Slash and Burn agriculture has traditionally had a bit of a bad reputation. Traditionally, it worked like this:
Actually, this is a very sustainable, low energy use, environmentally friendly form of agriculture. But not if too many people do it. Then you end up cutting down all of the RainForest. The problem is worsened because forest re-grows more slowly when big patches of it are removed than if individual trees or small areas are removed—the basic reason for this is that trees have no cover, so the shrub has to grow first, through which the trees then grow.
But, I think that we need to reconsider this form of agriculture, as it potentially could produce vast amounts of Energy, in an entirely Renewable fashion. It's clear that the system needs management, if we are not to get Forest destruction, but given that this is the case, it could be one of the best sources of Energy in the future.
Consider that there are many areas of developed world under woodland. Even the UK has a quite a bit of this. If we had less sheep, then even more upland areas in Scotland, Wales and the North of England would become wooded 1. But this pales in comparision to countries like Canada or Austrialia. The north of Canada is more or less entirely covered in wood, and the areas in question are enormous. More over, these areas have a tendancy to burn down, causing massive disruption, damage to property and loss of life.
If these areas were subject to a slow, steady, rolling process of clear cut, it would have no environmental impact in itself; these areas are going to burn down anyway and they will grow back from clear cut just as they do from fire. More over, if you clear cut only small areas or, perhaps, thin strips through these areas, they will grow back more rapidly and efficiently than following forest fire.
Of course, to do this cutting takes quite a lot of Energy itself. Trees are fairly hard to cut down. More over, if you want to do anything useful with the Wood, then you have to get it to somewhere useful. You can do this with a river, or a big road, but these are expensive to build and maintain. If you want to use the Wood as a BioFuel, then you run the risk of spending more Energy than you produce.
However, you can get around quite a lot of these problems. If you are using the Wood for Energy, you don't have to transport the Wood at all. Instead, you can gassify it on site, either using a AnaerobicDigestionPlant to produce Methane or burn it, perhaps with a FluidBedIncinerator, and use the Energy to generate Hydrogen. This can then be transported back to where it is needed via a HydrogenBalloonNetwork.
The image that I have in my head is a large, mobile, system, which moves through Woodland, cutting perhaps a 2 metre wide path, so the machine would be narrow and quite long. Behind the cutting device, there would be a incinerator 2. This would burn the wood 3, generate Hydrogen, which would be sent back to base. The path behind would be clear cut, but covered with an ash from incinerator, and should regrow quickly. The whole system would move forward relatively slowly, perhaps half a metre an hour. It would be relatively autonomous. When it had to be recovered or relocated, a Hydrogen AirShip could be used for transport.
There is a big problem with this sort of EnvironmentalSlashAndBurn, which is that clear-cutting parts of the wood land are likely to cause damage wildlife in the area. Quite a bit of research would need to be done to limit this damage. It also needs to be pointed out that this form of Energy production would help to prevent forest fires, which also damage the wildlife. Alternatively, the use of RobotPruners might be investigated.
1. This would actually have a second big advantage. The upland areas of UK allow water to flow off very quickly. More trees would hold the water on the land for a much longer time, which would make our rivers much less likely flood as often as they are doing so now.
2. The size of the incinerator would have to depend on the average size of the trees. You don't want to have to chop it up into too small pieces as this will, again, use all of the Energy that you are trying to produce.
3. You wouldn't want the incinerator to be too good—many plants use forest fire as part of their natural lifecycles and we would to mimic this.