Most are dying because of the emerald ash borer.
Officials say many of the trees stand in picnic groves and along trails where they could fall and hurt someone.
Petra Blix, a community activist who lives on Chicago's northwest side near the Pucinski Forest Preserve, says the action will destroy healthy trees and turn picnic areas into uncomfortable, desert-like islands.
Forest Preserve Superintendent Arnold Randall says only infested and dying trees will be removed.
The forest preserve is spending about $950,000 to remove 7,724 trees invested with emerald ash borer with another 1,238 trees being mostly elms with Dutch elm disease. Randal says the trees will be replaced.
The Associated Press contributed to this report.