Graduate Thesis Or Dissertation
 

Landscape Genomics and Climatic Responses of Willamette Valley Alders

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  • The climate of the Pacific Northwest is in flux, and existing forest ecosystems are stressed and poised to shift in fundamental ways, with or without human intervention. This dissertation probes the nature of forest responses to environmental change through investigations of morphology and genetics of three species of alder co-occurring in the Willamette Valley, Oregon. Chapter 1 provides an introduction to the genus Alnus and reviews some ways in which hybridization among species, including alders, may be relevant to forest responses to changing environments. Chapter 2 describes geographic and taxonomic patterns of relationships in the plastid-based phylogeny of 114 western alders, mostly from the Willamette Valley. Through genome skimming and whole-plastome analysis, we discovered that alders in our sampling tend to share plastids with neighboring trees irrespective of taxonomy. We found that the Willamette Valley is dominated by a single plastid lineage which is subdivided into geographically localized sub-lineages distinguished by just one or two SNP’s or indels. Some of the geographically-structured clades separate populations only 10 km distant, providing evidence of recent interspecific hybridization within the Willamette Valley, and suggesting that recent seed migration, particularly among white alder populations, is highly constrained except along the Willamette River. In Chapter 3, we built upon the findings of chapter 2 by adding analyses of differences in seed dispersibility and plastid diversity between red and white alder. In a wind-tunnel seed dispersal experiment, we found that red alder seed is the better disperser, which is supported in the general distribution of red alder plastid diversity within the Willamette Valley. White alder plastid populations, on the other hand, tend to be isolated from one another and more related to nearby red alders, suggesting a possibly dominant role of pollen-mediated migration in this species. We test this hypothesis with a simple model relating within-stand diversity to stand composition. We also attempt to differentiate pollen-mediated migration from seed-mediated migration by comparing plastid diversity in stands along mainstem rivers with diversity in tributary watersheds. Chapter 4 presents two comparative analyses of red and white alder within the Willamette Valley. Based on 19 distinguishing characters in 92 red, white, and putative hybrid alders, a morphological analysis arrayed field identifications against a morphological index. We found indications of hybridity not only in field-identified putative hybrids, but also in many putative red and white alder, suggesting widespread admixture within these populations. The occurrence of admixture was further supported by the inclusion of F1 progeny of known red and white alder parentage in hybrid analyses. A wood mass-spectrometry analysis designed to distinguish species and hybrids instead uncovered a high degree of metabolomic similarity, particularly between red and white alder, a finding consistent with introgression and admixture of the taxa. In Chapter 5, we present an initial analysis of three years of data collected from an array of four common gardens spanning an environmental gradient, from the floor of the Willamette Valley to the crest of the Oregon Coast Range. Each garden was planted with over 1000 progeny of known parentage of red, white and putative hybrid alders from 12 stands across the Willamette Valley. We analyzed survival and growth of progeny and examined responses along temperature and precipitation clines. We found striking differences in survival between species, with white alder out-surviving red alder in every garden, even in the Oregon Coast Range, well beyond white alder’s current limits. Red alder was the fastest growing in every comparison, and hybrids were intermediate, but growth of red alder was severely curtailed by mortality in valley-floor gardens. These findings are consistent with those of Chapter 6, which suggest a northward contraction of the range of red alder. Chapter 6 is a broad analysis of the changing climate and forest of the Willamette Valley. The current health of eight tree species, four with ranges extending northward from the Willamette Valley and four southward, are compared through an assessment of 792 individual trees from stands throughout the northern and central valley. We found that all four northward-ranging species, including red alder, were faring poorly relative to southward-ranging species, with elevated rates of severe crown decline and mortality. Southward-ranging species, by contrast, including white alder, were comparatively healthy. These findings suggest ongoing decline of temperate forest species as temperatures rise and summer precipitation decreases.
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  • Native Plant Society of Oregon, Portland Garden Club
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