Lake Sediment Pollen from Knob Hill Pond in Marshfield VT from 14000 BP to Present

The abrupt, range-wide decline of Tsuga canadensis ~5500 calibrated years before present (cal. yr BP) is one of the most-studied events in North American paleoecology. Little attention, however, has been given to an earlier Tsuga decline, dated to ~6000 cal. yr BP in southern Ontario, Canada. To investigate whether this event occurred elsewhere in eastern North America, we analyzed the middle-Holocene interval of a lake-sediment record from Knob Hill Pond, located in northern Vermont, USA, an area of historically high Tsuga abundance. A dramatic, short-lived drop in Tsuga pollen abundance does occur at ~6000 cal. yr BP in the Knob Hill Pond record, indicating that Tsuga populations declined in various parts of its range. We hypothesize that both middle-Holocene declines of Tsuga were caused by the deleterious effects of pronounced droughts on this moisture-sensitive tree. Close examination of pollen data from a transect of sites across New England reveals that the earlier decline of Tsuga is present in other records, although some aspects of the event appear to have varied geographically. While northern and higher-elevation sites exhibit a nearly full recovery of Tsuga populations between the two declines, records further to the south are characterized by a stair-step pattern of progressive decline. At sites near its southern range limit, relatively warm conditions between ~6000 and 5500 cal. yr BP were apparently not conducive to the reestablishment and survival of Tsuga, and thus it was unable to recover between the drought events.

距今约5500个校准年前（cal. yr BP）的加拿大铁杉（Tsuga canadensis）全分布区突发式衰退，是北美古生态学中研究最为深入的事件之一。然而，此前针对更早的铁杉衰退事件的研究甚少——该事件在加拿大安大略省南部的年代约为距今6000个校准年前。为探究该事件是否在北美东部其他区域发生，我们对美国佛蒙特州北部诺布希尔池塘（Knob Hill Pond）的湖泊沉积记录中的中全新世层段开展了分析，该区域历史上铁杉丰度极高。诺布希尔池塘的沉积记录中，确实在距今约6000个校准年前出现了铁杉花粉丰度的显著短期下降，这表明铁杉种群在其分布区的多个区域均发生了衰退。我们提出假说：两次中全新世铁杉衰退事件，均由严重干旱对这种水分敏感型树种产生的有害效应所引发。对新英格兰地区跨站点样带的花粉数据进行细致分析后发现，此次更早的铁杉衰退事件在其他沉积记录中也有体现，尽管该事件的部分特征存在地理差异。在两次衰退事件之间，北部及高海拔区域的铁杉种群几乎完全恢复；而更南部的沉积记录则呈现出逐步衰退的阶梯式模式。在铁杉分布区南部边界附近的站点，距今约6000至5500个校准年前的相对温暖环境，显然不利于铁杉的重新定植与存活，因此其无法在两次干旱事件之间完成种群恢复。