Mountain Hemlock Landscape Genetics on the Kenai Peninsula, Alaska

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Johnson, Jeremy Scott
landscape genetics; landscape genomics; seed dispersal; long distance dispersal; mountain hemlock; Tsuga mertensiana; Kenai Peninsula, Alaska; alpine treeline
thesis / dissertation description
Global ecological change is a serious threat to biodiversity. Changes in temperature, precipitation, and landscape fragmentation may reduce the ability of plants to persist within these changing environments. Knowing how plants have responded to past climate fluctuations, and accounting for alternative future responses will help conservationists, land managers, and policy makers plan for the future. This research takes a multiscale genomic approach to understand how a long lived conifer tree, mountain hemlock (Tsuga mertensiana Bong (Carr)), has responded to past climate variability, and assesses its potential to respond to future changes. I used biogeographic and landscape genetic approaches, based on double digest Restriction Associated DNA sequencing (ddRADseq), to address three research questions: 1) Are isolated stands of mountain hemlock on the Kenai Peninsula of Alaska glacial relicts or are they the product of long distance dispersal following glacial retreat? 2) What is the dominant mode of reproduction at mountain hemlock tree line? 3) Are seeds arriving at the alpine tree line ecotone the product of local dispersal or are they arriving from more distant sources? I found that genomic diversity and genetic structure differed between isolated stands of mountain hemlock and those found across the rest of the peninsula. A graph approach based on electrical circuit analysis identified high landscape connectivity and conductance across the peninsula. Genetic variation was primarily explained by landscape resistance and not geographic distance. These findings suggest that mountain hemlock colonized the peninsula via long distance dispersal and repeated founding events accompanied by high levels of gene-flow. At the local scale, seed dispersal ranged from 1.44 to 326.85 m with a mean dispersal distance of 73 m. Most seeds arrived as cryptic gene flow from beyond tree line. Long distance dispersal was quantified at the 99th percentile of the dispersal curve and accounted for dispersal at distances greater than 450 m. This analysis showed that under similar landscape configurations, mountain hemlock has the capability to track its shifting climate niche in response to future climate change. This research represents a novel integration of genomics and geography to answer a pertinent set of questions allowing us to have a deeper understanding of how plants may migrate under shifting climate conditions.