A large area of the National Trust Squirrel Walk at Formby was set alight by vandals on Saturday night.
Many trees were damaged by the fire and several have been felled because they were badly burnt.
A spokesperson for the National Trust said "Frustrating morning for rangers continuing clearing up after large fire deliberately lit"
Anyone with information is asked to call Merseyside Police on 101 or Crimestoppers on 0800 555 111.