 
     Should the average tree crown occupy less than 10-15 percent of the tree height, those trees will not be able to grow large crowns.  Thinning trees with small crowns will not cause new branches to grow below the existing crown because red pine doesn’t have the ability to do that, unlike a maple or an oak.  Thinning may simply result in more wind breakage. 
       Northern white-cedar stands can grow to remarkably high stand densities, in excess of 300 square feet per acre.  A sapling cedar under these very shady conditions may barely stay alive.  Yet, if released to increased light, that sapling will begin to grow aggressively.  Cedar is among the minority of tree species with this sort of light elasticity. 
     Trees, however, while dominant, are not the only plants in the forest.  Other plants must be adapted to the light conditions that a forest canopy provides.  There is more light under a mature aspen stand than under a mature cedar stand.  Understory plants will vary accordingly. 
     As openings occur, either natural or through management, the understory responds predictably.  The mix of ground plants, shrubs, young trees, and mature trees is called stand structure.  Stand structure, among other characteristics, can be manipulated by controlling light levels through forest management. 
     One of the few downsides of managing light is the potential invasion of exotic plant species.  Shrubs such as buckthorn, autumn olive, Japanese barberry, and honeysuckles occupy understories more quickly than most native species.  Some exotic ground plants, such as garlic mustard, may not even need a lot of light, but will gradually take over a shady forest floor.  These exotic invaders reduce native diversity, erode habitat quality, degrade ecological services, and significantly alter the course of natural succession.  Native plants are more difficult to grow. 
     Letting forests “go wild” will seldom result in positive outcomes.  Nearly all of our forests have been heavily impacted by past practices.  In some ways they are not “natural”.  Failure to manage light levels, and other ecological parameters, inevitably leads to serious declines in forest vigor and resilience. 
     Forest protection, restoration, and the provision of products and services are compromised by benign neglect.  Visual quality, while important, is a particularly poor measure of ecological integrity.  Forest owners have a wonderful opportunity to improve the conditions of their woodlands.  Management of these valuable forest resources is also a great way to bring a family together, have fun, and provide meaningful experiences.  Forest owners have a special opportunity to “share the light”. 

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Bill Cook is an MSU Extension forester providing educational programming for the Upper Peninsula. His office is located at the MSU Forest Biomass Innovation Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at cookwi@msu.edu or 906-786-1575.

Prepared by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005 J Road, Escanaba, MI  49829
906-786-1575 (voice),  906-786-9370 (fax),  e-mail:  cookwi@msu.edu

Use / reprinting of these articles is encouraged. Please notify Bill Cook.
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