How much space and time is needed to understand fire regimes: Using 12 lake sediment charcoal records to understand landscape scale wildfire controls and ecosystem responses across centuries and millennia
Wildfire is controlled by multiple processes at multiple scales. Charcoal buried in lake sediments can expand our understanding of the temporal scale of fire from centuries to millennia. When multiple lake records are combined across a landscape, wildfire can be examined at varying scales of space and time. Climate is thought to be the dominant process controlling fire regimes in subalpine landscapes but other factors, such as topography and vegetation can alter fire regimes at smaller spatial scales. We examined the effects of climate and vegetation change at varying spatial and temporal scales in a representative subalpine forest in northern Colorado. To do this we combed 12 charcoal records and 5 pollen records across a 90,000 ha landscape for the last 2000 years.
Between 2000-1200 BP (50-750 CE) and 1000- -60 BP (950-2010 CE), fires between lake sites behaved semi-independently, suggesting local and stochastic controls on fire at the smaller scales of charcoal source area (lake sediment charcoal preserves evidence of fires within a ~3 km radius of the lake). At ~1200 BP, mean temperatures warmed 0.5 degrees C. During this time, fires synchronized across the landscape with a median probability of 80% of the sites burning within a century (between 1150-1050 BP). These results illustrate that variation within any individual records often obscures the relationship between climate and fire. As records are combined increasing the spatial sampling, however, the climate signal in fire history data eventually exceeds the variability found across individual sites, and the climatic control on fire then becomes apparent.
Fire, however, did not remain widespread through the duration of warmth and declined before temperatures declined, suggesting a hysteresis relationship between fire and climate. Evidence of vegetation from pollen records suggests that woody cover decreased at the upper elevations of the landscape, possibly limiting further fire spread and decreasing fire size. The fire and vegetation results combined provide insights into the complex interaction of top-down and bottom-up controls on fires at different scales and different times.