Forestry Part 1 – Selecting Crop Trees

  • Posted on: 23 January 2016
  • By: David Trammel

Guest Post by David Coulter


This article is the first in a series on basic intensive small scale forestry for producing firewood and timber while improving the quality of your woodlot. My experience is limited to a woodlot in eastern Ontario Canada. As a consequence, what I have to say is most applicable to forests typical to northeast North America. I'm not a professional forester, just gifted amateur. I've logged my own woodlot to provide my own firewood for 17 years. Firewood is the principle means by which I heat my house in the winter.

Though my recommendations are based on my experience in eastern Ontario, I have nonetheless tried to write this article in the most general terms so that the concepts may be adapted to other environs. This article assumes you have few acres of mixed upland woods and want to sustainably produce firewood sufficient for your needs. Managing (conifer) plantations, sugar maple stands, and other special cases will be addressed but the focus will be on mixed woods and hardwood stands.

I think it is worth stating that firewood and commercial quality timber are certainly not the only values of a forest. They are, however, pressing ones, particularly firewood, when it comes to matters of basic survival in a northern climate. Basically what I'm trying to do is show a means of getting what you need from the forest without doing damage to the forest and perhaps even “improving” it. I have included a number of rule of thumb that I have found useful. The idea being to give enough practical knowledge to manage your woods without burdening you with too much theoretical considerations of limited practical value. I have allowed myself two transgressions in this regard for those who wish to deviate from what I recommend.

Before I start, I think I should discuss why burning wood is preferable to burning natural gas or fuel oil or heating by electricity. The most obvious is up front cost. If you don't include capital costs (land and equipment), your time, physical effort, and a few gallons of gasoline to run your chainsaw, splitter, and truck; wood heat will cost far less than a winter's worth of gas, fuel oil or electricity to heat your house. The downside are those things I just discounted plus the physical danger associated with saws, falling trees, moving wood, risk of chimney fire, and the general messiness of firewood and ash in your house. One significant positive by-product is the wood ash that can be used as a potassium fertilizer and adjust the pH of your vegetable gardens and fields.

I think it is worth noting that burning wood for heat is simply displacing one set of dependencies for another. So it is for most projects involving preparation for the long descent. The presumption is not lack of dependence, but that one set of dependencies is preferred over another due to greater resilience. The core issue in choosing an energy source or technology is how such may fail you. Does the energy source fail gracefully or catastrophically? Does the absence or interruption of an energy source or technology mean the task can no longer done at all or just that it must be done differently?

Another point worth noting is that the tools you use for a job determine the way the job is done to a fair extent. I assume the use of a chainsaw as a minimum A splitter is less essential but still pretty important. If such are not available, as may someday be the case, then coppice (for firewood) and standards (for timber) may be your best bet if your winters are not extreme. Fortunately the method of forest management I present here can be converted to coppice and standard. This means the method I present here is not a a dead end if power tools fail you in the long term. If power tools are not available, the method here could still be used but providing firewood for your home would no longer be a one man job.

Age Distribution

The first step is to spend sometime in your woods and get to know what species of trees you have and what their age distribution is. To find the age distribution, take a 1/10 acre plot (66 feet square) typical of your woods and measure the diameter at breast height (Dbh) of the main stem (bole) of all the trees greater than or equal to 4 inches in diameter in that plot. A forester's tape is handy for this as it provides the diameter reading directly when measuring the trees circumference. Dbh is really a proxy measurement for the diameter of the crown of the tree. Measuring the diameter of the crown of a tree directly is difficult. Measuring Dbh is easy. Fortunately there is a relationship between the two for most hardwoods that I will discuss later.

Once you have this data, plot Number of Trees against Dbh Range. The Dbh ranges I use are less than 4 inches (Saplings: not recorded); 4 to 9 inches (Polewood: nominally 6 inches); 10 to 15 inches (Youngwood: nominally 12 inches); 16 to 21 inches (Maturewood: nominally 18 inches); 22 inches and greater (Oldwood: nominally 24 inches). If your plot decreases gradually from left to right, you have uneven age woods. If your plot is roughly bell shaped (or drops immediately from Polewood), you have even aged woods. In northeastern North America, almost all woods are even aged woods. Such woods have been disturbed at least once, typically by clear cut, sometime in the last 100 years.

A word on the Dbh and its relationship to the age of the tree: Of course, if the Dbh is large, the tree is relatively old. It doesn't work the other way around. A tree can have a small diameter and still be relatively old; it just didn't do as well in the same time as an adjacent wider (and probably taller) trees of the same age In even aged woods (by definition), most of the tree are about the same age, i.e. they date from the disturbance, but there is a spread in tree width (and often height) as a result of their differing success in competition for light and nutrients.

I will address the case of an even aged wood first and in detail as this is the most common case and the one requiring the most attention. How to address an uneven aged wood will become obvious as we proceed.


Before we go further, we should talk a little about the species of trees you sampled. Generally speaking, if a large area has been disturbed by clearcut, fire, disease, or a blow down, the first trees to establish themselves are fast growing shade intolerant trees. Slightly slower growing mid shade tolerant trees will often succeed these; followed finally by slow growing shade tolerant trees. Fast growing trees generally have wood that is less dense i.e. it weighs less for the same volume. Slow growing trees have the most dense wood. The notion of succession is that light loving fast growing trees have the advantage at the start of the creation of a forest but the slower growing shade tolerant trees have the advantage later in an established wood. The result is that, all things being equal, (shade) tolerant trees tend to succeed intolerant trees.

Firewood and Timber

Wood density (weight per volume) matters for firewood since it is directly related to the energy density of the wood. The heat available per pound of wood is essentially the same for all types of wood save those that are extremely resinous. This value is 8600 Btu (2.5 kWh) per pound of dry wood. Generally there is a bout 3,000 lbs of wood in a full cord (8x4x4 feet) of wood. Since the amount of time and fuel required to fell, buck (cut up to length), split, move, and stack is roughly the same no matter the species of tree, you get more value for your work by taking trees with greater wood density. Yes, denser wood weighs more (by definition) and so takes more work to lift and stack. However, you will need less of it for the same energy required to heat your house through the winter. The net effect is less physical work and less space required if you use denser wood. A similar rule applies for choosing trees for timber. Denser wood is usually stronger and so generally considered of greater commercial value.

Even Aged Forest – Selecting Crop Trees

Back to managing your even aged wood. The next step is marking crop trees. “Crop trees” are the trees intended to be keep to maturity. You may never cut these trees down. In any case, you will not be cutting them down right away. The reason why they are marked is to avoid cutting them down right away. The trees to mark as crop trees are the healthiest trees with the straightest and tallest bole. A “mature” tree means a tree in the range of 16 to 21 inches Dbh. Such trees are roughly 100 years old and have generally stopped growing much vertically. If left to live without competition from neighbouring trees, a mature tree will continue to put on girth but the annual increase in timber does not generally make it commercially worthwhile to keep in lieu of one that might replace it. This is not to suggest other values might incline you to keep it.

How many crop trees do you select, mark, and perhaps record, per acre? 60 equally spaced hardwood crop trees per acre is typically recommended. This recommendation is based on the crown of crop trees just beginning to touch each other at the time of crop tree maturity. The condition of the canopy just starting to close when the crop trees reach maturity gives the maximum density of trees per acre at (potential) harvest while allowing good tree growth (due to gaps in the forest canopy) before the forest canopy closing. As I suggested earlier, Dbh is a proxy measurement for the canopy occupied by any particular tree. In the range of about 16 Dbh, the diameter of a hardwood tree's crown will be about 20 times the Dbh. At lower Dbh the factor is greater, and at higher Dbh it is lower.

Using the rule of thumb of 20, if all crop trees had a Dbh of 16 inches, each tree's crown would be about 26 feet in diameter. At that point in the tree's growth, the next closest crop tree should be no closer than 26 feet. Any closer and the two crop trees will have their branches growing into each other (i.e. the crowns of the two trees are just touching) thus slowing growth due to less light available. Any farther away and we are wasting space and so valuable crop trees. Crop trees spaced 26 feet on a rectangular grid gives 64 trees per acre (close enough to 60).

What happens to all the trees in the canopy before the crop tree maturity? You remove them before they touch the branches of the crop trees and so interfere with the growth of the crop tree. The trees you remove are your firewood and perhaps some timber. The basic principle here is “keep the best and take the rest.” What you are doing is effectively weeding and thinning the woods to favour the crop trees. This is the way to improve the health and quality of your herd of trees.

I believe that crop trees should not be selected only on the basis of what makes the best timber i.e. healthy trees with straight high boles of largely shade tolerant species. Selecting only shade tolerant trees pushes your woodlot towards a monoculture. There is a tendency for woods to work there way towards a monoculture over time, but one is not advised to accelerate this process. Monocultures are not resilient and so to be avoided.

The way to avoid a monoculture forest is to also select rarer tree species as crop trees. In a typical maple, hickory, and red oak forest, I would select and mark as crop trees rarer healthy trees in that particular stand. These would be species such as black cherry, supra-canopy pines, or white oak. Keeping the rarer (often mid tolerant) trees helps slow the natural tendency towards a monoculture of tolerant trees only. Maintaining the species diversity of your forest helps it be more resilient to disease. It also helps maintain the diversity of other beings in your forest from decomposers all the way to the apex species of the ecosystem.

Now lets say you don't want to harvest all crop trees when they reach 16 inches Dbh. No problem, harvest half the wood on the diagonal and the remaining crop trees are spaced at roughly the square root of 2 (i.e. 1.414) times 26 feet or 37 feet. The Dbh when the branches of these remaining trees touch will be about 23 inches. Or you can choose not to harvest these trees at all and let the wood go to old growth. In this condition of the canopy being completely closed in, the crop trees and any other trees that remained would eventually self thin. It would still have been worthwhile to select crop trees because now you have an old growth forest of healthy trees, though perhaps of better commercial form than you need. You can't go wrong by keeping the best and taking the rest.

Even Aged Forest – Self Thinning, Canopy Closure, Crown Touching

As just indicated, a closed canopy forest self thins. Trees that cannot compete for light gradually die off. Well before this happens, the trees slow their growth due to less light. At 26 feet between trees, complete canopy closure occurs at about18” Dbh. Though not essential to know, there is a relationship between Number of Trees and Dbh at self thinning, canopy closure, and crown touching that is just too fun not to share. Its the sort of thing that gives enthusiastic biologists a sudden rush of blood to below the waist. Another reason for sharing this information is my own frustration when an author holds back on providing the information that allows me to derive things from first principles. If the math bothers you; however, just skip to the paragraph below starting with “Now back to the real world.” Otherwise, indulge me. The relationships are as follows:

Nst = 230 (Dq/10)^-1.6

where Nst = number of trees per acre at self thinning and Dq = SQRT(Sum((Dbh)^2)/N) in inches, i.e. the square root mean Dbh squared of all the trees (N) per acre

Ncc = 164 (Dq/10)^-1.6

where Ncc = number of trees per acre at complete canopy closure

Nct = 136 (Dq/10)^-1.6

where Nct = number of trees per acre at crown touching

If N is plotted against Dq, these three equation will give you three straight lines of slope -1.6 on a log-log plot. Straight lines in nature are cool. These equations are special cases of the allometric (sometimes called “allomorphic”) principle in biology. The allometric principle states than in some cases the percent change in growth of one part of an organism (say your shoe size … but not your waist size) is related to the percent change in another part of the organism (say your hat size) by a constant. Yes, the tailor of Laputa in Gulliver's Travels comes to mind; nonetheless, we will proceed.

In the equations above, we are effectively relating crown diameter (given in terms of number of trees per acre) to Dq (called the “quadratic” diameter ... effectively a type of average Dbh for the forest) for even aged midtolerant hardwoods (oaks specifically).

The first equation above was determined empirically by biologist named L.H. Reineke (published in 1933. “Perfecting a Stand-Density Index for Even-Aged Forests” Journal of Agricultural Research 46:627-638). Note that at a Dq of 10 inches, the numbers are 230, 164, and 136 trees per acre respectively for self thinning, complete canopy closure, and crown touching. These numbers are, by definition, the respective Stocking Density Index (SDI) for the given conditions for upland oaks. Though determined for oaks, they are broadly applicable to hardwoods in middle latitudes. These SDI values are independent of the particular values of Dq and N. Thus, if you wish a different value of Dq on harvest than 16 inches, Nct will give you the appropriate crop tree spacing as follows:

C = 66*SQRT(10/Nct)

where C is the diameter of the crown in feet (and so crop tree spacing). Note: an acre is 66^2 x 10 sqft

Combining the last two equations we get

C/Dq = 34/Dq^0.2

where Dq and C are in inches (and so C/Dq is dimensionless)

This last equation is the source of the rule of thumb that the ratio of C to Dbh is about 20 when Dbh is in the region of about 16 inches. All this to show I didn't pull the rule of thumb out of the back of my pants.

Even Aged Forest – Marking Crop Trees

Back to the real world: Now of course natural life is not normally set on horizontal grids. Sometimes two very good trees are right next to each other. Good, mark them both and keep them. The health, species (keep rare species in your woodlot), and form of the crop trees are the primary values, in that order. Their spacing is a distant fourth. If a tree is open on three sides of an imaginary square around its trunk, that is sufficient. It will have enough light. Nonetheless, each of the 64 crop trees per acre can be thought of as occupying a square of about 26 feet on average.

How you mark your crop trees is up to you. I spray paint blue dots (one per size category i.e. trees between from 16 to 21 inched Dbh gets 3 dots for 3 x 6 = the nominal diameter of 18 inches) on the north side of each crop tree. The north side is less conspicuous and the paint will fade less. Blue is clearly of human origin but dark enough not to spoil the view. Yes I am suggesting spray painting your woods. If this does not appeal to you, find another way to mark these trees. Perhaps a hammer, some nails, and some trail tape will do the trick. I don't recommend blazing crop trees however.

I use a GPS to record the species of each crop tree and its Dbh. GPS or even maps are not necessary however. A rope 26 feet long, a magnetic compass, and a can of outdoor blue spray paint are all that is required for marking crop trees. Pen and paper are all that is needed to record the position, species, and Dbh. Even these notes are not required: the information is all in the woods after all. Nonetheless, UTM projection found on most topographical maps provides a convenient reference grid. Most such maps are marked off in 100 m square grids (one hectare in area by definition). If I plan to mark 64 trees per acre, each crop trees will be in a 26 foot sided square on average. 26 feet is about 8 meters. Thus about 9 crop trees arranged in a square are about 1/16 of a hectare (24 or 25 meters on a side). One square hectare would contain 144 crop trees. This gives 12 such trees per side of a square hectare. Checking the number of crop trees marked in a given UTM grid area (or distance) on your topo map could keep you on track.

(To Be Continued)

Further Reading:

Green Wizards Forum - Eighth Circle: Energy and Power

("Fairytale Forest" by Arrlxx1: