NOAA/WDS Paleoclimatology - Larson - Liebmann - QUSP - ITRDB WI039

New and updated multi-century tree-ring chronologies from living oak trees, remnants, and archeological beams from across the Driftless Area of southwest Wisconsin and northeast Iowa, USA, were developed to fill a spatial gap in the network of available tree-ring chronologies. We produced a robust 303-year summer drought reconstruction (June-August Palmer's Modified Drought Index (PMDI): r2 = 0.45) that identified clusters of extreme droughts and pluvials (PMDI <= -4.0 or >= 4.0) in the early 1700s and more even distributions of drought conditions, with the exception of the post 1930s period when drought became relatively infrequent. Compared to the Living Blended Drought Atlas (LBDA) and the North American Drought Atlas (NADA), our reconstruction more accurately represented moderate moisture conditions across the Driftless Area, the NADA and LBDA more closely represented extreme pluvials, and our reconstruction and the LBDA better represented extreme drought years. The three reconstructions largely captured the same high-frequency variability in drought conditions and differed most at low frequencies. Significant correlations were identified between our reconstruction and corn (r = 0.30, n = 91, p = 0.002) and soybean (r = 0.25, n = 81, p = 0.012) yields, with the strength of the correlations increasing over recent decades suggesting a tighter coupling of interannual climate variability and crop productivity in the region. Superposed epoch analyses indicated significantly wetter conditions in the Driftless Area two years after major volcanic eruptions. In the context of long-term climatic variability, the Driftless Oaks drought reconstruction demonstrated that drought and pluvial conditions more extreme than those experienced during the instrumental record have occurred in the past.

本研究构建并更新了采自美国威斯康星州西南部与艾奥瓦州东北部无冰川漂移区（Driftless Area）内活栎树、树木残体及考古木梁的多世纪尺度树木年轮年表，旨在填补现有树木年轮年表网络的空间空白。我们得到一段稳健的303年夏季干旱重建序列（基于6-8月帕尔默修正干旱指数（Palmer's Modified Drought Index, PMDI），决定系数r²=0.45），该序列识别出18世纪早期存在极端干旱与雨涝事件集群（PMDI≤-4.0或≥4.0），其余时段干旱状况分布相对均匀，仅1930年代后干旱事件相对罕见。与融合活体干旱图集（Living Blended Drought Atlas, LBDA）及北美干旱图集（North American Drought Atlas, NADA）相比，本重建序列更准确地反映了无冰川漂移区的中等湿度状况；而NADA与LBDA则更精准地还原了极端多雨事件，本重建序列与LBDA则更能表征极端干旱年份。三类干旱重建序列大体捕捉到了一致的干旱状况高频变异性，仅在低频变异性上差异最为显著。本重建序列与玉米（r=0.30，n=91，p=0.002）、大豆（r=0.25，n=81，p=0.012）产量间存在显著相关关系，且相关强度随近几十年呈上升趋势，表明该区域年际气候变率与作物生产力间的耦合关系愈发紧密。叠加年代分析（Superposed Epoch Analysis）结果显示，大型火山喷发两年后，无冰川漂移区的降水条件显著偏湿。在长期气候变率的研究背景下，本研究构建的无冰川漂移区栎树干旱重建序列表明，过去曾出现过远超器测记录极值的极端干旱与雨涝事件。