Phylogeography of Douglas-fir: testing hypotheses from the fossil record.

Abstract

Paleobotanical records and molecular data from modern forests can provide a synergistic understanding of the ecological and evolutionary history of an organism. I used the fossil record to generate hypotheses that I tested with statistical phylogeographic methods for Douglas-fir (Pseudotsuga menziesii). In Chapter 1, I describe alternative scenarios of glacial refugia and postglacial migration based on compiled fossil pollen and macrofossil evidence from the late Quaternary. In Chapter 2, I test those hypotheses using coalescent analyses of mitochondrial and chloroplast DNA sequence data. I also test the paleobotanical hypothesis that Douglas-fir’s two varieties diverged coincident with the Cascade orogeny in the late Pliocene. Finally in Chapter 3, I test whether Mexican Douglas-fir diverged from U.S. populations in the Miocene or Pleistocene, consistent with alternative interpretations of limited fossil evidence in the region. The present patterns of molecular variation in Douglas-fir are well-described by Pliocene (or early Pleistocene) divergence of its varieties, mid-Pleistocene colonization of Mexico, and restriction to multiple glacial refugia in the late Quaternary. Holocene expansion into Canada resulted in recontact among varieties and hybridization driven entirely by pollen dispersal but not seed dispersal. Douglas-fir populations have responded individualistically to past climatic and geologic change, such that some underwent expansions while others contracted to higher elevation and some diverged while others coalesced. These findings highlight the complementary insights that fossil and molecular data provide and can be used to inform the conservation and taxonomy of Douglas-fir.