Taking Back Sunday Singer's Leg Crushed By Falling Tree

A rep for the band tells TMZ, Adam was at a recording studio in rural Michigan Saturday ... and he decided to step outside for a cigarette. It was real windy, and we're told a tree collapsed on the singer out of nowhere, trapping him underneath.

The rest of the band ran outside when they heard his calls for help -- but couldn't lift the tree off him ... so they called 911, and the local fire department came and sawed the tree to pieces.