Taking Back Sunday Singer Adam Lazzara -- Leg CRUSHED By Falling Tree

Taking Back Sunday singer Adam Lazzara is lucky to be alive -- after a tree literally FELL on top of him this weekend ... shattering his leg.

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 (below).

Once freed, we're told Adam was rushed to the hospital -- where doctors put a cast on his broken leg. According to the rep, Adam's expected to be in the cast for the next 4-6 weeks.