Bow saws are wonderful things for getting firewood, and I carry one in the winter often. Normally you need at least 24" long to be relatively useful.

Having said that, for shelter building in an emergency (i.e. a lean to or something) an Axe is enough - for the small things you are typically cutting to do this a bow saw is more bother than it is worth for cutting green limbs and small stuff for this type of activity.

Now, having said that, I'll say *why do this?* Most of us try pretty hard to leave no trace (or as little as possible). Yes there are areas where doing this is fine, but realisticly, a siliconized nylon tarp or tarp/poncho will weigh about half a pound to a pound - a little more with stakes and lines. My 8x10 silnylon tarp weighs 600 grams (about a pound and a half) with 12 foot lines and 8 stakes. That is *far* lighter than either my axe or my bow saw, and I *guarantee* you will keep me a lot dryer and more comfortable than the best lean-to or bough shelter I can possibly build, and won't leave the area looking like it's been attacked by lawnmowers after 10 people who are doing the same thing camp there.