Tree-ring reconstruction of island and mainland fire events along a historic canoe travel corridor in Minnesota's Boundary Waters Wilderness

Abstract

Tree-rings of ancient, well-preserved red pine (<italic>Pinus resinosa</italic>) stumps provide an exceptional opportunity to examine the proto-historic fire frequencies of red pine stands in the Quetico-Superior region of northern Minnesota and northwest Ontario. The primary purpose of this research undertaking has been to determine the level of human influence on fire occurrence along a historic canoe travel corridor in Minnesota's Boundary Waters Canoe Area Wilderness (BWCAW). I approached this research challenge by testing three research questions in a small portion of the naturally fragmented BWCAW landscape. First, can island fire frequencies be explained by rates of lighting-ignited fire occurrence alone? Second, are surface fires within the focus area synchronous in time and space? And third, are local surface fires significantly associated with patterns of regional drought? I developed spatially explicit and annually resolved records of surface fires using dendrochronological methods, with 71 stump cross-sections from an approximate 2,170 hectare study area on the islands of eastern Lac La Croix (centered at 48°18'08"N, 92°03'15"W). Between 1590 and 2010 there were 79 separate fires within the study area recorded by tree-rings. All but two of the reconstructed fires occurred before 1922. Sixty-one fires were recorded at single sites with the other 18 burning at 2-5 sample sites. Ten of the 79 fires were recorded at both mainland and island sites. In the same area, from 1929 to 2010, thirteen lightning-ignited fires have been detected and suppressed by the Superior National Forest (only two in island settings). When comparing the conservative, tree-ring reconstructed estimate of pre-modern fire occurrence and the nearly 100% complete modern lighting-caused fire record (1929 -present), there is a noticeable change in the distribution and frequency of fires within the study area. The tree-ring reconstructed fires are spatially and temporally asynchronous and, as a whole, are not strongly associated with regional drought (p > 0.05). My results suggest that proto-historic Ojibwe land use activities in this part of the Boundary Waters landscape likely contributed to the frequency of proto-historic fire occurrence. These findings have important and direct implications for fire and wilderness management practices in the BWCAW and the greater Quetico-Superior region.