Allowing Crop Trees Room to Grow in Young
Hardwood Stands

The silviculture method known as crop tree release is when desirable trees are given room to grow faster and larger by removing adjacent, competing trees.  Crop tree release is a management tool that woodlot owners may use in young, even-aged hardwood stands.  When released, most young hardwoods will respond favourably by developing quicker and attaining a larger diameter size at maturity.

Crop tree release is intended to only cut trees that are competing with the crop tree, not selecting crop trees and cutting all other trees in the stand. 

In many cases, since the work is normally carried out in young stands (20 to 60 years of age), often the work is not cost effective under operational conditions and is undertaken on a pre-commercial basis.   The ultimate goal is to invest a little now in pursuit of producing high-valued trees that have long, straight stems free of defects. 

It is important to note, as with any harvesting activity, that if implemented improperly this treatment may be detrimental to the stand.  If in doubt, seek out the assistance of a local forest consultant to assist in your crop tree release project.

Read on to learn more about using crop release as a tool in managing your young hardwood stands.

What is an even-aged forest?

It is a forest composed of trees having no, or relatively small, age differences (i.e., within 20 years) exist between individual trees.

The trunk of a crop tree should be well formed with a straight bole, free from forks, crooks and/or seams.

Figure 1. Dominant / co-dominant (D/C): Trees with crowns forming the general level of the crown cover and receiving full sunlight from above, but little from the sides.  Intermediate (I): Generally shorter than D/C, with crowns extending into the crown cover, but receiving little direct sunlight from above and none from the sides.  Overtopped (O): Trees with crowns entirely below the general level of the crown cover, and receiving no direct light. 1

What Makes a Good Crop Tree?

A quality crop tree has a number of attributes.
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A crop tree should be a dominant or co-dominant tree that has a healthy crown (i.e., greater than 1/3 of the tree’s total height) and the tree should be in or above the main canopy.  Refer to Figure 1.
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The trunk of a crop tree should be well formed with a straight bole, free from forks, crooks and/or seams.
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When selecting a crop tree for sawlog production, choose the most valuable species that is well adapted to the site.
-

Select crop trees with signs of vigorous growth and trees that are disease-free.

Timing

When should crop trees be released?  The timing is species and site dependent.  Recommendations outlined in the extension note Managing Young Hardwoods for Sawlog Production suggest the following:
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For yellow birch, black cherry and other fine-branched species the release should be undertaken when the stand is 15-25 years old or the trees or have reached an average diameter of about 6 inches (or 15 cm) at breast height (dbh).
-

For maple, ash, oak and other coarse-branched species the release generally can be undertaken between 25-40 years of age or about 8 inches (or 20 cm) at dbh.

Figure 2. Try to space crop trees (C) 25 feet apart and you will end up with a sufficient number of trees per acre.  Some crop trees can be spaced 15 feet apart and others 35 feet apart. 1


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Spacing

In young polewood stands (4-10 inches in diameter) crop trees should be spaced approximately 20 to 25 ft. (or about 7 m) apart.   Crop tree spacing at this interval will provide you with about 80-90 trees per acre.  It is important to keep in mind that crop tree quality is more important than maintaining a constant spacing regime.  In some situations you may find yourself leaving quality crop trees closer together, spaced 15 ft. apart and others 35 ft. apart.

Because this type of treatment is carried out at a very young age (as a pre-commercial operation) it is of little value to release trees that will be removed in future immediate cuttings.  Concentrate your investment on the stems of the highest value and minimize your expense and labour. Do not release more than the recommended number of crop trees (refer to Table 1).

Table 1, from the extension note Managing Young Hardwoods for Sawlog Production, provides a stocking guide for even-aged hardwood stands.  For example, a young hardwood stand with an average diameter of 6 inches (15 cm), the ideal stocking level is 835 trees per ha, of which 200 to 220 should be crop trees.
Table 1: Stocking Guide for Even-aged Hardwood Stands

Average Stem Diameter   
1.3 m Above Ground Level (cm)

Number of Crop Trees per Hectare
Total number of Trees per Hectare
Average
Maximum
15
200 to 220
835
920
17
"
720
790
19
"
600
670
21
"
515
570
23
"
450
500
25
"
390
425
27
140 to 180
355
390
29
"
315
350
31
"
290
320
33
"
260
290
35
"
230
260

Where to Begin?

Identifying crop trees:  To begin, start in one corner of the forest stand and walk in a straight line through the stand marking (with blue paint or ribbon) suitable crop trees every 20-25 ft.  At the end of the line, walk 20-25 ft. at 90 degrees to your original line and then traverse back parallel to your first line, marking crops trees at the required intervals.  Repeat this process until you have covered the entire stand.  Refer to Figure 2.

To maintain diversity in your woodlot it is best to select a variety of species as your crop trees.  Do not mark and release sub-standard trees in areas void of good crop trees (e.g., do not release overtopped trees).

Identifying trees to be removed:  Once you have identified crop trees, it is necessary to provide them with room to grow.  This is achieved by releasing the crown, also referred to as a crown touching release, where trees whose crowns touch the crop tree are removed from all sides.

Figure 3. When identifying what tree to remove, start by dividing the crown of the crop tree into four quadrants.  Assess each quadrant whether it is competition free (illustration on the right).  Mark for removal those competing against your crop tree.  The illustration on the left shows the crop tree has been released from all four sides. 2
Girdling – to kill a living tree by encircling the stem with cuts that completely sever the cambium layer, interrupting the flow of food between the leaves and the rest of the tree

When assessing which trees are to be removed, divide the crown of the crop tree into four quadrants and assess whether the tree is free to grow from all four sides. Refer to Figure 3.  Mark for removal (for example, with yellow paint or ribbon) only the trees whose crowns are in the main canopy and are touching the crown of the crop tree.  Remember that the objective is to remove only those trees whose crowns are competing with the crop tree, not selecting crop trees and harvesting all the other trees in the woodlot.  Refer to Figure 4.

Figure 4.  Aerial view from above (from left to right) showing crop trees selected to be released (light green colour), trees marked for removal (orange colour) and after the marked trees have been cut. 2

If you encounter two crop trees whose crowns are close or touching, treat this crown grouping as one tree and release from all four sides.

With some species (i.e., maple, oak and ash) it is important to retain sufficient competition to encourage straight growth and to minimize epicormic branching (growth of lower branches) during their early growing years.  Release shouldn’t occur until the trees are 25-40 years old and average ~8 in. (20 cm).

Before you begin to cut, it is a good idea to walk through the marked area and evaluate the planned release.   Keep in mind that releasing from one or two sides is not enough, but you do not want to thin too much.   Adjustments to the intensity of the treatment should be done by increasing or by decreasing the number of crop trees being released, not by reducing the release given to each crop tree.  Refer to
Figure 5.

Figure 5.  Adjusting the intensity of treatment.  If the cutting is too heavy (illustration A) then adjust the intensity of the cut by reducing the number of crop trees selected for release (illustration B), not by reducing the amount of release given to each tree (illustration C). 2

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Trees designated for removal can be cut and utilized as firewood, felled and left on the forest floor as woody debris or deadened by girdling. (A combination of these is possible.)

Summary

As discussed in the introduction, often crop tree release is undertaken on a pre-commercial basis because this silvicultural treatment is carried out in young hardwood stands that generally yield very little commercial product (i.e., sawlogs), but may generate firewood for the woodlot owner.  However, in most cases trees released will respond favourably in the long term by increasing in value more quickly (i.e., in volume and quality), greatly contributing towards a landowner’s goals and objectives.

Recommended Reading

The following guides and extension notes will provide you with more in-depth information on this topic on crop tree management.

Central Appalachian Crop Tree Field Guide . USDA Forest Service. NA-TP-10-01.  Available online at <www.iowadnr.com/forestry/pdf/croptree.pdf>.

Crop Tree Management: A New Tool to Help You Achieve Your Woodland Goals.  Ohio State University.   Extension Note F-50-02.  Available online at <http://ohioline.osu.edu/for-fact/0050.html>.

How to Release Crop Trees in Precommercial Hardwood Stands.  USDA Forestry Service. Report NE-INF-90-88.  Available online at <www.na.fs.fed.us/pubs/howtos/croptrees/ctree.htm>.

Managing Young Hardwood Stands for Sawlog Production.  OMNR Extension Note.  Available online at <www.lrconline.com/Extension_Notes_English/pdf/swlg.pdf> or by calling the LandOwner Resource Centre (613) 692-2390.

Illustration Credits

1 How to Release Crop Trees in Pre-commercial Hardwood Stands.
2 Central Appalachian Crop Tree Field Guide

© 2009 Ontario Woodlot Association

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