Late‐Wisconsin Pollen and Seed Analysis from the Nebraska Sandhills

Between two lrge dunes near the northern edge of the grass—covered Nebraska Sandhills on the Rosebud Indian Reservation is an alluviated lowland containing more than 6 m of organic sand and silt. Pollen and seed analyses of the lowest 120 cm of sediment imply that a boreal forest with spruce grew around the site 12,600 c 1 4 years ago and was succeeded abruptly by grassland, with perhaps more pine than exists today (as Pinus ponderosa) in the nearby Niobrara River Valley and the Pine Ridge escarpment of northwestern Nebraska. Pollen analyses of surface samples were used to determine the modern pollen rain at different distances from the modern pine forests of the western plains. The presence of spruce fossils at the Rosebud site, along with similar finds in northeastern South Dakota and southern Minnesota, implies a late—Wisconsin boreal forest over much of the present prairie region. The major disjunction of Picea glauca in the Black Hills, 200 km northwest of the site and 700 km from from the spruce forest of Manitoba, may thus be explained as a relic of a semi—continuous distribution during late—Wisconsin time.