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I'd like to heat my house using self-grown firewood. How do I go about calculating the number of trees I'd need to plant each year to make it sustainable - same number of trees harvested each year as new ones planted?

I know how much firewood (m^3) I need to use per season to keep my house warm, but I have no clue how to convert that to number of saplings and time of growth. Also, what would be the best species for that purpose (climate: moderate, Central Europe), and optimal time (how many years) to wait until harvesting the trees for wood and replacing them with new ones?

Any other tips for growing and harvesting trees for firewood?

  • williw and poplar are the two species that come to mind for a moderate climate. – EnergyNumbers Feb 14 '13 at 10:15
  • @Mark: Electric chainsaw to cut them down and chop them up; it's close enough to the house that transport will be fully manual, wheelbarrow or such. Uprooting old stumps may require renting a backhoe once every few years if they don't decompose fast enough. – SF. Feb 14 '13 at 11:46
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    @Mark: I would if I saw how your question and my answer is relevant to my original question. Sure it's a valid concern overall but it doesn't affect the numbers. It's the same n m^3 from a 20-year pine cut down with motor chainsaw and brought by truck, as from one cut down with an axe, cut with hand-saw and carried on my back. – SF. Feb 14 '13 at 14:45
  • I hope you are nowhere near any population, for burning wood is extremely polluting and kills 250,000 people per year. – gerrit Sep 7 '18 at 16:59
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The old farmers' rule of thumb is that you can harvest a cord of wood per acre per year. (For everyone not in the US, a cord is a unit of measure equivalent to 128 cubic feet of tightly stacked firewood, or 3.625 m3. An acre is about 0.4 hectares. So the rule of thumb corresponds to about 9m3/hectare.)

(For anyone questioning the validity of this estimate, consider: a 100-year old maple tree could yield a cord of firewood, and you could plant about 100 maple trees on one acre at 20x20' spacing. With that said, it's just a rough way to estimate yields.)

Being a rule of thumb, there are some variables to account for:

  • Different tree species have more stored energy per unit of volume. E.g. Hop Hornbeam (Ironwood, Ostrya virginiana) has 26.4 MBTU/cord (7.685 MJ/m3), while Willow (Salix spp.) has 14.2 MBTU/cord (4.133 MJ/m3).
  • Different tree species grow at different rates. E.g. Salix x sepulcralis may grow to 45' (13m) in 20 years, while Ostrya virginiana might only reach 20' (6m). So while you can plant willow and get a ton of growth, you'll only get half the energy out of all the labor you'll have to do to harvest the firewood. I do the work to pull 6-8 cords off 10 acres every year with machines (tractor, winch, chainsaw, splitter) and it's a lot of work. I'd much rather have the denser wood, same energy, half the effort.

You ask "how many saplings?", but that also depends on your species type and how you plan to manage the woodlot.

You ask "what would be the best species?". Personally, I would select 5-10 different species with different stored energy values, growth rates, and pest/disease resistance. If you get a beetle infestation that wipes out one species, you will only lose 10-20% of the total.

You ask "optimal time to harvest?". Personally, I prefer to harvest trees that are 40-60 years old. In my woodlot, these are typically beech (Fagus grandifolia) or maple (Acer spp.) that are 18-30" (45-75cm) DBH. You might be able to harvest smaller trees on a 20 year rotation, but then you're only getting logs that are perhaps 6" (15cm) DBH. Again, that requires 3x the harvesting effort. Also, having mature trees in the woodlot means that they reseed themselves -- I never do any planting (just thinning), nor do I remove stumps.

You know how much you use, and once you know the mix of species that you're using you can make a rough calculation about how much energy (BTU or MJ) that works out to, and the volume (cords or m3) that you'll need (assume an average energy density based on your preferred species mix). Then when you figure out your planting density (see "other questions" below), you know how much land area this will occupy.

Other questions you might ask:

  • What are the labor requirements for harvesting a {cord,m3} of firewood using various methods? (all manual, with machines, with draft animals)
  • What kind of storage do I need to build for curing firewood and then keeping it dry?
  • What planting density should I use for planting firewood? (Include the species mix you're going to use.)
  • What management strategies work well for a firewood planting?
  • How can I get multiple uses out of land that is planted to firewood? (silvipasture / agroforestry)

My answer above assumes that you're heating with firewood "conventionally" -- relatively large (>12" DBH) trees that have been cut into ~16" rounds and then split and dried. This is how I deal with wood. I have heard of (but am not familiar with) stoves that are set up to use smaller chunks of wood from much younger plantings, e.g. densely planted, fast growing, possibly coppiced species like willow or poplar on 10 year rotations. This possibility suggests questions about:

  • Coppicing (you'll have to do some initial research to be able to ask reasonably answerable questions here)
  • Systems (stove/boiler/furnace, storage, and possibly harvesting tools) for heating this way
  • Probably other questions that I haven't considered that will come up in your research
  • So, which 5-10 different species would you recommend for optimal growth+energy density rate? – SF. Feb 14 '13 at 14:38
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    I don't know what grows best in your soils and/or in Central European climate. That's a good question to ask your local forester, the person you buy your wood from currently, or other local firewood growers. Or observe what is growing wild around you. – bstpierre Feb 14 '13 at 16:33
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    I know the vote system is supposed to accomplish this, but I just had to say, GOOD POST! – OCDtech Feb 22 '13 at 20:25
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While wood-burning fireplaces have a shorter carbon cycle than coal and gas derived heating and help you to get off-the-grid, they are in no way "clean".

The pollution emitted from a domestic fireplace is much worse than the equivalent pollution emitted from dedicated power generation plant for the same amount of heating. The pollution is also local to your environment, which means you will personally breathe much more of it than if you used electricity supplied from the grid.

If your goal is sustainability, a wood burning fireplace is not the best solution. Investing in passive solar heating and heat storage and release would be a cleaner use of your time (although this really needs to be done when designing the house, not after it's built).

Some references:

http://www.epa.nsw.gov.au/woodsmoke/#pollution
http://ehhi.org/woodsmoke/health_effects.shtml
http://burningissues.org/car-www/index.html
http://woodsmoke.3sc.net/wood-vs-cigarette-smoke

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    Do these studies include comparison to pollution of producing these "sustainable" solutions, transporting them, marketing, maintaining work environment for their manufacturers and salespeople, and their suppliers? Does it factor in the oxygen produced by the trees while they grow before I cut them down? (my rough bet is the amount of oxygen produced and CO2 removed from atmosphere that way outweighs the amount of pollution emitted by burning the wood by far.) – SF. Feb 14 '13 at 14:52
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    Oxygen emitted and CO2 removal are not directly comparable to particulate and gaseous pollution. They are different things and are completely independent. One cannot "outweigh" the other. Transporting and marketing costs are equivalent enough for fireplaces and passive cooling to not affect sustainability. Other costs such as maintenance tend to favour the passive heating option. Lastly, with passive heating you can still plant the trees and just not cut them down and burn them. – Ladadadada Feb 14 '13 at 15:39
  • I think you have a somehow too hi-tech image for how my fireplace looks like. And passive heating using solar energy would require something better than 4-5 days of poor sunlight a month during winter, with temperatures sometimes dropping to -25C at times. I did my maths for solar heating of the house. It's good for fall and spring but not for winter. (and considering how ridiculously overtaxed "cleaner" sources of energy like gas and electricity are, in this case ecology must yield to economy.) – SF. Feb 14 '13 at 16:05
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    @SF. We all have economy to worry about, but this is a sustainability site, so Ladadadada's advice is excellent. You are right, that harvesting your own firewood is great in one sense. For climate change, it's a great solution, because all the CO2 your fireplace produces was already in the air (minus chainsaw energy, etc.). But, as this answer said, from a particulate standpoint, wood fireplaces are absolutely horrible. It may work for one cabin in the woods, but if any significant % of the population did this, air quality would be terrible, so it's hard to call it sustainable. – Nate Aug 4 '13 at 3:00
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    SF: then you could also halve your fuel consumption by getting a decent wood-burning stove. An open fireplace is great for returning heat to the environment but not much else. A modern double-burning stove is more efficient as well as less polluting. – Móż Aug 5 '13 at 0:15
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True sustainability is not dependent on outside sources. Try to learn the art of coppicing for fire wood, along with growing self propagating plants such as black locust which grows fast and produces more saplings along the root system and then can be cut for wood within a year or two. You would not need to purchase trees often once the coppice method is employed and the stump sprouts new growth. These also make good handle, bean pole and fence posts plus other ideas.

  • This is the best answer IMHO. Coppicing trees produces way more biomass in a given time than clear cutting. +1 for the black locust as well. – Steven Gubkin Jun 24 '15 at 2:38
  • JimBo's suggestion of coppicing, especially black locust, is ideal. Usually 4-5 year cycle to recut the new growth. Also look up rocket mass heaters, an affordable DIY masonry heater that uses far less wood than a typical wood stove. Burns cleaner, stores the heat over many hours so you aren't trying to smoulder the burn overnight. You use anywhere from 1/4-1/10 the amount of wood. I recently built one at the Cob Cottage Company, and you can make one pretty quick for cheap. – JohnD Oct 21 '17 at 6:53
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I have been cutting poplar trees for firewood on my 11 acre mixed use property for a few years now , since I have retired . I discovered many 50 to 60 foot tall trees , straight with few limbs , and 12" to 15" and more at the base .

Last year I cut 30 down of these large trees and wound up with 6 cords of stacked firewood , cut and split. 5 trees to the cord . Enough for the winter's burn.

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    Give the age of these trees too, please. – SF. Jun 18 '15 at 8:37
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I don't think the number of trees really matters. A fixed amount of energy from the sun falls onto a certain area and plant life will emerge to cover all of that area and capture the energy. If you plant fewer trees, each will get a bigger share of the energy and end up larger. If you plant more trees they will each get a smaller share of the energy and end up smaller. A closed canopy is a closed canopy and more trees will not result in more energy/photosynthesis/firewood.

As far as what area is needed to cover your needs:

"Primary productivity" is the term used to describe the rate at which energy is converted into organic material via photosynthesis.

Deciduous temperate forests have a net (i.e. useful) primary productivity of:

  • ~6000 kCal/m2/year
  • ~16 kCal/m2/day
  • ~67 kJ/m2/day
  • ~0.0186 kWh/m2/day

The households in my country average about 24kWh/day of energy usage over the course of a year. In the temperate regions of the country, about 30% of that (7.2kWh/day) is spent on heating. To provide that energy would require:

7.2/0.0186 = 387m2 of deciduous temperate forest

Thus the yearly heating needs of a single household can be provided by 'logging' an area only about 20x20m in size — roughly double the size of the house itself.

Of course you wouldn't be logging the same 20x20m area each year, you'd be logging a different 20x20m area each year — that gives time for trees to (re-)grow in previously-logged areas.

Tweak the numbers above to suit your local situation and plant (or allow existing trees to sucker) at least that area each and every year and you'll be right.

If your trees reach harvestable age in (say) 20 years, then you're logging on a 20-year rotation and would need a woodlot 20x the annual logged area (387m2) for it to be sustainable (i.e. a woodlot at least (20x387=) 7740m2 (0.774Ha) in size).

As others have suggested, it's a good idea to mix up the species. Not only do you end up with different burn characteristics, but if your woodlot is hit by a disease or invaded by insects, there's less chance that all of your trees will be wiped out at the same time.

  • You assume 100% of energy received by plants as sunlight is stored as chemical energy in the wood. This is blatantly wrong. Imagine a single, 1000 years old oak with canopy diameter of 20 meters. Can it sustain heat in 1000 households for a year? – SF. Oct 22 '17 at 20:36
  • SF, almost everything you said was wrong. 1) Incident radiation at 40⁰ latitude is ~640W/m2 and you get about 6 solar hours a day for a total of ~3.84kWh/m2/day. The "primary productivity" value is ~0.0186 kWh/m2/day... which has been obtained by government fields tests and research... and implies that only ~0.48% of the energy received makes it to a useful form. I'm not sure why you would think that 0.48 equals 100. – Tim Oct 22 '17 at 23:50
  • 2) A mature tree with a 20m diameter canopy, once harvested, can easily produce enough firewood to provide an average 7.2kWh of heating per day (~2,628kWh per year). Oak (since you mentioned it) has about 2,000kWh/m3 stacked. So you'd need to harvest only 1.3m3 of oak to provide all of your heating. That about a single, 16" DBH tree. You can be pretty sure a tree with a 20m diameter canopy will be more than 16" DBH. A lot more. – Tim Oct 23 '17 at 0:11
  • 3) Why did you invent the "single tree needs to supply 1000 households for a year" strawman? I clearly stated in my answer that the 387m2 area only supplies the needs of a SINGLE household with a 7.2kWh/day average heating need. Your original question said nothing about having to supply 1,000 households. – Tim Oct 23 '17 at 0:25
  • 4) I have no idea why you are referring to a 1,000-year old tree. Sustainable wood heat requires regular harvesting, so you wouldn't be cutting down old-growth forests, you'd be (re-)planting your own woodlot. If, say, your trees reach harvestable age after 20 years, then that's when you harvest them (and subsequently replant them). A 20-year rotation means your entire woodlot would need to be 20x the annual harvest area (387m2), so about 7740m2 (0.774Ha) in the above scenario. – Tim Oct 23 '17 at 0:35

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