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Response of C3/C4 plants to mid-Holocene aridity in west-central Minnesota.
Nelson, David*,1, Hu, Feng Sheng1, Tian, Jian1, Brown, Thomas2, Stefanova, Ivanka3, Wright, Herbert3, 1 University of Illinois, Urbana, IL2 Lawrence Livermore National Laboratory, Livermore, CA3 University of Minnesota, Minneapolis, MN
ABSTRACT- Paleorecords of ecological response to aridity can offer insights into the consequences of pervasive drought that will likely accompany future climatic warming in the mid-continent of North America. To examine the effects of mid-Holocene (MH; ∼8.0 to 4.0 ka yr BP) aridity on the prairie-woodland ecosystems of west-central Minnesota we analyzed sediment cores from West Olaf Lake (WOL; 46° 37′ N, 96° 11′ W) and Steel Lake (SL; 46° 58′ N, 94° 41′ W) for 13C of charcoal, charcoal abundance, and pollen assemblages. Mineral composition (aragonite/calcite ratio) at WOL and 18O of bulk carbonate at SL suggest that the region experienced pronounced droughts during the MH, and that drought severity decreased with time. By applying a mixing model to charcoal 13C data we estimate that the abundance of C4 plants averaged 50% at WOL and 43% at SL during the MH. At WOL the abundance of C4 plants was negatively correlated with the aragonite/calcite ratio (r=-0.63). Severe and variable aridity suppressed C4 species in favor of weedy C3 species such as Ambrosia, and reduced fuel loads and fire importance. As aridity ameliorated, C4 abundance increased (from ∼35 to 65%), weedy species decreased, fuel availability increased, and fire became more important (charcoal accumulation rates increased from <10 to >60 charcoal pieces cm-2 yr-1). In contrast, further east at SL C4 abundance showed no correlation with 18O-inferred aridity. Climatic conditions were less arid at SL than at WOL, which resulted in a higher abundance of woody C3 species such as Quercus, and lower biomass flammability (<5 charcoal pieces cm-2 yr-1) at SL throughout the MH. Our results imply that C4 plants may not be adapted to climatic conditions characterized by high severity and variability of aridity.
Key words: aridity, Minnesota, C4, fire