Interactions of climate and fire at two sites in the northern Great Plains, USA

The relationship of fire and climate in explaining the origin and maintenance of the grasslands of the northern Great Plains has long been of interest. I examined the hypothesis that burning was more frequent during wet years using charcoal to reconstruct fire histories near Coldwater Lake in southcentral ND, and Rice Lake in northcentral ND and published records of ostracod Mg/Ca ratios as a proxy for climate at these same two sites. Over the past 10000 years, charcoal influx rates for Coldwater (0.04-5.68 mm super(2) cm super(-2) year super(-1)) greatly exceeded influx rates for Rice (0.01-1.91 mm super(2) cm super(-2) year super(-1)). Both sites showed strong, significant periodicities of similar to 1500-1800, similar to 800-900 and similar to 130-140 years but charcoal maxima and minima were only similar at similar to 4200 cal years BP. Charcoal influx during the past 2000 years also revealed strong periodicities of between 310-400 and 140 years at both sites. Comparison of smoothed charcoal influxes and ostracod Mg/Ca ratios suggests that both proxies are responding to changes in climate. When smoothed with a 320-400-year window, increases in charcoal influx typically preceded rises in ostracod Mg/Ca ratio by similar to 50-100 years, and further smoothing (140-year window) of the residuals suggested a variable relationship between Mg/Ca ratios and charcoal influx. These results do not clearly support or reject the fuel limitation hypothesis, and this lack of clear support may result from (a) climate-driven shifts in the mix of C3 and C4 grasses resulting in maximum productivity at intermediate moisture levels, (b) possibly different responses of charcoal and Mg/Ca ratios to summer vs. winter precipitation, or (c) ground-water driven lags in the response of Mg/Ca ratios to shifts in climate. Comparison of the results from this study with other studies suggests that links between fire and fuels at finer time scales (0-100 years) may be broadly constrained by longer term ( similar to 500-2000 years) patterns of climate in the northern Great Plains.