Wednesday, 25 January 2017

Logging, cutting down trees for wood products

This post is a part of the series An Acre of Sunshine.

Logging was by far the single biggest economic driver during the settling of the region of our property, as it was in many parts of North America. Ottawa, the nearest large city and capital of Canada, began as a logging town. Ottawa is situated at the Chaudiere waterfall on the Ottawa River so as to take advantage of all of the hydropower available there, used to cut logs that were floated down the rivers. Our own property lies near the Gatineau River, which flows right into the city of Ottawa, and our area was first logged in the mid to latter part of the 1800s. Everything about our local region has been shaped by logging, down to the location of the villages up and down the Gatineau River. There is a small village every 8 to 10 miles, spaced just at the distance that a horse-drawn sleigh could move per day in the winter, with a small hotel and stable springing up at each camp that later developed into a village of its own.

While I don't have a complete history of our actual property, a look through the history of the region and the tell-tale signs left behind in our woods tell much of the story. Before the arrival of Europeans, our property was heavily forested, mostly old-growth white pine, sugar maple, basswood, white spruce and red oak. On the first pass of logging in the mid 1800s, loggers took only the large white pines. These trees make for great construction lumber and were also a favorite for ship masts, with the trees growing to four feet across and as much as 150' tall. Only the pines were cut at first, in part because these trees could be floated down the river and be brought to market; all of the maple and oak were so dense that they would sink. Starting at the end of the nineteenth century, softwoods like spruce were sent down the rivers to be made into pulp and paper. The arrival of the railway and logging trucks just after the turn of the century opened up the possibilities of cutting the denser hardwoods. Following these waves of cutting, there were many openings and clearings left behind, and these areas filled in with what is called 'secondary' forest, made up of a greater variety of species, including those that need much more sunlight, like aspens, white birch, and black cherry. Our property today is a mature secondary forest. Secondary forests similar to ours abound today throughout the northern states and eastern Canada, from Minnesota east to the ocean.

Along with these bigger logging operations, our place has been farmed since the 1870s, and every farmer throughout the region has used their woodlots to provide a steady supply of wood to build and heat their homes, barns, and workshops. Our property was commercially cut once more in 2008, harvesting some of the fast growing and sun-loving trees like aspen, as many of these trees were reaching the end of their roughly 80 year lifespans. Our forest is now in the process of moving very slowly back towards a more old growth condition similar to what came before the waves of logging.

In eastern forests like ours, it is usually best practice to do what is called selective cutting (for more information see here, here, and here). In a regime like this, one cuts only a modest portion of the trees at any given time, while leaving the rest to grow and fill into the gaps left by those that are removed. This can preserve a relatively natural looking landscape and maintains much of the wildlife, understory, and ecological relationships of an unmanaged forest. Done properly, one cuts out those trees that are sick, weak, or poorly formed, as well as some of the 'good' trees, while leaving some healthy trees of all sizes. This allows the straightest and healthiest trees to grow with relatively little competition. Unfortunately many loggers, if left to their own means, 'high-grade' when they do selective cuts. This means that they take only the most valuable trees while leaving everything else behind, which can leave a forest without good growing stock for many decades to follow. A well managed selective cut should take approximately 1/3 of the trees at a time, and can be repeated approximately every 15 years in perpetuity. This means that one is then harvesting 15 years worth of growth and energy on each pass through the forest. One could just as easily cut less trees more often, which homesteaders and farmers often do, but for commercial logging the amount of heavy equipment used necessitates doing much bigger harvests to justify bringing in the equipment. Our own property has most likely been cut in successive 'high-grade' cuts, where loggers went through and took only the best, while leaving the rest. The forest still holds promise, but is not what it could have been, had it been taken better care of.

Clear cutting is another, and often much more vilified, approach to logging. In clear cutting, loggers go through and remove every tree, or at least every tree worth harvesting. While this can be a reasonable thing to do in certain situations and areas, clearcuts require many decades before the forests can recover, and if one wants to encourage slower growing species like oak and maple, it can take even longer. In the early years after clearcutting, there is such a profusion of young trees that they end up wasting much of their energy in competing with each other, rather than turning that energy into growth. On the flipside, this is a tremendously efficient way to harvest. One can collect all the accumulated energy of decades worth of growth, and loggers don't need to be careful about working around any trees that are to be left behind.

What does an acre of forest actually include?

As we are trying to keep our understanding of energy in the human scale of one acre, it is worthwhile to talk about what that would actually mean in terms of individual trees. This amount could of course vary tremendously between a developing forest of young trees versus an old growth forest holding only a few giants; a forest could vary from just a few dozen huge trees to many thousands of seedlings in an acre. For the purposes of illustration I will go ahead and describe what an acre of our own forest looks like.

I did a tree survey of our property a couple of years back so I actually have a quite good idea of what is there. In doing tree surveys, it is not usually worthwhile to count the thousands of small saplings, so the only trees counted are those that are 4" or more in diameter at chest height. Foresters randomly sample small areas around a property, and then can extrapolate to estimate an entire site.  My sampling estimate is that on each of our acres there are a total of 318 trees bigger than 4" (as of 2012). The large majority, 257 trees, are of the smaller sizes between 4" and 8". Trees of this size are often left unharvested. Then there are the medium size trees, those 10" to 14", of which we have about 55 per acre. These are getting up to what would be a respectable size for a tree in a suburban yard, and are starting to be valuable for logging. Then finally there are the larger trees, those larger than 16" across, and our farm property has 6 of these trees per acre. It is these larger trees that are the favored target of logging in eastern forests like ours. Of all our trees, about a quarter are sugar maple, with 9 different other species each making up 5% or more of the forest, and finally another dozen species present in smaller proportions.

The numbers above make it easy to overestimate how skewed the population is towards small trees. There are almost always many more small trees than large, and while these are the growing stock of the future, many of them won't survive to reach large size. Even though 80% of the trees in our  forest are in the small sizes (8" or less), once you account for how big the average tree is, they make up only around one third of the total amount of biomass present. Each of the big trees can weigh thousands of pounds, while the smaller trees may only be one or two hundred pounds. As mentioned above, our own forest has been overharvested in the larger sizes of trees. If our forest were in peak health according to best forestry practices, it would have a much higher proportion of the biggest trees, perhaps as many as 20 per acre that were 16" or more, instead of the 6 that we currently see. With continued good management, we should return to peak condition over the next few decades; forest management requires an enormous amount of patience.

Embodied solar energy in wood.

It is easy to see that there is a lot of energy bound up in wood. An individual tree in our area can reach three or more feet across, and some reach to 100 feet or more in height. Almost everyone has sat next to a crackling campfire and felt the heat rolling off just a few small pieces of wood. Though it makes up only a small portion of the energy used in developed countries today, one needs go back only a bit more than one hundred years to reach a time when wood provided the majority of world energy needs, and wood is still a primary fuel throughout the developing world.

Wood really is an excellent energy resource. It is relatively energy dense, it grows for free, is very widely distributed, is easy to harvest and process, it stores well, and can be used on demand. The main reason that wood can't directly compete with fossil fuels is because of the sheer magnitude of fossil fuel use, but before the industrial revolution the total amount of energy being used was vastly lower than today.

Firewood is of course only a small proportion of what timber is used for today, with the lion's share of it going to lumber, paper, or other wood products. Most of the energy embodied in wood is bound up in cellulose and lignin, primary building blocks in the make-up of woody tissue. The molecular structure of these materials has very useful properties, giving cohesion, strength, compression resistance and rigidity. Most wood products take advantage of these properties, whether the final product is construction lumber, fiberboard, or paper. As long as the molecular structure is preserved, most of the energy stays locked up in the wood. In a way, all of the wood used to build your home is energy that has been frozen in time, to be released only at such a time as that wood decays or burns at some point in the future.

From an energy perspective, one of the most interesting things about forests is their ability to store energy for a relatively long time. For no other land use is it possible to store years, and even decades, worth of solar energy in the form of biomass, all in a form that is very manageable for people to process, store and use. In some ways, one can think of those long straight trunks as huge living batteries, storing up energy until such a time as that energy is needed. For the sugar maple which are the most abundant on our property today, it is not uncommon for individual trees to live 300 years or more. All of this means that one can extract tremendous amounts of energy on a single pass through a woodlot.

Energy estimate #1. From first principles.

To estimate how much energy is turned into trees in a given year, we first need to determine the steps that would reduce the total amount of energy that goes into the final product that we are interested in, wood. We have already established an estimate for the total amount of energy that plants are able to harness from sunlight at 36,000 kWh/acre/year. This is the amount of energy that our acre of forest has to grow, create leaves, produce seeds, and live the rest of its life.

The first thing that every tree needs to do is to support itself through each day, moving around water and nutrients, keeping all tissues healthy, which is known as 'maintenance respiration'. Trees, being very large and very long lived, have a lot of maintenance that they need to do to allow health and vigor for decades and even centuries. I wasn't able to get a lock on a precise number for this that would apply to our northern forest, but I did find some related information here, here, and here. I imagine that there is an expert that could give a more precise figure, but the numbers here indicate that from 50% to 80% of a tree's energy is used for maintenance, leaving the rest for growth. For the sake of argument, we will use the figure of 2/3, 67%.

The next thing to consider is where all of the growth is actually happening within each tree. A tree has to build all of its component parts, the trunk, branches, leaves, flowers, seeds, fruits, roots. Let us start with reproduction, flowers, seeds, and fruits. As will be discussed in a later section on orchards, nut and fruit orchards can be quite productive, growing up to several thousand kilowatt hours worth of nuts or fruits per year. These are, however, an extreme case of breeding and domestication for the purpose of maximizing fruit and nut production. Native tree stands don't produce anything like this in terms of seeds. The closest thing found in numbers around our property is red oaks, which can grow quite large crops of calorie-rich acorns. Acorns will produce something like 800 kWh of acorns per year in a pure stand of oaks (see calculation below). All other local trees produce much less total seed and fruit. Counting all of flowers, fruits, and seeds for the native trees, let us estimate that on average an acre of trees spends about 1000 kWh/acre/year to reproduce, about 10% of the energy that the tree would have left after respiration.

1000 pounds acorns/acre * 40% of acorn is edible * 1755 calories/pound * 1 kWh/860 calories = 816 kWh/acre/year of acorns

Then there are the varying parts of the tree itself. I found a couple of estimates for the relative sizes of different parts of trees, here and here. The second of these articles even gives a distribution for how much each part of a douglas fir tree weighs (not found in our area, but it should work as an approximation). This states that the breakdown is as follows; leaves 3%, small branches 8%, main trunk accounts for 62% in the wood and 10% in the bark, and finally 17% in the stump and roots. Most of the time, logging is aimed almost exclusively at harvesting the wood of the main trunk, and relatively little use is made of the rest of the tree. Occasionally small branches are chipped and bark is used for heating, but for the most part, it is that 60% of the tree that is the trunk which is the desired product. For the sake of argument, let us assume that each part of the tree takes the same amount of energy to produce, meaning that it took an equal amount of energy to make the same total weight of roots, trunk, or leaves.

Putting the numbers above all together, one gets the following:

35790 kWh/acre/year productivity * (1 part growth/3 parts total respiration) * .9 (10% energy needed for seeds, flowers, fruit) * .6 (proportion of total tree that is the trunk) = 6440 kWh/acre/year of wood

Energy estimate #2. Measured sustainable wood harvests in forests like ours.

In reading about the topic and speaking with guys who sell firewood as a business, I have come across the same estimate for sustainable firewood production many times, at least for our northern forest (areas further south with longer seasons and better conditions can have higher yields). This estimate is that a well-managed woodlot can produce about half of one cord of wood per acre per year indefinitely. For those of you who don't burn wood at home, a cord is a volume measurement, equal to 128 cubic feet, often thought of as a stack of wood 4' wide, 4' tall, and 8' long. Different types of wood have very differing densities, but if this were sugar maple, which is a very commonly used firewood, this half cord would weigh around 2300 pounds. Being that folks have long been interested in how much heat (energy) they could get out of firewood, there are plenty of resources that list the amount of energy that can be wrung out of a cord of firewood. That calculation yields the following:

.5 cords harvest/acre/year * 24 million btu/cord of sugar maple * .000293 kWh/btu = 3516 kWh/acre/year.

While almost all wood could be turned into firewood or wood pulp, other lumber products can only be made with the 'best' wood, the straightest, most sound, with the fewest knots and imperfections. It turns out that the figures for lumber are also easily available, and that somewhere between 1/3 and 1/2 of harvested wood tends to be good enough quality for lumber products.

Being conservative, and given that there is a lot of guessing going on in the 'first principles' estimate above, we will go forward with this second more conservative estimate of firewood yield as our best guess for the amount of energy that can be sustainably harvested from our forest in a given year.

Energy estimate #3. How much energy is actually harvested in a selective vs. a clear cut?

I mentioned two different approaches to harvest above, and just wanted to quickly highlight the differences between them for the long term management of land. The established best practice for eastern deciduous forests would be a selective cut each 15 years or so, taking out about 1/3 of the volume of wood each time. However, after each cut, there would be healthy trees of all sizes, so that the forest is constantly in a sweet spot where it is putting on 'good' growth. This is the sort of state that would allow that 3500 kWh/acre/year, though it would actually be harvested in 15 year increments, taking out over 50,000 kWh of wood at each harvest.

Contrary to that, the clearcut takes a mature forest and removes all of it at once. The yield is great, as much as 150,000 kWh of wood. The problem is regeneration time. When the forest begins to regrow, there are far too many small trees, and they will 'waste' a lot of their energy competing with each other. What is more, if one is hoping to harvest relatively slow growing trees like maples or oaks, it will take at least 100 years for the forest to have lots of larger trees of these species. One could then clearcut again to restart the process. The quick and dirty math here shows that this would be a total yield of only 1500 kWh/acre/year, less than half the total rate of selective cutting. Unfortunately, it is difficult for people to put long term planning and interests over short term gains, and this certainly isn't a problem limited to forestry.

Visualizing wood growth

So what does this amount of wood end up looking like when gathered all in one place? As mentioned just above, that half cord is a good place to start, a pile that is 4' cube of stacked firewood pieces.
the pile in the foreground is a bit over half a cord

If you had that one year of growth all turned into lumber, one could build the frame for about a 10'x12' shed, with floor joists, stud walls, and roof trusses (siding, flooring, and roof would be extra).

this shed frame is 10'x12'

To give one more version, here is a picture of the wood that each acre of our forest produces each and every day (averaged over the year).

For a much more in-depth and technical look at this same process discussed above, see the following book chapter:
Pretzsch, H. From Primary Production to Growth and Harvestable Yield and Vice Versa: Specific Definitions and the Link Between Two Branches of Forest Science. In Forest Dynamics, Growth and Yield: From Measurement to Model, 2009.

Estimate for total wood production: 3516 kWh/acre/year

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