Ecological history of a long‐lived conifer in a disjunct population. (20th July 2017) - Herring, Erin M.
Abstract: In northern Idaho (USA), more than 100 vascular plant species are disjunct >160 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. 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 have influenced its modern distribution. The modern distribution of T. mertensiana was analysed by constructing a range map and modelling the potential species distribution. The presence of outlier populations surrounding the Idaho disjunction along with broad areas of unoccupied suitable habitat indicates that 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 analyse the probability of detecting T. mertensiana pollen as a function of distance from its geographical range limit. Consistent T. mertensiana pollen occurrence at ≥1% abundance is likely only within 42 km of its range limit. Tsuga mertensiana first appears in the pollen and macrofossil record at the highest‐elevation site at c . 4, 100 cal yearbp, then at the next highest‐elevation site at c . 1, 600 cal yearbp, and last at the mid‐elevation site at 800 cal yearbp . Tsuga mertensiana pollen occurs continuously at ≥1% at all three sites by c . 300 cal yearbp 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 160 km away. Synthesis . Comparison with palaeoclimate reconstructions from the broader region suggests 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 a limiting range infilling in the interior mesic forests today. Abstract : We examined palaeoecological records to address the establishment of Tsuga mertensiana in an interior disjunction >160 km separated from its main coastal distribution. Lack of any fossil evidence >4, 000 years ago suggests surprising long‐distance dispersal. The timing of first detection coincides with climate changes consistent with a warmer climate limiting its interior distribution. 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.