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Article #16, October 1998
By Bill Cook

When we think of the word “intolerant”, I suppose most of us think of nasty people with bad habits.  However, “tolerance” and “intolerance” in forestry refers to a critical characteristic of trees.  Tolerance is mostly used to describe a tree’s ability to tolerate shade.  Most trees can tolerate variable degrees of shade, or in some cases quite a bit.  Other trees just stress-out or die, even under light shade.

Trees, of course, have many survival requirements.  However, tolerance is arguably the most important.  All trees require light to produce the sugars needed to live.  Tree species such as jack pine, red pine, aspen, and paper birch require nearly full sunlight to maintain vigorous growth and effectively fend off insects and diseases.  When these species occur in shade, even partial shade, they do not do well.

On the other hand, sugar maple, beech, basswood, cedar, and balsam fir can tolerate fairly high levels of shade.  Of course, deep dark shade will prevent any tree from surviving to adulthood.  Tree species that are “shade tolerant” are generally capable of reproducing themselves without a large forest disturbance.  Seedlings in the understory will take advantage of a hole created when a big tree dies. 

Shade tolerant tree species can sometimes cast their seeds under stands of “intolerant” trees.  In the long term, the intolerant species will disappear and the tolerant species will take over.  This is called “succession” and is happening across much of the eastern United States, including here in the Upper Peninsula.  The area of our “northern hardwood” forests of sugar maple, beech, and basswood are on the increase.  Some of the intolerants, especially paper birch, are on the decline.

To make things more interesting, the tolerance of a tree sometimes changes with age.  Seedlings and saplings may be more tolerant of shade than mature trees of the same species.  This, of course, makes some sense.  Trees are more likely to be shaded when they are small than when they are tall.  However some species, such as cedar, remain quite tolerant of shade and an abrupt increase of light regardless of age or size.  Quaking aspen never has any degree of tolerance.  It needs full sunlight from the word go.  Older hemlock, when exposed to large increases of light, will often decline and die.

Tolerance to shade, and the reaction of rapid increases of light, are two of the key factors that determine how we manage different kinds of forests.  Forests composed of largely intolerant tree species are naturally same-aged and usually managed and harvested with some variation of clearcutting.  Without management, these tree species must depend on a natural disaster to regenerate, such as wind, fire, flooding, or an insect or disease epidemic.  Society today spends a lot of effort to minimize these kinds of natural disasters.  This doesn't favor the reproduction of shade intolerant species.

Tree species that possess shade tolerance can be managed through shelterwood and selection systems.  The level of light reaching the forest floor and canopy layers can be manipulated to encourage both regeneration and tree quality.  Sometimes the balance is hard to obtain. 

There are other factors in addition to tolerance that influence tree regeneration, health, quality, and the choice of forest management systems.  But understanding a forest’s response to light is one of the most important.  Similar to people, not all trees have the same requirements and preferences.  Foresters have designed management systems to meet both the needs of different kinds of trees … and the needs of different kinds of people.

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Bill Cook is an MSU Extension forester providing educational programming for the entire Upper Peninsula. His office is located at the MSU Upper Peninsula Tree Improvement 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.
By-line should read "Bill Cook, MSU Extension" Please use the article trailer whenever possible.

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