Response of C₃ and C₄ plants to middle-Holocene climatic variation near the prairie-forest ecotone of Minnesota

Paleorecords of the middle Holocene (MH) from the North American mid-continent can offer insights into ecological responses to pervasive drought that may accompany future climatic warming. We analyzed MH sediments from West Olaf Lake (WOL) and Steel Lake (SL) in Minnesota to examine the effects of warm/dry climatic conditions on prairie-woodland ecosystems. Mineral composition and carbonate δ¹⁸O were used to determine climatic variations, whereas pollen assemblages, charcoal δ¹³C, and charcoal accumulation rates were used to reconstruct vegetation composition, C₃ and C₄ plant abundance, and fire. The ratio of aragonite/calcite at WOL and δ¹⁸O at SL suggest that pronounced droughts occurred during the MH but that drought severity decreased with time. From charcoal δ¹³C data we estimated that the MH abundance of C₄ plants averaged 50% at WOL and 43% at SL. At WOL C₄ abundance was negatively correlated with aragonite/calcite, suggesting that severe moisture deficits suppressed C₄ plants in favor of weedy C₃ plants (e.g., Ambrosia). As climate ameliorated C₄ abundance increased (from ≈33 to 66%) at the expense of weedy species, enhancing fuel availability and fire occurrence. In contrast, farther east at SL where climate was cooler and wetter, C₄ abundance showed no correlation with δ¹⁸O-inferred aridity. Woody C₃ plants (e.g., Quercus) were more abundant, biomass flammability was lower, and fires were less important at SL than at WOL. Our results suggest that C ₄ plants are adapted to warm/dry climatic conditions, but not to extreme droughts, and that the fire regime is controlled by biomass-climate interactions.