NOAA/WDS Paleoclimatology - Porter - Thomas Creek TH1 - PCGL - ITRDB CANA550

We present a new 23-site network of white spruce ring-width chronologies near boreal treeline in Old Crow Flats, Yukon Territory, Canada. Most chronologies span the last 300 years and some reach the mid-16th century. The chronologies exhibit coherent growth patterns before the 1930s. However, since the 1930s, they diverge in trend and exhibit one of two contrasting, but well-replicated patterns we call Group 1 and Group 2. Over the instrumental period (1930-2007) Group 1 sites were inversely correlated with previous-year July temperatures while Group 2 sites were positively correlated with growth-year June temperatures. At the broader northwestern North America (NWNA) scale, we find that the Group 1 and Group 2 patterns are common to a number of white spruce chronologies, which we call NWNA 1 and NWNA 2 chronologies. The NWNA 1 and NWNA 2 chronologies also share a single coherent growth pattern prior to their divergence (ca. 1950s). Comparison of the NWNA 1/NWNA 2 chronologies against gridded 20th-century temperatures for NWNA and reconstructed northern hemisphere summer temperatures (ad 1300-2000) indicates that all sites responded positively to temperature prior to the mid-20th century (at least back to ad 1300), but that some changed to a negative response (NWNA 1) while others maintained a positive response (NWNA 2). The spatial extent of divergence implies a large-scale forcing. As the divergence appears to be restricted to the 20th century, we suggest that the temperature response shift represents a moisture stress caused by an anomalously warm, dry 20th-century climate in NWNA, as indicated by paleoclimatic records. However, because some sites do not diverge and are located within a few kilometres of divergent sites, we speculate that site-level factors have been important in determining the susceptibility of sites to the large-scale drivers of divergence.

本研究构建了一套全新的23个站点白云杉轮宽年表（ring-width chronologies）网络，采样区域位于加拿大育空地区奥克劳平原的寒带林线（boreal treeline）附近。多数年表的时间跨度覆盖过去300年，部分年表可追溯至16世纪中叶。1930年代之前，所有年表均呈现一致的径向生长模式。但自1930年代起，各站点年表的生长趋势出现分化，呈现出两种截然相反但重现性优异的模式，本研究将其分别命名为第1组（Group 1）与第2组（Group 2）。在器测时期（1930-2007年），第1组站点的径向生长与上一年7月气温呈负相关关系，而第2组站点则与生长年6月气温呈正相关关系。在更广阔的北美西北部（NWNA）尺度下，我们发现第1、2组的生长模式也存在于多个白云杉年表中，本研究将这类年表分别命名为NWNA 1与NWNA 2年表。NWNA 1与NWNA 2年表在分化发生前（约20世纪50年代）同样具有一致的生长模式。将NWNA 1、NWNA 2年表与北美西北部网格化20世纪气温数据以及重建的北半球夏季气温（公元1300-2000年）进行对比后发现，在20世纪中叶之前（至少可追溯至公元1300年），所有站点的径向生长均与气温呈正相关关系；但此后部分站点转为负相关（对应NWNA 1组），其余站点则维持正相关关系（对应NWNA 2组）。生长趋势分化的空间分布范围表明，该现象受大尺度驱动因子调控。鉴于该分化现象仅出现于20世纪，结合古气候记录（paleoclimatic records）的相关证据，我们推测气温响应模式的转变是由北美西北部20世纪异常暖干气候引发的水分胁迫所致。但部分未出现分化的站点与分化站点的空间距离仅为数公里，因此我们推测，站点尺度的因子在决定各站点对分化相关大尺度驱动因子的敏感性方面发挥了关键作用。