A 1994 study estimated that old-growth forest grew on just 3.9 percent of Minnesota's timberland, whereas it made up 51 percent of Minnesota's forest in 1850. The reason is simple: widespread clear-cut logging was often followed by fires set to burn discarded tree limbs or ignited accidentally.
Today the old growth that survives was either young or poorly formed when the loggers arrived, or consisted of species that didn't have high value, such as jack pine and black ash. Some forests were too hard to reach—in ravines, on steep slopes, or in the middle of vast wetlands, for example. Some survive because they lie in the Boundary Waters Canoe Area Wilderness and Itasca State Park and were protected by government order.