| Silica phytoliths, can improve our understanding of past environmental change and supplement pollen data by providing records from and regions and by identifying taxa that cannot be distinguished with pollen analysis. The vegetation history of the Columbia Basin Province was reconstructed for the last 100,000 years. Vegetation reconstructions were based on phytoliths preserved in four loess sections located along an elevation gradient in southeastern Washington State. Age determinations were based on thermoluminescence dates and tephrochronology. Interpretation of past assemblages was based on modern phytolith collections of plant and soil samples in eight vegetation types in the Pacific Northwest. Vegetation records were compared with published pollen data and paleoclimatic reconstructions for the late Pleistocene and Holocene.; In the modern plant collection, Pinus ponderosa, Pseudotsuga menziesii, Carex, Aristida, Stipa, and members of the Asteraceae were identified by diagnostic phytoliths. Other taxa, including Abies, Agropyron , Artemisia, Calamagrostis, Festuca, Koeleria, Larix, Picea, and Poa, were identified by the combination of their phytolith morphotypes. Sagebrush steppe; Agropyron-, Festuca-, Stipa-, and Elymus-dominated grasslands; Pinus ponderosa-, Pseudotsuga-, and Abies lasiocarpa-dominated forests had distinct phytolith assemblages in soil samples.; Changes in the proportion of different phytoliths in the four loess sections indicate large shifts in vegetation during the last 100,000 years. A low-elevation section near the center of the basin provided a record of alternating sagebrush steppe and grassland that correlated well with paleoenvironmental reconstructions based on cicada-burrow and stable-isotope data from the same section, a nearby pollen record, and global ice-volume variations. Higher elevation sites supported mesic grassland, forest, and parkland at different times in the last 100,000 years. Variations in vegetation are explained as a response to changes in large-scale climatic controls. Grasslands and steppe were widespread and forests were more restricted than today during the early and mid-Wisconsin intervals, probably as a result of cooler and drier conditions. At the Last Glacial Maximum, Artemisia shrub steppe with Stipa and Poa grasses was present at low and high elevations. Middle elevations supported non-analogue parkland of forest and steppe elements, including Abies, Picea, Calamagrostis, Artemisia, and Stipa. This dissertation includes unpublished co-authored materials. |
