## Thursday, 9 February 2017

### Food from the land - hunting and gathering

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

Natural wild ecosystems are amazingly beautiful and complex. Though northern forests may not be as diverse as many areas nearer to the tropics, there are still thousands of species found on our farm alone, plants, animals, insects, fungi, micro-organisms and more. These species all exist in a delicate balance, each with its own niche in the environment, feeding on and being consumed by others. I am not alone in my love of being out at the lakes and mountains, forests and fields, just to soak it all in. On the other hand, the energy moving through this system is not well optimized to produce food for people. In the time before the invention of agriculture, humans everywhere existed primarily by hunting and gathering, traveling over large areas and collecting what food they could find naturally occurring in the wild.

At first blush, it seems like there would be lots of food available right out in the woods. There are many different species that have historically been hunted available right in our area. Over the last few years, just on our own property, I have seen white-tailed deer, snow-shoe hares, squirrels, ground hogs, porcupines, skunks, beavers, otters, coyotes, ruffed grouse, spruce grouse, wild turkeys, ravens, Canadian geese, several species of ducks, and even a couple of black bears. While only some of these are hunted today, they were all on the menu when times were tougher than today1.

Then there is a great diversity of wild plants that one can eat in the form of leaves, grasses, tubers, berries, nuts, and more. There are also edible mushrooms growing in our woods, including such species as morels, chanterelles, chicken of the woods, and puffballs. While I haven't been that much of a forager myself, I have dabbled in wild berries, dandelions, the occasional hickory nut. I will instead defer to other sources of expertise, and my understanding is that a couple of the very best foraging books for plants in my climate in eastern Canada come from Samuel Thayer, who lives and forages around Wisconsin, The Forager's Harvest and Nature's Garden.
Pin cherries are a bit tart but still tasty

Of course lots of variety doesn't mean that there is large total availability. We don't see too many people living as hunters and gatherers today, and there are some very good reasons for it. Though there is a bewildering array of diversity just on our farm, the flora and fauna are spread rather thinly across the landscape, with animals ranging across many acres of land to find their food. For the plants, there are only a relative few concentrations of those that provide good food sources to people. Another major problem with great diversity is the harvest. With hundreds of different species to be hunted and gathered, it takes both an incredible amount of time and depth of knowledge to find it all and harvest it efficiently. There are very few peoples in the world who have continued a hunting and gathering lifestyle for any great length of time after they have been exposed to the food concentration that comes with farming. Finally, a climate like that found in eastern Canada makes food very seasonally limited, with many types of food only available during a short window each year.

So how far will an acre get us? Unlike some of the other land uses that I discuss throughout this piece, I couldn't find any precise estimates of the total availability of wild food. So, I've taken a couple of methods and done some 'back of the envelope' calculations.

Estimate #1: Density of Native Americans prior to European colonization

Now I realize that it isn't being fair to native Americans to classify their lifestyle as solely hunting and gathering, as they did a great deal to modify their environment and practiced many forms of agriculture. However, archeological and historical knowledge of our area of eastern Canada2 suggests that hunting, fishing and gathering accounted for most of the food of the local Amerindians. The history that I have read suggests that immediately before the arrival of Europeans the local natives summered in large camps along the Ottawa River and hunted their way through the hinterlands during the winter, relying mostly on small game and stores from the fall's harvest. Our property falls squarely in the middle of those historical winter hunting grounds, being forested hills near a major navigable river. This same text suggests that the population density of these peoples was only one individual per each 27 square kilometers.

We know how much food energy each person needs, and we have an estimate of the population density, so we can make an estimate of the total food energy production per acre per year for native peoples:

(2.32 kWh/person/day) * (365 days/year) *(1 person/27 square kilometers) * (1 square kilometer / 247 acres) = .12 kWh/acre/year

This estimate suggests that each acre produced on average much less one day's worth of food each year. Most years a given acre of forest probably didn't provide any food, while others would give up a few handfuls of mushrooms, some berries, or in a lucky year, some wild game.

This sort of estimate doesn't account for seasonality or technology. There was much more food available in the summer and fall with all of the ripening plants and young of the year animals, but pre-European peoples did not have the same abilities to harvest, preserve and store food that we do today, nor did they have modern weapons that would allow them to harvest all of the available game. To account for that, and the amount of time that would really be needed to harvest all of the hundreds of edible species throughout the year, let us say that the actual population was only able to fully take advantage of 1/100th of the total potentially available food. This brings us to an estimate of:

.12 kwh/acre/year * (100 units food available/1 unit fully utilized) = 12 kWh/acre/year

Estimate #2: Ecological estimates of the carrying capacity for wild edible species
Another way to come at this same question would be to take a look at the science of ecology, in that biologists have long been studying the populations and distributions of native flora and fauna. I will start with the wild game, and then move on to a semi-educated guess about available plants and fungi.

For almost all of the larger animal species found on our property, especially those that are hunted, there are relatively good estimates of the population densities. These are especially useful for natural resource agencies and are used to evaluate the health of populations and set hunting regulations to maintain the health of those populations. To make the estimates below, I found what sources are available for the number of animals/acre. Many of these species actually have ranges of tens of acres or more per individual, so the numbers can be quite small.

Providing a few ecological estimates:
White tailed deer -  30 deer/mi sq *(1 sq mi/640 acres) * (1 of 3 deer harvested per year) * 40 pounds meat per deer * (.7 kWh/pound) = .4375 kWh/acre/year of venison
Ruffed Grouse - 50 grouse/mi sq*(1 sq mi/640 acres) * (1 of 2 grouse harvested in fall) * .5 pounds meat per bird * (.6 kWh/pound) = .012 kWh/acre/year of grouse meat
Other calculations end up being similar, for species such as wild turkeys, small mammals, bears, etc. Being generous, wild game could add up to something like 2 kWh/acre/year of meat in our region.

Edible wild plants that I have seen locally: Berries (blueberries, raspberries, strawberries, hawthorne berries), tree fruit (wild crabapples and plums, edible tubers (e.g., cattails), nuts (acorns, butternuts), stems and leaves (dandelions, basswood leaves), young growth (fiddlehead ferns, wild leeks), mushrooms (morels, chanterelles). This is of course not a comprehensive list, nor could I find precise figures on harvest rates of wild plants and edible fungi, but I did come across many admonitions to avoid overexploitation of these species, as it can cause their decline. For the sake of argument, one could imagine that it may be possible to harvest 40 pounds of plants and mushrooms per acre in unmanaged forest, field, and marsh, which would yield:
(40 pounds/acre/year) * (500 Calories/pound on average) * (1 kWh/860 Calories) = 23 kWh/acre/year

This total estimate of 25 kWh/acre/year is roughly in line with the first calculation above.

Estimate #3. Percentage of total available photosynthetic energy

From here we have an estimate of 35790 kWh/acre/year worth of energy harvest by plants on our property. This energy is the original source for all of the wild edible species available, whether they be plants, animals or even fungi. From the estimates above, this means that something on the order of one part per thousand of the total energy captured by plants in the forest reaches a form that could reasonably become food for someone who is hunting and gathering. If it truly were one part per thousand, then we would have:

35790 kWh/acre/year * .1% efficiency at creating human food = 36 kWh/acre/year

As all of these estimates are so small as compared to other land uses, we will go ahead and use this last and largest estimate going forward.

How far would hunting and gathering get me and my family?

36 kWh hours worth of tubers, berries, leaves, seeds, meat, and fungi. This really is not very much on the scale of human needs, as this is only 15 days worth of human food for each acre of land. At 150 acres, our farm property could then support the food needs of 6 people, if those people had the skills and time to harvest, process, and store all of the naturally available foods available on the property.

(36 kWh/acre/year of food produced) * (150 acres) / (849 kWh/person/year of food) = 6.4 people supported.

Even with the proper skills and knowledge, the harvest and processing of all of this food would be close to a full-time job, leaving very little time to, say, hold down a job to support those needs other than food. This would be bare subsistence and there would be no excess to sell or trade. On top of all that, even though I consider myself fairly knowledgeable in such things, I don't have anywhere near the expertise necessary to find and process all this variety of food. I think that the best takeaway from this analysis is to show that in today's world, hunting and gathering belongs in the place where it sits, as a pastime for those who enjoy being out in nature and like to eat in a more adventurous way.
Wild plums in early August

Previous Page: Harnessing the sun's energy - Photosynthesis
Next Page: Coming soon.

1 I have a copy of a classic cookbook called the Joy of Cooking that I inherited from my grandmother. This edition is from 1950. The most fascinating section to me in this book is on wild game, with instructions for skinning, cleaning and cooking a wide variety of species that I have never heard spoken of as dinner possibilities, including raccoon, porcupine, and beaver.
2 Gaffield, Chad et. al. History of the Outaouais. Laval University Press, 1997.

## 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 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, taking 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 if it had been taken better care of.

Clear cutting is another, and often much more villified, 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. 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 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).

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

Previous Page: Harnessing the sun's energy - Photosynthesis
Next Page: Coming soon

## Friday, 20 January 2017

### Harnessing the sun's energy - Photosynthesis

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

Almost all life on earth is supported by solar energy that is originally harvested by plants and algae. Though there are some notable exceptions, such as the life found at deep sea vents, that isn't really applicable to a discussion of a farm in Quebec. In photosynthesis, plants and algae capture the energy of photons (particles of light), and use that energy to cause a chemical reaction, taking water, carbon dioxide and solar energy as inputs, which allows them to produce simple sugars and oxygen gas as outputs. The chemical process is as follows:

6CO2 + 6 H2O + photons → C6H12O6 + 6O2
More simply: carbon dioxide + water + sunlight  → sugar + oxygen

These sugars are the primary 'stuff' that plants are made out of. As was discussed here, sugars can be thought of as chemical 'springs', that are built and compressed by adding energy from the sun, and can be released at a later time to access the energy (motion) that they contain.

These same sugars are then the source of energy for nearly all other forms of life. The plants use the sugars themselves to support their ongoing needs, to grow and build their stems, leaves, roots and trunks, and to reproduce, making flowers, fruits and seeds. All animals and fungi are in turn supported by these plants, eating and recycling the energy that the plants have captured and stored away. As both the plants and their consumers go about their business, the energy stored in these sugars is converted many times into other forms, used to build other tissues, and is stored as many types of sugars and fats, and often actively used more actively in the form of proteins. Some of that energy is also converted into heat and motion, such as when a person heats up when they are out for a jog.

As with every type of energy capture and conversion, photosynthesis doesn't capture all of the energy coming in from sunlight, it actually converts only a small fraction of it into sugars. Without going into a long lesson about biochemistry, the takeaway is that the maximum possible efficiency of photosynthesis is around 11%, but that the typical efficiency of actual plants is more like 2%. If you would like to know more about those details, start here and here. This means that out of all the energy that is in the sunlight striking a plant, the plant only captures and uses two parts of each one hundred.

But it is even worse than that. This 2% figure is for plants that are growing in a good environment, with just the right soil, temperature, nutrients, rain, etc. At our hilly farm, we have conditions far from this optimal. The local soil is sandy and rocky, and doesn't have all the nutrients that are available in the good black soil found in local river bottoms. Of course soil can always be augmented with fertilizers and other nutrients, though this is not been done at our property. Fortunately rain is not an issue for us, as our area consistently has 10 or more days per month with at least some precipitation. For the sake of argument, lets say that these imperfect conditions reduce by half the total energy capture of our local plants.

Finally, it is cold and snowy for nearly half of the year. The only time that plants can capture the sun's energy is when they have leaves. We have at best 180 growing days, from early April to early October. Fortunately the summer is also when there is more sunlight, so only having leaves for half the year means that plants can take advantage of 70% of a year's sunlight (see monthly energy availability here). Putting all of these factors together, we can figure out how much energy an acre worth of plants can actually harvest in a year.

5,112,000 kWh/acre/year * .02 photosynthetic efficiency * .5 losses for imperfect conditions * .7 of annual solar energy captured = 35790 kWh/acre/year of energy harvested by plants at our land

How numbers fall, don't they? We are now down to a figure in the same ballpark as our annual energy needs per person. And remember, this is all the energy that plants harvest, not just the parts that we want (known in biology as gross primary production). That energy includes everything, the metabolism of the plant, the fighting off of disease, pests, and predators, and the growth of leaves, stems, flowers, seeds, and roots. No matter which part of the plants that people want to use, the useful amount is always going to be much less. Say we were growing corn (we'll get back to this in a later section on crops). In this case, we only really want the kernels of corn. The rest of the plant, the stem, the leaves, and the roots, are usually not harvested or used. What if we only wanted the walnuts from a walnut tree?

If we wish to be sustainable, these considerations set the limit for how much energy from plants could be harvested per year. As long as we eat plants and use wood products grown under the sun, these limits are going to constrain us. It is possible that with better plant breeding, irrigation, soil quality, fertilizers, pesticides, etc., that this figure could perhaps be doubled or even tripled, so that our plants could capture a much higher yield. Also keep in mind that these numbers are for a northerly clime - our solar resource, and our growing season, isn't as long or as bountiful as what might be found in other areas of the world. With more sunlight and a year-round growing season, fertile tropical areas could be much more productive than our farm in the north.

Now that we have an estimate for the amount of energy that plants can obtain from the sun, we can look at all of the different plants, and sometimes the animals that feed on them, that we could grow and harvest on our farm.

Estimate for photosynthetic production: 35790 kWh/acre/year

Previous Page: Insolation (aka Sunshine)
Next Page: Logging, cutting down trees for wood products

## Wednesday, 18 January 2017

### Insolation (aka Sunshine)

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

If we are going to live within the energy budget that we could generate renewably, would we be able to support our energy needs from what we can harvest at our farm? As was mentioned in the introduction, the basic realization that I had was that for most, if not all, sustainable uses of our property, that sunlight was the energy source from which all our other activities would spring. In terms of sustainability, this is as close as it comes to getting 'free' energy - sunlight is going to be shining down on the earth's surface regardless of what we do and will continue to do so for a few billion years.

The amount of solar energy that arrives at any particular spot on the earth's surface is quite variable. This is largely due to the combination of the location of the sun in the sky as it moves throughout the day as well as the weather. For the orientation of the earth to the sun, the closer the sun is to straight up in the sky, the more energy reaches the surface. Annually, the maximum amount of solar energy is generally for those places where the sun is most directly straight up in the sky for the greatest part of the year. This is mediated by clouds, which are the biggest way that weather has an impact on solar energy. When it is cloudy, much of the sun's energy is intercepted, with only a relatively small amount reaching the surface in a form that can be captured for other uses.

The equator is where, not counting weather differences (cloudiness), the most light energy comes down each year. The Ottawa region, where our farm is, is halfway between the equator and the north pole, at 45 degrees latitude. There is an excellent sun visualization tool from the University of Nebraska, from which I've made a couple of schematics below.

Summer solstice (June 21) sun path in Ottawa
Here in Ottawa, the sun never is straight up in the sky. The yellow line with the sun on it shows where in the sky the sun passes across the sky through the day. As can be seen in this first image, the summer sun in Ottawa reaches fairly close to straight up in the sky at noon on the summer solstice, 68 degrees up off of the horizon (90 degrees would be straight up). And in the winter, the sun never gets very high in the sky, as you can see in the image below. On the shortest day, December 21, the sun reaches only 22 degrees above the horizon.

Winter Solstice (Dec. 21) sun path in Ottawa

When one actually compares total annual solar energy to other areas around the world, Ottawa doesn't fare as bad as one would think. Ottawa receives about 2/3 as much solar energy as Cairo, Egypt, which receives as much solar energy as any large city on earth. How does this happen, you ask? Ottawa, and any other locations far from the equator, have something very interesting going on. They have very long days at the same time as the sun shines from highest in the sky; we call this time 'summer'. This also means that Ottawa gets most of its solar energy in those few summer months.

Because of the variability in the energy available from sunlight, we will describe the amount of sunlight over the course of a complete year. In Ottawa, the total insolation (solar energy) over the course of a year is: 1263.4 kWh/m2/ year. This is energy per square meter per year. A square meter is a bit smaller than a typical door inside a home. Though the annual total is important, notice in the chart below that the amount of solar energy at our location varies tremendously, with July getting over 4 times the energy from sunshine as January. As discussed above, this is due to sun angle, day length, and prevailing weather.

(chart data courtesy of http://www.gaisma.com/en/location/ottawa.html)

To discuss land use, one seldom speaks in units land as small as one square meter. A more typical unit that is discussed in land use, property ownership and agriculture is the acre (especially in the US). One acre is roughly the size of a football field, minus the endzones. This is a decent sized chunk of space, but very manageable when discussing forests and wheat fields. Here is a picture of one of our fields with lines added to show exactly how big one acre is.

Converting out from our square meter, this single acre, as well as every other acre of property in the area, receives:

1263.4 kWh/ m2 / year * 4046.86 m2 / acre = 5,112,000 kWh/acre/year

Over five million kilowatt hours per acre per year, that is an enormous amount of energy! This is especially true considering that we calculated that my family uses 80,000 kWh/year in our personal lives, and that the total annual energy use per capita in the US is only about 90,000 kWh/year. By this math, it looks like one acre should be able to support the energy needs of over 50 people on each and every acre. It almost seems like there is no problem to be solved, that there is so much energy that we can be as inefficient as we want. If only it were so simple. The problem is, as was discussed here, that we have to have some way to capture, convert, and make that energy useful for fulfilling our needs, and we have to account for all the inefficiencies of converting energy from one form into the next.

What do we have at our disposal that can capture that solar energy so that we can put it to good use? Though there are some technological options that we will discuss later on, we will start with the solution that life on earth figured out billions of years ago, photosynthesis.

Estimate for solar energy reaching the ground: 5,112,000 kWh/acre/year

Previous page: So how much energy does a person really use?
Next page: Harnessing the sun's energy - Photosynthesis

## Saturday, 14 January 2017

### So how much energy does a person really use?

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

I've seen quite a number of different ways of calculating real world energy use, and the different techniques certainly have different merits. The simplest of them simply involves an estimate of the total amount of energy used by a group, such as an entire country, divided by the number of individuals in that group. This sort of estimate gives the average amount of energy that is used per person to keep society functioning. It has the big advantage of including all sorts of things that are outside of personal consumption such as the cost of building and maintaining infrastructure, running government, commerce, hospitals, manufacturing, etc., which are not easily visible to the individual. The downside of course is the lack of specificity to a given individual or family; some families may use vastly more or less resources than the average based on their wealth, geography, and their consumption habits.

Another pathway is to try to make specific estimates of a given individual's energy use, which allows a fine-grained analysis of the particulars of each situation. For the questions that prompted me to begin my investigation, it was these personal level energy considerations that I initially thought about. Of course this sort of approach leaves out everything outside of one's direct consumption, and can only measure those things that an individual comes into direct contact with.

One other form of energy that we should account for is the embodied energy in the major 'things' that we own, and there are existing estimates that I can piggy-back on of how much energy is needed to make things as well as how long they last. For my family's energy consumption, I will assume that each of the 4 individuals in my household, including two small children, each count as needing the same amount of energy. This is of course not entirely accurate, but provides a good first approximation.

What I've decided to do here is to follow multiple paths, to start with some rough calculations of the broader-scale averages, and then dig deeply into the situation of the family that I know best, my own. For the personal energy consumption, I need to make some caveats about what I am and am not including in this list. I'll look only at our personal consumption, of food, shelter, transport, etc., but to not even begin to account for the energy that was needed to grow and transport that food, or to refine and ship gasoline to my local station. It is absolutely true that this external energy use will be significantly greater than the amount of energy that reaches the end user. Likewise, for this first pass I will ignore the question of where this energy actually comes from, and discuss the sustainability of my energy use in a later section.

Overall energy usage estimates
In starting to write this section, I realized that I was just dipping a toe into an existing area of sustainability research. For instance, I found an entire book on energy usage by Dr. David MacKay, titled "Sustainable Energy - Without the Hot Air", which digs far deeper than I intend to on power sources and power usage. So rather than reinvent the wheel, I will suggest that you look at this book for a deeper analysis of these average energy estimates. The book is written about Britain, which uses less energy per person than Canada or the US, but other than that the analyses should be much the same. So without further ado, I will give his estimates of energy use per person, of 195 kWh/day for the British, and 250 kWh/day for Americans. Another analysis by the Economist puts together similar figures, giving 242 kWh/day for Canadians, and 233 kWh/day for Americans. It is no big surprise that the North American lifestyle is more energy intensive than the British, if only for the reasons of the larger distances that people must travel and the more severe climates that North Americans must deal with. As mentioned above, a given individual does not have personal control over a lot of this energy use, as it includes their share of the roads, government and commercial buildings, the military, and more.

Household energy use
Statistics Canada data shows that the average detached home in Canada uses about 40 kWh/day per person for such things as heating, cooling, and electrical loads. All home energy uses and sources are lumped together in this estimate, so this number includes the total energy used within the home by electricity, natural gas, heating oil, etc.

For my own family, we actually need to account for the household energy that we use both at our rental in the city as well as our place out at the farm. I have already run some analyses of our household energy use at our country house here, where I showed that this house consumed a total of 15,200 kWh for the year of 2014-15, giving a total of 10.4 kWh/day for each member of my family. This home was intentionally built to be highly efficient, and so it is good to see that this house needs only a fraction of the energy of a typical home.

I've also reviewed the electrical and natural gas bills for our rental in the city, which is a 3 bedroom duplex that we are in about half of the time. For our natural gas bill, we used 1480 cubic meters of natural gas over the last year, which with a bit of calculation puts us at 15318 kWh, or 10.5 kWh/day per person. Our electricity bill shows that we used 7075 kWh over the last year, and that adds up to 4.8 kWh/day per person. The total for this place is then 15.3 kWh/day per person, which is still quite a bit lower than the average. There are at least a couple of contributing factors here. One is that heating loads are much reduced over a typical detached home, because of a shared wall with the neighbor, as well as because of significant updates to the insulation and windows. Second, as we are not there all of the time, many energy uses are reduced when we are away.

Putting this all together, my family used a total of 25.7 kWh/day per person to run our household for all heating, cooling, and electrical loads for the year of 2015. While this is a reduction by almost half over the national average, but I would still like to bring down this number further into the future, or at least shift to most or all renewable energy sources.

Embodied energy in our homes

It takes a lot of energy to build a home, and it is also true that a building doesn't last forever. This means that one should consider the amount of energy that is 'used up' each year due to the slow and steady aging and decay of a home. Some homes may end up lasting hundreds of years if someone takes the time and invests the energy to do regular updates and maintenance, whereas some others, if poorly cared for or if someone decides the land should have a different use, may not even last a decade. Putting these together, it is a reasonable rough assumption to say that a house lasts 100 years. While it is beyond the scope of this piece to go into the ins and outs of calculating this embodied energy, some analyses that I found here, here, and here all point to figures that are in the ballpark of 300,000 kWh for a 2000 square foot home. Spaced out over 100 years for a family of 4, this means that each of the homes that we occupy are consuming 2.1 kWh/day per person for each of the two buildings that we inhabit, due to the slow process of aging, decay, and obsolescence.

Gasoline used in our automobiles
Transportation is another large source of energy consumption. Americans, in 2013, used 392 gallons of gas per capita. Put into our terms, this is 36 kWh/day per person. This is a lot of energy. So how does my own family stack up? We have 2 cars, one that is a 2003, the other is a 2015 model, they both happen to be Subaru Outbacks. Fortunately, my wife has a commute of only 4 miles, and my commute is short enough that I can usually walk or bike. We do often go from the city to the country, but usually take only one car, and the trip is about 50 miles each way. In 2015, we put 8000 miles on the 2015 car, and only 1000 miles on the 2003. While I don't have gas receipts, I can calculate our total energy use based off of the odometers and the published mileage estimates for our cars. Our 2003 Outback should get a combined 22 miles per gallon, while the 2015 car gets 28 miles per gallon. This gives us a total of 331 gallons of gasoline containing 11,155 kWh, or 7.6 kWh/day per member of my family to get us around in our cars. This is again a pretty dramatic reduction over the American per capita use. However, do keep in mind that the overall American figure does include all sorts of commercial traffic, as well as all those poor souls who have daily long commutes in gridlocked traffic to deal with.

The 'typical' American driver drives for 13,476 miles per year. The average gas mileage of the American passenger car fleet is in the same ballpark as our own vehicles, so if our family drove the typical mileage, this would consume 1077 gallons of gas for 35972 kWh, or 24.6 kWh per day per family member.

Embodied Energy in our cars
Just as with homes, cars have a finite lifespan and require considerable energy to manufacture. In looking around, I found various figures (here, here, and here). None of these analyses are for our vehicles in particular, so I will approximate our cars as having about 40,000 kWh of embodied energy in each of them. Considering a car lasts about 15 years and that we have two of them, this means that car aging accounts for about 3.7 kWh/day for each of our family members.

Embodied energy of other possessions
On top of the big items of home and car, we of course have all the other trappings of modern life, including appliances, furniture, electronics, clothing, and more. I wasn't able to easily track down good estimates of the embodied energy of each of these myriad items, so for the sake of argument, I will simply say that we have an amount of 'other stuff' that is about the equivalent of our cars, which a quick run through my imagination suggests is roughly reasonable both to the amount of stuff that we have as well as how long things last. With that estimate, we then have an additional 3.7 kWh/day to account for.

Air travel
In recent years I have heard rumblings about the sheer amount of energy is needed for air travel, and there appears to be a considerable literature on the topic (e.g., here). Fortunately I was able to find a source that lists the FAA estimate of energy expended per passenger mile traveled, at .778 kWh/mile. A look at the US Department of Transportation data shows that there were 607,772,000,000 passenger miles flown domestically in the US in 2014, and there were 318,900,000 Americans at that time. This works out to an average of 1906 miles flown per person in 2014. In energy terms, this is 1483 kWh/year, or 4.1 kWh/day.

A few minutes with our calendar and a map show that my family traveled 2714 miles by air in 2015 with a couple of trips to visit friends and family, which puts us at a total of 8446 kWh for the year, or 5.8 kWh/day per person. Not as much energy as we burned in the car engines, but still quite considerable, and a bit above the national average.

Food
Food is easy to account for if you simply estimate that each member of my family ate the typical recommended 2000 calories per day. Again, this doesn't account for a lot of things, like any food that we may waste, or the fact that my young kids certainly don't eat this much, but it is close enough for our needs. There is also the consideration that meat requires much more energy to produce than fruits, grains, and vegetables, but we will for now leave that outside of our calculations. 2000 calories per day means that my family consumes about 3395 kWh worth of food, or 2.3 kWh/day per person.

Table summary
First, keep in mind that the figures for my family and the averages aren't directly comparable, as they were calculated in different ways in the descriptions above. That said, the table above summarizes most of the energy that my family is in direct control of, we get to decide where we live, how much and how we travel, what sorts of goods we buy, etc. It is also easy to see here that of the energy that we have direct control over, most of it is for transportation and home energy needs, and that a smaller proportion is for all of the 'stuff' that we have.

You can also see that the estimate of personal average energy use is 80.5 kWh/day, while the total national energy use is 233 kWh/day. This means that the average individual only has direct control over about 1/3 of the energy that is being used 'on their behalf'. Two thirds of the energy is from all of the infrastructure and services that surround us, the roads, the stores, the hospitals, the police, inputs to food production, all of it. All of this other energy being used on your behalf is a part of what it means to be an American, or a Canadian, or whichever nation you may live in.

A later section of this blog will look more deeply at my ideas about what we could aspire to do in terms both of reducing total energy usage as well as to make the energy that we do use to be truly sustainable in the long term. However, I'll give you a hint of where I'm going, and suggest that I think that my family may over time be able to reduce our energy usage by as much as one half. This still leaves a big job to provide for enough energy to run our busy household. Now we have to hope that sustainable and renewable sources can provide for those needs, and will begin to look at those numbers next.

Family energy use and the amount that our farm would need to produce

So these final numbers in this section are the numbers that we need to shoot for, if we were going to produce enough energy from our farm to provide for my family. My original question was, "Does our farm property produce enough usable energy to sustainably provide for all of my family's energy needs?" In the table above are three different statements of the same information, and all three scales make different kinds of sense. Energy use per day makes sense when thinking about all of the things that we do on a daily bais, from commuting to work, eating meals, watching TV, sleeping, and more. It is a scale that is very human. Then, there is the much larger number of annual energy consumption. This makes sense in light of yearly cycles. Plants only grow in the summer, home heating is only applicable to the winter season, and all of the ups and downs of daily activities and energy use are smoothed out when looking at the course of a full year. Finally, there is average continuous energy use, watts. When you get right down to it, this is still energy use per unit time (joules/second), but we often think of watts as 'instantaneous' usage. 8500 watts, this would be like constantly supplying the needed power for 85 laptops, or 8 drip filter coffee pots, or 2 large domestic hot water tanks. It is a lot of energy, but by no means unimaginable. Following sections will examine how much energy different land use decisions could actually generate.

Previous page: Measuring energy
Next page: Insolation (aka Sunshine)

### Measuring Energy

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

It is very easy to get lost in the morass of terminology surrounding the measurement of energy. There are some good reasons for why there is so much terminology, but much of it results from the history of what was discovered when, by whom, and used for what purpose. To try to keep things as simple as possible, we will stick to only 2 units of measurement, those that I find to be most familiar to people and that are easy to work with at the human scale. The first of these is the Calorie1, which is how we generally speak of the energy that we get from food. The average person consumes about 2000 to 2500 Calories per day. The meals, snacks, and beverages that a person consumes each day provide all of this energy. The second unit that we will use is the kilowatt hour (kWh). This is the standard unit of measurement for electricity, so all of you readers who pay an electricity bill should be used to seeing this measurement. Depending on where you live, the cost of residential electricity can be anywhere from a few cents to a dollar or more per kilowatt hour, with current 2015 prices in North America mostly being between $.10 and$.30 per kWh.

A couple of examples can explain how the kilowatt hour is measured, and put it into perspective. Say that you have a 100 watt incandescent light bulb that you want to use to light up your kitchen (a laptop computer working hard uses about the same amount of power). That rating, in watts, is a measure of how much energy is required to get the bulb to light up for any given second. With anything electrical, it is the motion of electrons  through the parts of the device, be it a lighting filament in a light bulb or a transistor in a computer, that allows them to function. It is beyond the scope of this piece to go into a deep look at the physics of electricity, but if you are so curious, here is a place to start. So our light bulb needs a continuous supply of 100 watts to stay lit. What if we wanted to talk about the amount of energy needed to keep the bulb lit constantly for a full 24 hour day? To measure it, we can just say how much time that energy was being used, and this is commonly done in hours. So to run a 100 watt light bulb for 24 hours, one multiplies watts by hours and gets 2400 watt-hours. To make the numbers more manageable, this can then be switched to be 1000 times smaller by adding the suffix kilo-, making it 2.4 kilowatt hours (kWh). 2.4 kWh is the amount of energy it takes to keep that light bulb lit for a full day.

A second very different example can be from biology. How much energy, in kWh, does it take to run a person for a day? As mentioned above, a person needs roughly 2500 calories each day. Since they are both units of energy, it turns out that one can directly convert Calories into kilowatt hours; 1 kWh is equal to 860 Calories. Applying this to our daily intake of food, a person needs 2.9 kWh of energy to do our usual goings on. So it takes just a bit more energy to run you, the reader, for one day as it does for a 100 watt incandescent lightbulb. Put another way, a person runs on about 120 watts. The kilowatt hour then acts as a very human scale measurement, in that a person typically uses some relatively manageable number of kilowatt hours of energy for a day's activities.

All of the different units for measuring energy are potentially interchangeable, and I've provided a table below that shows a few of the common units that you may be familiar with and their conversion with each other.

Now that we have established a common language and an idea of scale for discussing energy, another table can show the amount of energy found in many of energy sources we encounter day to day. Many of these particular ways of storing energy will come up again the following sections. One caveat to make about the whole fresh foods here, like chicken or potatoes, is that much of the weight of these is actually water, which doesn't contain any useful energy. So while the energy in chicken comes mostly from protein and fat, much of a whole chicken is actually water.

Previous page: Energy capture, conversion, and storage
Next page: So how much energy does a person really use?

1 This blog will always be speaking of the dietary calorie, or kilocalorie, see here for a more in-depth explanation.

### Energy Capture, Conversion and Storage

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

Each time one form of energy is captured and converted into some other form, only some of the energy is converted to that new form, the rest of the energy is lost. This isn't to say that the energy disappears, as energy is neither created nor destroyed (see the 1st law of thermodynamics). Instead, I mean that only some of the energy goes to the targeted next step, and the rest goes somewhere less useful. Let's take the example of a typical car running on gasoline. The desired target for the energy from the gas is to move the car, but there are some unavoidable problems. In this case, the issues are such things as friction on the highway, which heats up the tires (converting some energy to heat), and wind resistance which slows the car by transferring some of the motion of the car into the air. The engine turns much of the energy of the gasoline into non-useful heat, doesn't burn all the gas perfectly well, and so on and so forth. For the example above of a gas-powered car, only about 20% of the total energy in the gasoline is converted into motion of the car. And this doesn't even include the fact that  energy is often converted many times between different forms before it is used for the motion that we want, with losses at each and every step. For that gasoline example, there are inefficiencies and losses in taking the oil out of the ground, refining it into gasoline and other products, as well as transport and storage losses.

Energy storage is also a key concern; What happens if there is available energy and motion now, but you don't want to use it until some time in the future? Well, you need to find some way to convert the energy into some stable form, and be able to also have some way to convert it again for the end use. Stored energy is 'potential energy', meaning that one has the ability to cause things to move at some point in the future. An easy way to imagine potential energy is with hydropower. Say that there is a river running through a valley. There is a lot of water motion that is there, that could potentially be harnessed and put to use. But what if the river runs dry every summer, and you wanted to be able to have a steady supply of power? You could then build a dam. Instead of the water moving now, the dam stops it and holds it still in a lake above the dam. All of that water has potential energy because gravity would like to pull it down, but the dam stops the water from moving. In this situation, much of the energy (motion) of the water in the river is now stored in the lake, and can be utilized whenever it is needed. The motion of the water, when it is released, is used to spin a turbine (basically a big wheel), and this spinning motion can be used to create electricity (though in the past it was for things like cutting wood or grinding wheat into flour). Other forms of energy storage act by the same underlying principle. They capture and freeze motion in place, setting up a situation where motion can be re-directed in the future.

A huge amount of the energy coming from the sun is captured and harnessed by living things, and passed between different living organisms via a food web - big fish eat small fish, small fish eat insects and other tiny creatures, the tiny creatures eat algae. Though the details are vastly different, the same basic principles of energy storage and conversion apply across all ecosystems. All living things store their energy largely in the form of proteins, fats, and carbohydrates (such as sugars). Each organism needs a steady supply of new energy inputs to metabolize, to grow, to reproduce. The base of this food web in most places is of course the process of plants converting energy from the sun into sugars and then on into other more complex forms. A good rule of thumb is that each level needs 10 times more of the level below to support it. So, 1 pound of large fish requires: 10 pounds of small fish, 100 pounds of tiny creatures, 1000 pounds of algae. In biology these are known as 'trophic levels'.

Animals and plants both stock up on energy reserves in times of plenty to prepare for the times of scarcity. This is both for the short-term as well as the long term. In the short term, plants need to have the energy to make it through the nights, as well as at least a few cloudy days. For animals, there are times when food isn't always available. Predators in particular may go relatively long periods between feeding. And of course there is the winter. Here in eastern Canada, the lead up to the long cold winter has animals putting on heavy layers of fat, and the trees building up stores of sugars through the summer and early fall, then dropping their leaves while sending most of their nutrients down into their roots.

Previous page: What is energy?
Next page: Measuring energy