Poisonwood is a species of flowering tree in the sumac family. It produces the irritant urushiol much like its close relatives poison sumac and poison oak. The tree grows abundantly in the Florida Keys and extends its reach to the Bahamas and throughout portions of the Caribbean.
My friend Scott (former Indian Key guide) and this park ranger have told me some horrifying stories about poor souls that have come in contact with this venomous-type tree. Be aware, this toxin eats through clothes and attacks your skin like no other leaving severe lifetime scars in some cases.
Seen here is a ranger from Lignumvitae Key explaining its effects. Also pictured is a stand-alone tree properly marked on Indian Key. The posted sign is from John Pennekamp State Park in Key Largo alerting its patrons. Most maintained paths mark these trees so stay on the trails or pick your own poison if chosing not to!