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.

本研究构建了加拿大育空地区旧克劳平原（Old Crow Flats）寒带林线附近的23个白云杉（white spruce）轮宽年表网络，为全新数据集。多数年表的时间跨度覆盖过去300年，部分年表可追溯至16世纪中期。1930年代之前，所有年表均呈现一致的生长模式；但自1930年代起，年表的生长趋势出现分化，形成两种截然相反但复现性良好的模式，本研究将其分别命名为第1组（Group 1）与第2组（Group 2）。在器测时段（1930年-2007年）内，第1组采样点与上一年7月气温呈负相关，而第2组采样点与生长年6月气温呈正相关。在更广阔的北美西北部（northwestern North America，缩写NWNA）尺度下，本研究发现第1组与第2组模式也存在于多组白云杉轮宽年表中，本研究将其命名为NWNA 1与NWNA 2年表。NWNA 1与NWNA 2年表在分化之前（约20世纪50年代）同样拥有一致的生长模式。将NWNA 1与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数据（公元1300年-2000年）进行对比后发现，在20世纪中期之前（至少可追溯至公元1300年），所有采样点均对气温呈正响应；但部分采样点的响应转变为负响应（对应NWNA 1组），其余采样点则保持正响应（对应NWNA 2组）。这种分化的空间分布范围暗示其受到大尺度驱动因子的影响。鉴于该分化仅出现在20世纪，结合古气候记录的证据，本研究认为气温响应模式的转变，是由北美西北部20世纪异常暖干气候所引发的水分胁迫所致。但部分未发生分化的采样点与发生分化的采样点相距仅数公里，因此本研究推测，采样点自身的局地因素，是决定其对分化大尺度驱动因子敏感性的关键因素。