Data from: Ecological history of a long-lived conifer in a disjunct population

1. In northern Idaho (USA), more than 100 vascular plant species are disjunct <200 km from their main distribution along the Pacific Northwest coast. It remains unclear whether most species within this interior forest disjunction, including Tsuga mertensiana, survived the last glacial period in a north-Idaho refugium or whether these species colonized the region via long-distance dispersal during the Holocene.
2. Sediment cores were extracted from three mid- to high-elevation lakes within T. mertensiana dominated forests in the Northern Rocky Mountains of Idaho. Pollen and macrofossils were used to reconstruct forest composition, determine the timing of T. mertensiana establishment, examine the hypothesis that the region was a glacial refugium, and contrast how climate, competition and/or dispersal limitation has influenced its modern distribution.
3. The modern distribution of T. mertensiana was analyzed by constructing a range map and modeling the potential species distribution. The presence of outlier populations surrounding the Idaho disjunction along with broad areas of unoccupied suitable habitat indicates the range of T. mertensiana is currently expanding. To assess the accuracy of T. mertensiana pollen at detecting its range limit, a network of pollen surface samples was used to analyze the probability of detecting T. mertensiana pollen as a function of distance from its geographic range limit. Consistent T. mertensiana pollen occurrence at ≥1% abundance is likely only within 42 km of its range limit.
4. T. mertensiana first appears in the pollen and macrofossil record at the highest elevation site at ca. 4100 cal yr BP, then at the next-highest-elevation site at ca. 1600 cal yr BP, and last at the mid-elevation site at 800 cal yr BP. T. mertensiana pollen occurs continuously at ≥1% at all three sites by ca. 300 cal yr BP suggesting regional presence. The timing of arrival suggests that T. mertensiana is a recent component of the forests of Idaho, having arrived during the Holocene via long-distance dispersal from coastal populations over 200 km away.
5. Synthesis. Comparison with palaeoclimate reconstructions from the broader region suggest that climate was a greater limiting factor than dispersal in the Holocene establishment in the interior, indicating little difficulty overcoming a large dispersal barrier. However, T. mertensiana remained at low abundances for millennia until Little Ice Age climates promoted its recent increase in abundance. Unoccupied areas of suitable habitat suggest that competition, rather than climate or dispersal, is limiting range-infilling in the interior mesic forests today.

1. 在美国爱达荷州北部，有超过100种维管植物（vascular plant）的间断分布种群，其分布区距离其在太平洋西北海岸的主要分布范围不足200公里。目前仍不清楚该内陆森林间断分布类群中的多数物种（包括山地铁杉（Tsuga mertensiana））是否在末次冰期于爱达荷北部的冰期避难所中存活，亦或是在全新世通过长距离扩散定植于该区域。
2. 研究人员从爱达荷州北落基山脉地区、以山地铁杉（Tsuga mertensiana）为主的森林中的3处中高海拔湖泊提取了沉积物岩芯。通过花粉与大化石分析，重建了区域森林组成，确定了山地铁杉定植的时间，验证了该区域曾为冰期避难所的假说，并对比了气候、种间竞争和/或扩散限制如何影响其现代分布格局。
3. 通过构建分布范围图并模拟物种潜在分布区，分析了山地铁杉（Tsuga mertensiana）的现代分布格局。爱达荷间断分布区周边存在孤立种群，同时存在大面积未被占据的适宜生境，这表明山地铁杉的分布范围目前正处于扩张状态。为评估山地铁杉花粉对其分布区边界的检测准确性，研究人员构建了花粉表层样品网络，以分析距离地理分布区边界的远近与检测到山地铁杉花粉的概率之间的关系。结果显示，仅在距离分布区边界42公里以内的区域，山地铁杉花粉丰度才大概率维持在≥1%的水平。
4. 山地铁杉的花粉与大化石记录最早出现在海拔最高的采样点，时间约为4100校准年BP（cal yr BP），随后出现在海拔次高的采样点（约1600校准年BP），最后出现在中海拔采样点（800校准年BP）。至约300校准年BP时，三个采样点的山地铁杉花粉丰度均持续维持在≥1%的水平，表明该物种已在区域内广泛存在。其定植时间表明，山地铁杉是爱达荷州森林中的新近类群，于全新世期间从200公里外的海岸种群通过长距离扩散抵达该区域。
5. 综合分析。将本研究结果与更大范围区域的古气候重建结果对比后发现，在全新世内陆定植过程中，气候因素相较于扩散限制是更强的限制因子，这表明山地铁杉跨越较大扩散障碍的难度较低。然而，山地铁杉在数千年间始终维持较低的丰度，直至小冰期（Little Ice Age）的气候条件推动其丰度在近期出现上升。当前未被占据的适宜生境表明，如今内陆中生森林中限制山地铁杉完成分布区填充的因素是种间竞争，而非气候或扩散限制。