Sensitivity of fire regimes to landscape context and climatic variation over the past 13,000 years in northwestern Wisconsin

Background/Question/Methods

This research compares fire records from two sites with vegetation and lake-level history of the surrounding landscape. The study focuses on Warner and Ferry lakes, both located in the Northwest Wisconsin sand plain.  At both sites soils are xeric with low cation exchange capacity. Ferry Lake is at a transition between jack pine (Pinus banksiana) communities and savannas with more oak (Quercus spp.), while Warner is in a jack pine area.  In the Public Land Survey conducted in the mid-1800s 51% of the bearing trees within 5km of Ferry Lake were jack pines, with 19% red pine (Pinus resinosa), while Warner had 62% jack pine, with 14% red pine (Pinus resinosa).  In the area surrounding Warner there are few lakes that could act as fire breaks, while lakes are more abundant near Ferry, allowing us to compare how fire regimes respond to changes in climate and vegetation in landscapes that differ in the potential for fires to spread. We used contiguous samples of charcoal and pollen samples from 50-100 yr intervals to reconstruct fire and vegetation histories for the two sites.

Results/Conclusions

A relatively uniform fire frequency prevailed over the 13,000-year period at Warner Lake, while fire at Ferry Lake was more variable. Fire frequency increased between about 5000 and 4000 years ago and declined 700 years ago at both sites.  The concentrations of different charcoal morphotypes changed around 4,500, 7,000 and 10,500 BP at Warner Lake, suggesting changes in the composition of vegetation (+/or in fire intensity) coincident with subtle changes in fire regimes.  The fire history of Ferry Lake was much more variable, with very high rates of charcoal accumulation between about 13,000 and 12,000 cal yr BP, a period of low accumulation rates before 5000 cal yr BP, and an abrupt increase in accumulation rates at about 5000 cal yr BP. The vegetation near Warner Lake experienced only subtle changes in the late Holocene. The vegetation at Ferry Lake was more dynamic, with a sudden state-shift from oak- (Quercus spp.) to pine- (Pinus spp.) dominated communities about 1400 cal yr BP and an increase in white pine (Pinus strobus) about 700 cal yr BP. The fire breaks in the vicinity of Ferry Lake may have led to greater variation in both fire regime and vegetation during the Holocene, while Warner Lake’s consistent vegetation may have been reinforced by a strong fire feedback enhanced by the open landscape.