Forestry Part 1 – Selecting Crop Trees

  • Posted on: 30 January 2016
  • By: David Trammel

Guest Post by David Coulter
Part One - Forestry Part 1 – Selecting Crop Trees

This would be a good point to go over how trees grow. If they are kept densely packed initially, they will tend to grow straight and tall just like some un-thinned vegetable in your garden. They will be spindly and, beyond a certain point, prone to disease though. Kept too long in that condition and the entire stand may die off. This is particularly true in even aged conifer plantations. If less than 1/3 of the crown is green in a conifer plantation, the stand as whole is in danger of dying.

Even Aged Forest – How Trees Grow

At about half the rotation time (i.e. half the time to maturity; meaning reaching it maximum height), a hardwood stand should be released (thinned) to allow the trees to start to grow in girth. At that point, the maximum vertical growth rate of the tree will have halved. The idea here is to first get the trees to grow straight and tall with a large bole for the first half of the rotation life by having all the trees compete against each for light. Once they are about half way to rotation age, they will be about 2/3rds of their mature height. Whenever you release an even aged hardwood stand, you more or less fix the height of the bole (distance from the ground to the first fork). If you release a tree too early, you get a tree with a short bole and huge crown. The extra crown allows the tree to put on greater girth (and visa versa). You get a fat tree with a short bole. Such a tree is good for maple syrup production but not especially good for fire wood (hard to split a wide trunk) or timber (bole too short). If you release too late, you get a tree with a tall bole and small crown. This will be a spindly and likely unhealthy tree due to an insufficient crown of leaves to feed the tree. From the point of view of creating healthy commercial timber and/or good fire wood, the middle case of equal bole and crown height is preferred. See the figures below.

Even Aged Forest – Weeding and Thinning

Weeding the stand is just that, getting rid of the sick and unwanted trees. You take these trees regardless of their position in the canopy or position relative to crop trees. In fact you don't need to have marked the crop trees to start weeding. Thinning is a bit more subtle. Here you are taking only those trees that are in the canopy and that are (or will before the next thinning) touching the crowns of the crop trees. The idea of thinning (or “releasing”) is to remove those tree that are or will compete with crop trees for light. The remaining trees are “released” in the sense that now they can start growing faster due to more light. When you thin a wood by the crown touching method, you are creating and maintaining a condition where each crop tree is effectively unaffected by competition for light with neighbouring trees. Release by thinning is most effective if the tree has grown vertically some but still has some room to grow further. Generally this condition occurs at half the rotation age i.e. at about 40 to 50 years of age.

What happens if you cut down all trees in the canopy save the crop trees right away? In this case you may be letting in too much light into the forest. If the lower part of the some trees such as maples and oaks are exposed to direct sunlight, they form “epicormic” branches off the top of the bole that effectively diminish the height of the bole. These epicormic branches may reduce the commercial value of the tree. In practice the problem is really that removing all non-crop trees is just too much work all at once and gives you way more poor quality wood than you can use immediately.

There is no need to cut down healthy trees in the understory as these will likely die back in any case, though, there is no harm done if you did so. In an even aged wood, these trees are of the same age as the crop trees but are effectively stunted for lack of light. These often show lack of “apical dominance” i.e. the tops of these little trees have flattened out indicating they are no longer growing vertically. Such stunted trees are not the best candidates for regeneration of the forest and may be cut down when harvesting nearby crop trees. The tree I do not cut down are supra-canopy pines and other unique or under represented species, especially mast trees. Mast trees are trees bearing large seeds (mast) such as acorns that are a source of food for wildlife.

Why leave legacy trees alone? Well, for the older trees I think its just respect more than anything. They survived the successions and have earned their keep. If that's not enough, think biodiversity. The more diverse the tree species, the more biodiverse the entire woodland ecology. I'd say that's an end in itself but if that's not enough again: more biodiversity generally means a more resilient ecosystem. The entire wood will be less prone to complete devastation by fire, wind, and disease. The truth is those old timers from the first succession will eventually die without replacement. Left alone, a forest will tend towards tolerant trees only and so, all things being equal, less biodiversity. By keeping legacy trees you are at least not speeding that process. Dead trees should also be left standing or left fallen. Neither generally provide good wood and both provide habitat for your woodland friends thus adding to the forest biodiversity.

Forest Biodiversity – Tree, Stand, Forest

A few more words on biodiversity are in order. Ultimately biodiversity is a matter of scale. One tree with its associated species is not as biodiverse as a mixed forest. A large climax forest containing only one species of shade tolerant trees is also not as biodiverse as a mixed wood mid-succession forest. Though such a climax forest may be natural, a monoculture is still a monoculture with all its attendant risks. The issue is scale. A diverse forest comprised of small patches of pure stands of trees is biodiverse on a large scale but not on the scale of each pure stand. The point here is to take the big picture; to see the forest as well as the trees. Aim for biodiversity but don't try to force the forest to go in direction it cannot go. If you have a pure stand of white oaks, great. There is little point in trying to change this but you could balance this stand by encouraging another species over oaks in a nearby stand where oaks do not dominate yet. Thus if your oaks fall prey to disease, you only lose a small stand not the entire forest.

Even Aged Forest – Harvesting Crop Trees

If we remove all crop trees at once, say at the end of the even aged hardwood rotation, if we clear cut in other words, we are initially selecting for shade intolerant trees if any seed sources are present. As these are less valuable for firewood and timber, this is not desired to say nothing of the complete destruction of the forest ecology you spent years cultivating. If we remove crop trees one by one, we are selecting for shade tolerant trees to replace each crop tree (thus perhaps lowering biodiversity). Also, though tolerant of shade, such trees do not necessarily grow quickly in the shade. Further, a very slow removal of crop trees means most will be “over mature” on harvest. These “over mature” trees (I prefer “Oldwood” as this is a more neutral term) are no longer growing much vertically and so the rate of change of wood volume is not as high as smaller trees.

A good compromise is to harvest small groups of adjacent crop trees. One rule of thumb is that if midtolerant trees are to be regenerated, a square area on the side equal to between one half and one times the height of adjacent trees is required. The issue here is allowing enough light in (at middle latitudes) to the forest floor to allow regrowth of the desired species. If we assume adjacent trees about 78 high, crop trees spaced 26 feet, and we wish to regenerate midtolerant trees, the removal of 4 crop tree in a square gives enough space (a square of about 52 by 52 feet) to regenerate tolerant species. A square area of 9 trees will create openings of 78 by 78 feet would be even better. The next square up of 16 trees is perhaps too large. One solution is to harvest your first crop trees early (when adjacent trees are about 80 feet high or less). The point here is that the choice of harvest groups is as important as crop tree spacing as these choices are directly related to each other. Attempting to regenerate midtolerant trees implies you have such trees to begin with. Simply providing the space for these when no seed source or stumps to regenerate off are available will not result in midtolerant trees regenerating. In that case, you might as well harvest smaller batches of trees and so favour what you in fact have: tolerant trees.

The actual height on maturity of your trees of course this depends on the species. It also depends on the site. Site Index is a numerical value used by foresters to evaluate the potential of a location. It is usually defined as the height in feet at age 50 years of the canopy of a stand of even aged trees. The fact that Site index is defined in terms of the height of trees alone underscores the point that it is the site that is the single most import factor in the ultimate height of a stand of trees of a particular species. A Site Index of 70 is about mid range for upland oaks. The final height of such oaks will be about 85 feet. A Site Index of 82 gives a final height of 100 feet.

If we harvested one group out of four of crop trees in a grid pattern, we could harvest a quarter of our crop trees at a time. Over maturity would still occur after the first harvest for the crop trees left standing, but larger cuts would ease removal of more trees than removing only specific trees and not their neighbours. Such opening would also favour coppicing from the stumps of the harvested crop tree. The decision to maintain the coppice for small dimension fire wood, waddle fencing, long handle tools, etc. or letting the coppice go to standard is now available. If regeneration off stumps is intended, these stumps should be cut as close to the ground as possible. Such short stumps allow more and healthier coppicing. The crop trees will also be already appropriately spaced for the next harvest.

What is happening in this approach to small scale intensive forestry is that we are gradually changing a even aged forest into an uneven aged forest. We are doing this by creating a number of relatively small disturbances in the forest to forestall a very large one from clearcut, disease, wind, or fire. There is a lesson here on how to address potential societal collapse. The analogy is exact if you assume that natural complex communities of living things fail in the same way human communities do. Better to cut a little and heal, cut a little and heal, etc, than to bleed out all at once.

Even Aged Forest – Basal Area

A forestry professional would note that I did not mention Basal Area (BA). The reason is that it is not a terribly important idea for even aged woods. It is good for impressing friends and relatives though, so here we go: If you cut down an acre of your woods at breast height (defined as 4 ½ feet from the ground) and took the area of all the stumps 4 inches in diameter or greater, you would have the BA of that section of the forest. Of course you don't actually need to clear cut your wood to determine BA.

In the Imperial system of units, BA is measures in square feet (of stump area) per acre (of forest). BA is used to measure the amount of canopy closure. When used for even aged forests, it must be used in conjunction with the number of trees per acre or the (square root mean squared) Dbh of the stand to find where the even aged forest is on a stocking chart. Such charts fill academic journals but are largely useless from a practical perspective. You want to know the extent of canopy closure? Look up. Want to know how many trees to cut? Count those touching the crowns of you crop trees. Still its cool to pull out a forestry prism and do a 360 degree sweep counting trees to get the BA. For those interested in such things, a stocking chart (BA versus N) for oaks (and other midtolerant hardwood even aged stands) can be derived from the equations given above. One rule of thumb is not to reduce BA by more than a third to avoid epicormics. This gives the lower limit of your cutting. Thinning based on the crown touching method gives the same result but with less math and bookwork. The forest itself is your book.

Uneven Aged Forest

So what if you have an uneven aged wood? Great, you probably have an old growth forest or something approaching it. Even if the area was logged well over a century ago, it has now reverted to a more natural state and is self thinning. That means the canopy is completely closed in and the rate of growth of individual trees is probably very slow due to lack of light. The wood will be wonderfully dense as a result of the slow growth. It also means there may be relatively low biodiversity given only a few tolerant species of trees are likely present. Now slow growth is not really a problem because you already have plenty of mature, and over mature, trees. In this case there is no need to mark crop trees, you've got plenty of mature crop trees already and they are obvious. All you have to do is harvest. You can harvest in much the same way as with even aged stands. Decide on the area to be harvested, based on what may replace your crop trees, and take all the trees out in the area to be harvested save those that might regenerate the area (i.e. still have apical dominance).

For those not comfortable with the math, please skip to the next section now. Otherwise allow me one more mathematical digression. Some people recommend using a “J-Curve” to determine how many trees in each each size category to thin to. A typical J Curve is of the form:

n = q^(b-d/a) where

n = number of trees in the interval
d = the Dq of the interval (nominally an integer multiples of a)
q = the ratio between intervals (typically 2.5 for a = 6 inches)
a = the interval (6 inches)
b = log(number of polewood)/log(q) +1

The equation above basically indicates that for any given size category, divide by 2.5 to find the ideal number of trees in the next category up, or multiply by 2.5 to find the same in the next category down. The idea is that once the number of trees in a particular interval (i.e. size category) exceeds the ideal value given by the equation above, you thin to get the number down to the ideal. This approach assumes you are dealing with an uneven aged tolerant hardwoods and you are selecting individual trees not groups of adjacent tree. Typical resulting profiles to the nearest integer for an acre (all with q=2.5) are as follows:

The idea here is to allow enough trees in each size category (or “interval”) so there are enough to allow recruitment into the next category up. If you are thinning to a Sugar Maple profile, you could mark the best 4 Oldwood, the best 7 Maturewood, the best 18 Youngwood, and the best 45 Polewood per acre. Thus you would mark a total of 74 trees per acre. You then cut everything else down. To make your work easy, you could just mark the top three categories for a total of 29 trees and only cut trees in those categories (i.e. don't cut Polewood or smaller).

This approach is fine for the uneven aged Sugar Maple profile, but I think it is too much effort for the larger numbers involved for the other profiles. Its easier to just take out all the trees in a number of small squares. For tolerant hardwoods each such square would be about 50 feet (i.e. half the canopy height) on the side. Each square would be on a gird of the same dimensions. For each block of 4 such squares, you would remove all the trees in say the southwest corner. Thus you would remove no more than ¼ of the forest every 25 years.

Even Aged Forest - Sugar Maple Stand

One rule of thumb for tapping sugar maples is to put in a tap for every 6 inches in diameter above 4 inches Dbh rounded down to the nearest integer. This works out to the same as taking the number of dots on my tree marking scheme less one i.e. Youngwood (10 to 15 inches) gets 1 tap, Maturewood (16 to 21) 2, Oldwood (22 and greater) 3 or more. Each tap should produce about 10 imperial gallons of sap per season. At a ratio of 40 to 1, each tap then produces about 1 quart of syrup per season. My family of 5 uses about 12 quarts of syrup per year. In theory I need only 12 taps but I double that to increase the flow to reduce the time I spend gathering syrup by hand.

Notwithstanding the rules of thumb just given, trees with larger crowns produce more and sweeter sap per tap. Such trees have short boles relative to the height of their crowns. This suggests that if we wish to maximize syrup production per acre, we should thin early to create trees of great girth and short boles. There is little point in doing so if all that is desired is enough syrup for your family; just put in more taps and manage as a normal even aged stand spaced at 26 feet but keep the Oldwood if its no good for timber (bole too short) or difficult to get firewood from (bole too wide).

If you wish to maximize sap production for a commercial operation, we would have to start thinning well before the vertical growth rate drops off to half its maximum value, say at 20 years instead of 40 to 50 years of age. The crop trees would continue to grow vertically at about the same rate they would have if you had not thinned early, but their crown and so bole diameters would be greater earlier at the expense of shorter boles. If we spaced the crop trees 26 feet apart, they would reach 16 inches Dbh sooner but then stop growing in girth well before maturity because the canopy closed in early.

To accommodate the crowns being wider sooner and to ensure the crop trees continue to gain girth, we could space our crop trees farther apart. Let me give an extreme example of say 52 feet apart giving 16 crop trees (¼ of 64) per acre. At maturity at 100 years these trees would not yet have reached a girth of 38 inches (from SDI =136 equation given way above) corresponding to their spacing. If we assume 6 years per inch of Dbh, the crop trees will be be about over 200 years old before their crowns touch.

At that point, each of the 16 crop tree should hold about 5 taps giving 80 taps per acre. Compare this against 64 crop trees per acre (at 26 feet apart) with Dq = 16 inches and so 2 taps per tree and so 128 taps per acre. Though the 16 trees per acre will likely produce more per tap to generate perpas about the same overall flow of sap, the whole process of such drastic thinning may not have been worthwhile due to all the extra work involved. Also you now do not have the timber you might otherwise have had since the boles are so short. Likewise the crops trees would also not be easy to get fire wood from given the greater width of the boles makes it harder handle and to split.

At the other extreme, we can now look at how you might thin a mature even aged stand for maple syrup production exclusively. Lets say most of the trees are 100 years old and were thinned using the crown touching method from about 50 years and so have a Dq of about 16 inches. This again assumes about 6 years per inch of Dbh which is not unreasonable if the crowns never touch. We could again select crop trees at an interval of 52 feet and thin using the crown touching method. In 100 or so more years these 16 crop trees per acre would be the only trees left in the stand if no crop trees were harvested and the next owner continued the process.

Though the 16 trees per acre will again likely produce more per tap to generate about the same overall flow of sap, the whole process of further thinning would again not have been worthwhile. Though you have more timber per acre (as the bole reached their commercial height), you had to wait 100 years to get it. You, of course, are now dead and the next owner now has no choice but to clearcut as the trees may not live much longer anyways.

My conclusion is that it is generally a better approach to space crop trees at 26 feet and take ¼ of your crop trees in groups of 4 every 25 years as you would for any even aged tolerant hardwood forest. The only real difference is that you would not remove Oldwood that already has a short bole or a bole too large to easily work with. This process of harvesting ¼ of your crop trees in groups of 4 every 25 years could continue indefinitely with a continuous harvest of syrup, firewood, and timber.

Even Aged Forest - Conifer Plantation

Conifer plantations are an extreme case of an even aged forest. Generally the individual trees are planted in rows on a square grid at a uniform distance apart. A plantation needs to be thinned before the crowns have died back to less than 1/3th the height of the tree. The first thinning generally involves removing every 2nd tree in each row with such trees staggered between rows i.e. removing all trees in every second diagonal row. The trees remaining are now on a diagonal grid spaced as 1.4 (square root of 2) times the original distance between trees. The second thinning generally involves removing every second diagonal row again; leaving ¼ of the original trees spaced at twice the original distance (1.4 times 1.4). As always, health and form take priority over exact spacing. Keep the best, take the rest. Final harvest could also be done in groups and started early to encourage midtolerant trees unless you wish to start from scratch again by clear cutting. I do not recommend this. Often conifer plantations offer excellent shelter for midtolerant trees. These trees should be released if not too old to move the stand in the direction of a mixed wood mid-succession forest.


Despite the length of this article, what you actually do is really quite simple. Determine if you have an even aged hardwood stand and, if you do, select and mark crop trees. Health, rare species for your woodlot, and form take priority over exact spacing. Having marked these crop trees, don't cut them down. Weed and thin the trees between the crop trees to ensure no crop tree has its branches touching another tree. When the crop trees mature, harvest them in blocks of adjacent trees of no less than 4 per block. There are some modifications of this basic plan for uneven aged stands, sugar maples, and conifer plantations, but the same basic principles apply in all cases.

Further Reading:

Green Wizards Forum - Eighth Circle: Energy and Power

("Man Chopping Wood" by HalfPoint: