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.