Pollen Sequence from the Columbia Basin, Washington: Reappraisal of Postglacial Vegetation

Abstract

During the early postglacial time of eastern Washington (10,000 to 18,000 years B.P.) treeless vegetation dominated by Artemisia may have occupied extensive areas of stony patterned ground, while conifers (haploxylon pine with Abies and Picea) were restricted to nearby loess hills. Decrease in Artemisia, Abies and Picea, suggesting a warming trend, began over 9000 14C years ago and coincided with a marked increase in diploxylon pine. INTRODUCTION The sequence of vegetation since the last recession of the Cordilleran ice sheet in eastern Washington (Richmond et al., 1965) (see Fig. 1) long has been inferred to show three stages. Most recently, Heusser (1965) has outlined these stages as follows: III. Pinus ponderosa dominant 4000-0 years B.P. II. Grasses, chenopods and composites dominant 8000-4000 years B.P. I. Pinus contorta parkland 13,000-8000 years B.P. This sequence is based on the numerous pollen diagrams constructed by Hansen (1947) in his pioneering work concerning the Quaternary vegetation history of the Pacific Northwest. Dates used by Hansen in the interpretation of his data were of necessity only relative orders of magnitude, being based on estimated rates of peat accumulation before the advent of radiocarbon dating. Pine pollen is omnipresent in these diagrams. Hansen reported that at least three species (P. ponderosa, P. monticola and P. contorta) could be distinguished in the fossil pollen record based on their different corpus breadths. Fossil pine pollen having a corpus breadth within the size range thought to represent modern species was assigned to that taxon in the pollen diagrams (Hansen, 1947). Re-evaluation of this technique as applied to pines in the Pacific Northwest suggests the method has low reliability (Mack, 1 Present address: Department of Botany, Washington State University, Pullman 99163.