Variability and persistence of post-fire biological legacies in jack pine-dominated ecosystems of northern Lower Michigan

On the dry, flat, jack pine (Pinus banksiana Lamb.)-dominated ecosystems of the northern Lake States and eastern Canada, wildfire behavior often produces narrow, remnant strips of unburned trees that provide heterogeneity on a landscape historically shaped by stand-replacing wildfires. We used landscape metrics to analyze a chronosequence of aerial imagery to examine these “stringers” of mature trees within historical wildfires in northern Lower Michigan. Our major objective was to describe the natural range of variability of stringer patterns and their persistence and change during the fire-free interval. Field studies were then used to examine stringer composition and structural variability. Stringers were found to occur in all fires >1000 ha, in about one-third of wildfires >80 ha, but never in fires <80 ha, likely because of the lack of fire intensity on smaller fires that is necessary for stringers to be formed. Stringers were typically composed of many small, well-aggregated patches that represented 3–14% of the area within the burn perimeter, and stringer formation was relatively independent of pre-fire forest structure or composition. Stringer patterns changed mostly in the first decade after the fire that created them and then stabilized. Major changes that occurred in stringer patterns after this period were most often due to human activities, highlighting their natural persistence through the fire-free interval. The historical persistence and importance of these features also highlights their importance on modern fire-prone landscapes, particularly in northern Lower Michigan where a high proportion of land management is focused on jack pine plantations for breeding habitat for Kirtland’s warbler (Dendroica kirtlandii Baird), an endangered species.

► Wildfires in jack pine forests often leave unburned strips of trees (“stringers”).
► Stringers are found in 2/3 of wildfires >80 ha since the mid-1960s.
► When they occur, stringers occupy 3–14% of the area burned and are composed of many small patches.
► Formation is driven by fire behavior rather than pre-fire forest characteristics.
► Stringers change naturally, but abrupt changes are primarily driven by humans.