I dont have a quantitative answer, but trees are living things, like us - and stabbing items into the tree and leaving them there *do* hurt the tree. They introduce external microorganisms into the sap, among other things. You want to keep the tree-nailing to a minimum...but a few shouldn't hurt most trees.
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Thanks everyone. I would not want the trees to get sick or hurt. I'm kind of a tree-hugger that way. Decided against it. Thanks!
The other solution is to build it so it rests on existing branches as much as possible, possibly with brackets on either side of (or around) the branches to keep it from sliding off. If you do that you may not need to nail anything into the tree itself. Only works if the tree is already roughly the right shape, of course.
Well the four trees that used to hold up my treehouse are still alive and thriving (25 years later). There are still four 4" nails in each.
That said, they're black spruce which are very hardy. As well, the treehouse's floor was constructed to fit tightly in the space between the trees, so it applied more weight where the boards met the tree than on the nail.
Get someone who is good with knots, as well as some proper rope, and you may very well avoid the problem.
I just had a brief conversation with my very wise grandfather on this subject... and I'd like to retract my suggestion of using rope or wire.
I think you'd be fine as long as you don't pound that tree full of iron. I mean. I've seen chains put around trees and the tree just engulfed the chain. I don't think nails really hurt trees too much. that said, I wouldn't just nail as much as you can. make it safe then stop.