Reconstruction of the late Pleistocene grassland of the Columbia basin, Washington, USA, based on phytolith records in loess

Silica phytoliths preserved in three loess sections in southeastern Washington State revealed a 100 000-year history of the Columbia Basin grassland. Changes in the proportion of different morphotypes indicate large shifts in vegetation composition during the last 100 ka. A low-elevation section (677 m asl) near the center of the basin provided a record of alternating xeric Festuca–Poa and mesic Festuca–Koeleria grassland. The middle-slope section (1095 m asl) supported Picea–Abies or Pinus ponderosa forest or non-analog parkland at different times. Some trees were present at or near the site even during the Last Glacial Maximum. The highest site (1220 m asl) supported Stipa-, Festuca- and Poa-dominated grassland with some Artemisia shrub during most of the late Pleistocene, but supports a coniferous forest today. Variations in vegetation can be explained as a response to changes in large-scale climatic controls. Grasslands and shrub steppe were apparently more widespread and forests more restricted than today during the marine isotope stages 2 and 4, probably as a result of cooler and drier conditions. The three new records are well correlated with previously published paleo-reconstructions based on phytolith, cicada burrow and stable isotope data from a nearby KP-1 loess section, Carp Lake pollen record, and global ice volume variations.