The Boundary Waters Canoe Area that we know can only exist with prescriptive fire that sustains its historical mix of trees, plants and wildlife. Here's a look at fire year intervals and burn percents by cultural history periods for the virgin forests of the BWCA, 1542 to 1972. Major fires and intervals have been in decline since the 'settlement' era began, according to this data collected by U.S. Forest Service ecologist Miron Heinselman in the early 1970s.