Ecological history of a long‐lived conifer in a disjunct population

Abstract

Abstract1. 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.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 have influenced its modern distribution.3. 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. 4.Tsuga mertensiana first appears in the pollen and macrofossil record at the highestelevation site at c. 4,100 cal year bp, then at the next highest-elevation site at c. 1,600 cal year bp, and last at the mid-elevation site at 800 cal year bp. Tsuga mertensiana pollen occurs continuously at ≥1% at all three sites by c. 300 cal year 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 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 untilPaper previously published as Standard Paper

Using paleo-archives to safeguard biodiversity under climate change

“…These episodic climatic events govern rates and patterns of range expansion and contraction(12). Studies of tree populations, for example, reveal the importance of non-stationary Holocene climate variability and its interactions with long-distance dispersal, local demographic processes, and species life-history traits(56)(57)(58)(59). Moreover, Holocene records show population expansions and declines are not necessarily accompanied by changes in geographic distribution(60,61), as is often assumed in conservation assessments(62).…”

Postglacial migration across a large dispersal barrier outpaces regional expansion from glacial refugia: evidence from two conifers in the Pacific Northwest

“…Geographically, the coastal sites that cluster with the interior are adjacent to previously-hypothesized mesic migration corridors between the coastal and interior distributions (26), suggesting that these coastal locations were sources of hemlock migrants into the interior. While habitable patches may have been available in an otherwise inhospitably arid rain shadow to facilitate their dispersal inland, exhaustive mapping has not identified such patches in the modern landscape (32). Despite recent dispersal into the interior, there is incomplete infilling of mountain hemlock into its potential distribution between the separately-established northern and southern interior disjunctions; this may have been a result of limited dispersal both across patchy mountain-tops in a direction orthogonal to the dominant winds, and from upwind coastal areas (32).…”

“…Mountain hemlock is well known for its tendency to leave weak pollen evidence in the fossil record, even on occasions where it was prevalent in the local watershed (80)(81)(82)(83). We therefore considered hemlock to be locally established when its pollen percentage reached at least 1% of the counted total, indicating its presence within 140 km (32). Records were then classified into five time slices (>20 kyr, >15 kyr, >10 kyr, >5 kyr, and >0 kyr) according to the basal age of the record and the age at which mountain hemlock first reached the 1% pollen threshold.…”

Growth and Its Relationship to Individual Genetic Diversity of Mountain Hemlock (Tsuga mertensiana) at Alpine Treeline in Alaska: Combining Dendrochronology and Genomics

“…Additionally, the species has long-distance seed dispersal capacity [34,35] and is highly outcrossed due to extensive pollen dispersal [36][37][38][39]. The species is found principally in wet cool environments on the Pacific coast of North America, where its range extends from the Alaskan Kenai Peninsula in the north to the Sierras in northern California [37,40], with disjunct populations occurring as far as Idaho, 160 km away from the main distribution [35].…”