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）的分布区与它们在太平洋西北海岸的主要分布区间断相距不足200公里。目前仍不清楚这类内陆森林间断分布类群中的多数物种（山地铁杉（Tsuga mertensiana）），究竟是在末次冰期（last glacial period）于爱达荷北部避难所存活至今，还是在全新世（Holocene）通过长距离扩散定植于此。
2. 研究人员从爱达荷州北落基山脉山地铁杉主导森林中的3处中高海拔湖泊中采集了沉积物岩芯（sediment cores）。借助花粉与大化石记录重建森林群落组成，确定山地铁杉定植的时间，验证该区域曾为冰期避难所的假说，并对比气候、种间竞争以及/或扩散限制如何影响其现代分布格局。
3. 研究通过构建分布范围图与物种潜在分布模型，分析了山地铁杉的现代分布。爱达荷间断分布区周边存在异地种群，同时存在大片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过程中。为评估山地铁杉花粉用于探测其分布边界的准确性，研究团队构建了花粉地表采样网络，分析了距其地理分布边界不同距离下检测到山地铁杉花粉的概率。结果显示，仅在距分布边界42公里范围内，山地铁杉花粉的丰度才会稳定达到≥1%。
4. 山地铁杉最早出现在最高海拔点位的花粉与大化石记录中，时间约为距今4100校准年（cal yr BP），随后在次高海拔点位出现于距今1600校准年，最晚在中海拔点位出现于距今800校准年。到距今300校准年时，三处点位的山地铁杉花粉丰度均持续达到≥1%，表明该物种已在区域内广泛分布。其定植时间表明，山地铁杉是爱达荷州森林的新近组成物种，是在全新世时期从200公里外的海岸种群通过长距离扩散而来的。
5. 研究总结。将本研究结果与更大区域范围内的古气候重建结果对比后发现，在全新世内陆定植过程中，气候因素比扩散限制起到了更强的限制作用，这说明该物种克服大型扩散屏障的难度较低。不过，山地铁杉在数千年间始终维持较低的种群丰度，直到小冰期（Little Ice Age）气候推动其种群丰度在近期实现增长。现存未被占据的适宜生境表明，当前内陆中生林的分布填充限制因素为种间竞争，而非气候或扩散限制。