I recently bought 13 acres of land in south-western Pennsylvania. The land was basically clear-cut prior to my purchase, so there are few trees large enough to attach no trespassing signs to except property line trees. Is it legal to staple No Trespassing signs to these trees?

It is your land.

The trees are on the property line and thus are only partially on my property. That is why I was asking the question.

Think about it . . .

you'll actually have to stand on your neighbor's land(trespassing), while you staple your signs on his/hers side of the line-tree.

Buy some 5'-6' long "T"posts and use them