Phytoliths in plants and soils of the interior Pacific Northwest, USA

Phytoliths are a useful paleoproxy in the arid environments. This modern analog study assessed variability of silica phytoliths in 38 species of plants and 58 modern soil samples from 24 locations in the interior Pacific Northwest. Phytoliths were grouped into 20 broadly defined morphotypes based on their 3D shapes under light microscope and presumed anatomical origin within the plant. Grasses (all C3) have most diverse forms. Most examined conifers, sedges and some shrubs produce identifiable phytoliths as well. Eight different community types can be distinguished based on their modern phytolith record in soils, including shrublands, four regional grassland types, and three forest types. Low percentages of grass phytoliths and high incidence of non-grass forms correspond to forest vegetation in the region today, while certain grass phytoliths allow further differentiation among different grasslands. Phytolith assemblages were further compared to 5 environmental variables, including elevation, mean annual temperature, mean annual precipitation, a moisture index and a growing-degree days index. Some morphotypes tend to occur within relatively narrow environmental windows, which could enable direct paleoenvironmental inferences from phytoliths in geological sediments from the region.