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While wood can - in theory - be a carbon neutral fuel, in practice burning wood creates a huge carbon debt and it may take a long time for all the carbon released by logging, soil degradation and so on to be reabsorbed. Additionally, one has to take into account how fast the wood will regrow (though the absolute regrowth will probably be faster than forming of new soil etc. for any given forest) and habitat destruction.

My question is, say I have a healthy forest of x size (I don't) in a temperate climate zone, how much wood can I log and burn so that ...

  • I don't release CO₂ faster than my forest can reabsorb it
  • I don't burn wood faster than it regrows
  • The carrying capacity of my forest for the various animals and so on stays roughly constant, there's no long term decline in habitats.

The background of the question is the following: Biomass is sometimes heralded as a green energy source, a claim that is highly dubious given the prevalent industry practices. So I wonder: How much energy could we actually gain from biomass from a given forest size if it was actually green?

  • Just as a side note: you are actually likely to increase biodiversity by selective logging by creating empty patches where new life can settle. – Stockfisch Apr 25 '14 at 13:22
  • Do not burn wood for space heating if you care about sustainability. The particulate matter is atrocious, worse than a diesel engine and killing 250,000 people per year. – gerrit Sep 7 '18 at 17:22
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Well, I'm by no means an expert on giving this answer, and yet I'll take a go at providing something reasonable. Assuming a 1ha biodiverse, properly hydrated, young stage forest with optimal leaf coverage that gives us a solar catchment area of 10,000 square metres. Subtract say 20% to account for immature trees amongst the larger growth, another 20% for damaged trees and other incidental issues like leaf blight or whatever else might go wrong and you're at 6,000 square metres of catchment. A single mid-stage growth tree can capture and sequester 48kg of carbon per year. Assuming a mid-stage tree head diameter of 2m that means that you have 3000 trees sequestering 144,000kg of carbon per hectare per annum. Using techniques like coppicing and pollarding in the forest increases the efficiency of aboveground tree growth which means a more rapid turnaround to go from a bare patch to a tree catching sunlight. As such we can probably up that 48kg per year figure to in the realm of say 60kg/yr pushing us up to 180,000kg per annum unless I've totally gone wrong with my maths somewhere.

That 180,000kg is largely bound up in the form of cellulose and lignin. When you have good, seasoned wood ready for use in a furnace/fireplace it ends up representing about half of the weight of the wood as much of that is still bound up as water/sap and other compounds.

So, a single hectare can supply about 9m3/hectare according to this answer here on this site. That's enough firewood to supply about 3 families with 3m3 each of wood for home heating and cooking needs, or 9 families 1m3 provided they are using fuel efficient heating like Rocket Mass Heaters or Kachelofens for heating and rocket stoves for cooking. Assuming we are talking moderately efficient kinds of woods dehydrated to 12% moisture content, that averages out to be in the ballpark of 600kg/cu.m which works out to be 5,400kg. Assuming a carbon content of 49.5% for the wood, that works out to be 2673kg of carbon re-released per year. This means in net, a single sustainably harvested 1ha forest will affix 67 times the amount of carbon that is released from the burning of the wood that comes from it.

Does that help?

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How much energy could we actually gain from biomass from a given forest size if it was actually green?

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 primary productivity of:

  • ~6000 kCal/sq.m/year
  • ~16 kCal/sq.m/day
  • ~67 kJ/sq.m/day
  • ~0.0186 kWh/sq.m/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 = 387sq.m 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.

Since deciduous trees around here mature and thus can be harvested on about a 30 year rotation, for true sustainability, your woodlot needs to be at least:

387 * 30 = 11,610sq.m == 1.16Ha == 2.87ac in size

Tweak the numbers above to suit your local (or hypothetical) conditions and calculate the minimum woodlot size that will work for you.

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