Well, there's gotta be some cottonwoods in the Rocky Mountain West that are not that weak as those were the only kinds of trees available for "stringing" up in some areas I know. Still are almost the only kidn of tree in some spots in Wyoming and Colorado, that I know of. Plus, I had a rope and wooden seat swing in a big old cottonwood when I was a kid, that my dad climbed the tree to put up for me.