Fire is a complex phenomenon that responds to climate and vegetation and may become more difficult to manage in the context of global climate and land use change (Bowman et al., 2009). Understanding dynamics of fire over extended periods of time that incorporate climate variability and forest change could help inform management of forests in the 21st century (Whitlock et al., 2008). Our study focused on fire-drought interactions in a small landscape of northern Wisconsin over a 200-year period characterized by dramatic shifts in human settlement and forest use and management.
We generated high-resolution (2-5 yrs sample-1) macroscopic (125 mm) sediment charcoal records from five lakes in a 100 km2 study area within the Northern Highlands Lake District, WI. We separated charcoal records into peak and noise components (Clark, 1988) and applied a locally-moving threshold over a background window of 75 years to identify peaks in charcoal accumulation rates significantly above background levels (Higuera et al., 2009, Kelly et al., 2011). Mean annual Palmer Drought Severity Index (PDSI) values for north-central Wisconsin from the Wisconsin State Climatology Office and the North American Drought Atlas (Palmer, 1965, Cook et al., 1999) were used to define drought years (mean annual PDSI < -0.5). We used bivariate and multivariate K-function analyses (Ripley, 1977) modified for one dimension (i.e., time; Doss, 1989; Gavin et al., 2006) to examine synchronicity between fire and drought records and to compare fire activity between five lakes over three different periods: Pre-Clearcut: 1810-1889 C.E., Clearcut and Land Clearance: 1890-1929 C.E., and Forestry and Fire Suppression: 1930-2005 C.E.. K-functions were transformed to L-functions to facilitate plotting.
Over the Pre-Clearcut period (1810-1889 C.E.), the record of fire episodes within the study area (including episodes from all five lakes) was significantly synchronous (± 3 years) with years of drought and a fire episode occurred somewhere within the study area on average every 6.5 years (range, 1-19 years) (Fig. above). Over the Clearcut and Land Clearance period (1890-1929 C.E.), the compiled record of fire episodes from all five sites showed independence from drought (Fig. above). Fire episodes occurred at three or more different lake sites within short intervals between ca. 1890-1894 C.E. and ca. 1914-1916 C.E. (Fig.) and, when comparing records between lakes, fire episodes were significantly synchronous (± 3 years) between sites (Fig. below). Over the Forestry and Fire Suppression period (1930-2005 C.E.), the combined record of fire episodes is independent from drought within a lag of 4 or less years, although the two series show attraction and fire episode years between ca. 1934-1936, ca. 1976, and ca. 1989, coincide with severe to extreme drought years (Fig. below).
Our sediment charcoal records agree with prior tree-based fire histories suggesting that fire was an important and common disturbance in mixed hardwood conifer forests prior to Euro-American settlement. Fire-drought synchronicity has been interrupted over time and dampened by fire control efforts; however, the landscape remains fire-prone during years of severe to extreme drought.
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