10

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?

  • 1
    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
8

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?

2

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/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.

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 = 11,610m² == 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.

0

Zero.

And by that, I don't mean I'm against the economical sustainable use of forest -- forest should be used as much possible as is sustainable to limit carbon dioxide emissions. The problem is, there's not enough forest, unless you cut 100% of rainforests on Earth and replace them with oil palm plantations.

Let us assume that there is a population that uses firewood. Wood that can be burned grows 0.1 zettajoules per year. The population uses less firewood than 0.1 zettajoules per year. Sustainable? Yes, except read on...

However, the population grows and due to simultaneous economic growth per capita, energy consumption increases to 0.5 zettajoules per year. Seeing that the limits of growth and sustainability are near, it has become trendy to burn firewood instead of burning coal, and it has also become trendy to own forest.

However, no matter how much people want to burn firewood, it cannot be burned sustainably more than 0.1 zettajoules per year. Thus, by burning firewood (regardless of whether you own forest or not), you are increasing the price of firewood due to laws of supply and demand, which causes somebody who can't afford firewood anymore or just doesn't care enough to stop burning firewood and start burning coal.

Also, no matter how much people want to own forest, not everyone can own so much forest that it would cover their energy consumption. By buying and owning forest (regardless of whether you burn the wood or sell it to somebody else), you are increasing the price of forestland due to laws of supply and demand, which causes somebody to sell it and use the money for something else, thus ceasing to be forest-owner. Not everyone can own enough forest.

The marginal energy is provided by coal. Forest owners won't all decide to cut their forest immediately, but coal mine owners are more than happy to sell some of their limited coal reserves to you. Forestry is sustainable, coal mining is not.

The important thing to understand is that everyone is using marginal energy. Marginal energy is coal. Thus, every kWh of electricity used, every MJ of heat produced, etc. effectively comes from coal.

In a hypothetical future world, marginal energy could be supplied sustainably by advanced small modular nuclear reactors (by that, I mean fission breeders, not fusion), or perhaps even wind power stored in hydrogen or batteries. However, today marginal energy comes from coal, and thus every energy user is effectively using coal.

If you reduce your energy use by 1 MJ, you reduce coal use by 1 MJ. It's as simple as that.

Also, needless to say, sawlogs shouldn't be burned; they should be used for constructing houses. Pulpwood could perhaps be burned, but a better option is to bury it. Unfortunately, there are no economic incentives for burying wood.

Wood is a very dirty fuel. If you bury 1 MJ worth of wood underground and burn 1 MJ worth of coal, less carbon dioxide is produced than would be produced by burning the 1 MJ worth of wood directly -- for the same energy output. In fact, wood is probably the dirtiest fuel of them all, very likely dirtier than peat. Let's bury the wood and burn the best and cleanest of the fossil fuels, natural gas! And perhaps some oil in airplanes and heavy trucks that use jet fuel or diesel, respectively, and have no viable replacement like electricity.

Disclaimer: I own 25.6 hectares of boreal taiga forest. It's an excellent investment even in these northern latitudes. If you can, consider investing in forest!

  • Perhaps it's me but after reading this 2 times, I still don't understand your reasoning. – THelper Feb 20 at 16:03
  • What do you mean by "marginal energy"? In the U.S. coal plants are shutting down due to economic pressure from cheap natural gas specifically because it can't compete on the margins. In Europe renewable energy prices and policies are putting similar pressure on coal generation. – LShaver Feb 20 at 16:57
  • I agree, in US marginal energy is natural gas. In Europe, it's coal currently. – juhist Feb 20 at 17:40
0

To be ecologically truly sustainable, no live trees should be cut. That will most closely mimic a natural ecosystem instead of turning them into managed tree plantations (which ain't properly forests).

"Only take dead wood"

Now forests naturally create also a lot of biomass in the form of dead coarse wood, dead fine wood and dead leaves. That would slowly release its carbon as CO2 through natural decay, so we might as well collect and burn that. We can extract (let's say) half of that, while leaving the other half as habitat for animals etc.:

Dead wood, whether standing (snags) or down, plays an important role in forest ecology, acting as habitat for animals such as cavity-nesting birds, plants and microbial lives. (source: Carbon sequestration via wood burial (2008))

Also let's say we only would take the coarse wood (typically understood as having 10 cm stem diameter and up), as that is easiest to handle. In that case, a rough estimation can be based on this:

For example, a 1 km × 1 km area (100 hectares) would accumulate about 100 tonne of carbon per year for a typical coarse wood production rate of 0.1 kgC m-2 y-1 […]. 500 tonnes of carbon [are equivalent to] […] about 1000 tonne dry wood mass […]. (source: Carbon sequestration via wood burial (2008))

So 100 t y-1 km-1 of carbon yield from coarse deadwood in a mature forest, based on the naturally occurring tree deaths. Which is equivalent to about 200 t y-1 km-1 of dry wood mass.

Finally, taking out only half of that wood results in a production of 100 t y-1 km-1 of dry wood mass.

Comparing results

For comparability with the answer by m.w.jacobsen: my number of 100 t y-1 km-1 is equivalent to 1 t y-1 ha-1, while m.w.jacobsen had 5.4 t y-1 ha-1, both of dry wood mass.

The difference is that (1) I only include half the coarse deadwood (otherwise it's 2 t y-1 ha-1 when including all coarse deadwood, or roughly 4 t y-1 ha-1 when also including fine deadwood), (2) I don't include any clearcutting or cutting of live trees – but it is not too clear what impact that has on forest productivity, if any and (3) most importantly, the numbers from the study I quote are quite rough. They are derived from global average forest productivity instead of that for only the temperate or tropic climate zone. Boreal forests have a lower productivity (see), lowering the average.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.