Thinnings and woodland stand improvement

April 18, 2007 at 9:37 am Leave a comment

Thinned oak stand near Mora, MN. Flickr photo by esagor. Click for original.

Thinned oak stand near Mora, MN. Flickr photo by esagor. Click for original.

Thinnings and timber stand improvement operations are designed to improve the vigor and value of existing forest stands.  There are many ways to do this.  As in choosing the right silvicultural system, choosing the right kind of thinning for your woodland can be somewhat complex.  That decision depends on a number of factors, including site characteristics, stand age, species composition, stand acreage, and landowner objectives.

Thinnings can be precommercial or commercial.  In a precommercial thinning, harvested material can not be sold and is generally not removed from the site.  Precommercial thinnings are made entirely as investments in the future growth of the stand.  Commercial thinnings, on the other hand, yield products that can be sold.  Commercial thinnings sometimes merely break even in the sense of covering harvest costs, but in older stands can be fairly profitable.

This section provides a basic overview of different thinning and timber stand improvement strategies.

Row thinning

Row thinning in red pine near Mora, MN. Flickr photo by esagor. Click for original.

Row thinning in red pine near Mora, MN. Flickr photo by esagor. Click for original.

As the name suggests, row thinning means removing trees in rows or strips.  Row thinnings are most often conducted in plantations, where trees are planted in rows.  Row thinnings are relatively efficient.  They also open up lanes for harvesting equipment to enter and move around within the stand, which may not be possible in unthinned plantation stands.  In most plantations, the first thinning is a row thinning.

Selection thinning
Selection thinning involves removal of certain trees based on their characteristics, spacing, and other considerations.  Selection thinning is often used in plantation stands for second or subsequent thinnings.  The advantage of selection thinning is that it allows more flexibility than row thinning, allowing for the removal of low-value trees.  This focuses growth on high-value trees, improving the health and long-term value of the stand.

Selection thinning can also occur in hardwood stands.  However, if not applied carefully, this approach can lead to high-grading, which can damage the stand and severely set back its vigor and long-term value.

Crop tree thinning

Crowns of unthinned (top) and thinned red pine, Cloquet MN. Flickr photo by esagor.

Crowns of unthinned (top) and thinned red pine, Cloquet MN. Flickr photo by esagor.

Crop tree thinning involves carefully maximizing the growing space available to the best trees in the stand.  Crop tree thinning is an excellent approach to use while cutting firewood on your property.  Crop tree thinning involves the following steps:

  1. Walk through your stand and identify the trees with the greatest potential future value.  These are your crop trees.  Mark your crop trees using flagging tape, spray paint, or some other marker.  “Value” need not mean “timber value.”  You might mark den trees, trees of rare species, or any other kind of tree that you want to thrive in your woodlot.
  2. Identify any trees that are in direct competition with your crop trees.  To identify competing trees, don’t look down.  Look UP at the crowns.  Any tree with a crown that’s touching the crown of your crop tree is competing with it.
  3. Remove some or all of the competing trees.  Be careful not to overdo it.  If your crop trees have been growing under intense competition for years, they could be shocked and stressed if all trees around them are removed at once.  Consider a multi-stage approach where competing trees are removed over a several-year period.
  4. Monitor crown closure after thinning.  Repeat these steps as needed.

High-grading involves harvesting only the highest-value trees from a stand and leaving all the rest.  High-grading can severely degrade the health, visual appearance, and long-term value of the stand.  High-graded stands tend to contain suppressed trees of poor form and/or species.  It can take decades for the stand to recover.  Diameter-limit cutting is one common way to high-grade a stand.

Although a partial harvest may seem preferable to a clearcut, in many cases you’d be far better off clearcutting the stand.  Clearcutting will generally lead to vigorous new growth of an early successional stand.  By contrast, high-grading leads to stagnant growth of a low-value stand.

Woodland stand improvement

    One approach to woodland stand improvement. Adapted from Woodland Stewardship: A guide for Midwestern landowners. Click for a larger version.

One approach to woodland stand improvement. Adapted from "Woodland Stewardship: A guide for Midwestern landowners." Click for a larger version.

Woodland stand improvement refers to any treatment designed primarily to improve the composition and condition of an existing stand. Stand improvement treatments require a financial investment in the future of the stand.

There are many different types of stand improvement treatments. One common approach is to kill or remove undesirable species to allow desired species more room to grow. Another approach is a targeted release of the best young trees in a stand. Trees are released by removing competing vegetation.

For more info
Another excellent source of information on Minnesota silviculture, including thinnings and stand improvement, is the North Central Forest Management Guides.


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