Nelson David M, Hu Feng Sheng, Tian Jian, Stefanova Ivanka, Brown Thomas A
Program in Ecology and Evolutionary Biology, University of Illinois, 265 Morrill Hall, 505 South Goodwin Avenue, Urbana, IL 61801, USA.
Proc Natl Acad Sci U S A. 2004 Jan 13;101(2):562-7. doi: 10.1073/pnas.0307450100. Epub 2003 Dec 30.
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 delta(18)O were used to determine climatic variations, whereas pollen assemblages, charcoal delta(13)C, and charcoal accumulation rates were used to reconstruct vegetation composition, C(3) and C(4) plant abundance, and fire. The ratio of aragonite/calcite at WOL and delta(18)O at SL suggest that pronounced droughts occurred during the MH but that drought severity decreased with time. From charcoal delta(13)C data we estimated that the MH abundance of C(4) plants averaged 50% at WOL and 43% at SL. At WOL C(4) abundance was negatively correlated with aragonite/calcite, suggesting that severe moisture deficits suppressed C(4) plants in favor of weedy C(3) plants (e.g., Ambrosia). As climate ameliorated C(4) abundance increased (from approximately 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, C(4) abundance showed no correlation with delta(18)O-inferred aridity. Woody C(3) 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.
来自北美大陆中部全新世中期（MH）的古记录可以为生态系统对未来气候变暖可能伴随的普遍干旱的响应提供见解。我们分析了明尼苏达州西奥拉夫湖（WOL）和斯蒂尔湖（SL）的MH沉积物，以研究温暖/干燥气候条件对草原 - 林地生态系统的影响。利用矿物成分和碳酸盐δ(18)O来确定气候变化，而花粉组合、木炭δ(13)C和木炭积累速率则用于重建植被组成、C(3)和C(4)植物丰度以及火灾情况。WOL的文石/方解石比率和SL的δ(18)O表明，MH期间发生了明显的干旱，但干旱严重程度随时间降低。根据木炭δ(13)C数据，我们估计WOL的MH时期C(4)植物平均丰度为50%，SL为43%。在WOL，C(4)丰度与文石/方解石呈负相关，这表明严重的水分亏缺抑制了C(4)植物，有利于杂草类C(3)植物（如豚草）生长。随着气候改善，C(4)丰度增加（从约33%增至66%），杂草类物种减少，这增加了燃料可用性和火灾发生频率。相比之下，在更东部气候较凉爽湿润的SL，C(4)丰度与δ(18)O推断的干旱程度没有相关性。木本C(3)植物（如栎属）在SL更为丰富，生物量可燃性较低，火灾在SL的重要性低于WOL。我们的结果表明，C(4)植物适应温暖/干燥的气候条件，但不适应极端干旱，并且火灾状况受生物量 - 气候相互作用控制。
