Ancient Divergences of the Maritime Alpine Tree Larix lyallii (Pinaceae) Contrasts With Patterns in Other Pacific Northwest Coastal Disjuncts

ABSTRACT

The disjunct mesic forests of the Pacific Northwest, occurring on the west side of North America's Coastal Ranges as well as in the Northern Rocky Mountains, have emerged as a model system for comparative phylogeography. Less well‐known, however, are the alpine and subalpine plants that mirror this distribution—here referred to as the Maritime Alpine Disjuncts. The phylogeographic histories of lowland coastal disjunct species are the result of either the Cascade orogeny, which separated once‐contiguous mesic habitats, post‐Pleistocene dispersal of species between the northern Interior and coastal forests, or a complex mix of these two processes. In the current study, we investigate the phylogeographic history of Larix lyalii (subalpine larch)—a narrowly distributed Maritime Alpine Disjunct species—to determine whether these geographically related disjunct communities are the result of processes similar to other Pacific Northwest disjuncts. We sample the entire range of L. lyalii and use ddRAD data to explore hypotheses on the timing of these disjunctions. Using a dataset of 28 individuals and 30,425 loci we find three major clades within the species: a Cascade Mountain clade sister to northern and southern Interior clades. We estimate the major clade divergences to be on the order of one to three million years with evidence of population admixture between the two Rocky Mountain clades surrounding the Flathead Valley in Northwestern Montana, USA. L. lyallii 's present phylogenetic structure has origins predating the Pleistocene. While the timing of this structure is largely consistent with an ancient vicariance event present in some lowland coastal disjunct species, the mechanisms for this are likely different in this high elevation tree species.