Applying the usual rules to an unusual ecological situation: Fire rotation in Great Lakes Pine Forests

Abstract

Fire regimes in Eastern North America are often determined from historical data because land-use change and natural resource policy have confounded natural fire processes. It is good practice to combine multiple historical data sources, which can serve to both corroborate findings and fill knowledge gaps that might exist when trying to gain a full picture of historical ecological processes. In the Great Lakes Region (GLR) fire rotation, or the number of years it takes to burn an area equivalent to the area of interest, is an extensively used metric that has been calculated based on the Euro-American settlement era General Land Office Public Land Survey (GLO) records. However, fire rotation methods and GLO records are best suited for understanding high-severity fire, and low- to moderate-severity fires have received less attention in forested ecosystems in this region. We used dendrochronological (tree-ring) data to evaluate GLO data and fire rotation methods in relation to low-severity fires. Tree-ring and GLO data were well-aligned in some ways, with high concurrence of tree species, tree density, and common fire dates. However, GLO data did not identify fires for survey points closest to any of our sites (n = 26), though 71% of sites burned within one year and all sites burned within 8 years of surveys. Mean fire return intervals for our sites ranged from 2 to 9 years for all fires and 6–20 years for fires recorded on ≥25% of samples within sites (1602–2018) with relatively minor effects of filtering on return intervals. Thus, fires were historically frequent and widespread within sites. We estimate that fires burned on average 858 km2 to 2564 km2 per year within five ecological landscapes with rotation intervals ranging between 11 (Northeast Sands) and 34 years (Northern Highlands; µ = 22 years across all five landscapes). We found 25 regional fire years that were synchronous among multiple (2–5) ecological landscapes over a 218-yr period with evidence that drought plays a role in regionally widespread fire years. High-severity fire was likely limited in the GLR; however, low- to moderate-severity fires were abundant, large-scale, widespread, and an important forcing mechanism shaping forests of the GLR over millennia.