Late Quaternary Vegetation History at Waits Lake, Colville River Valley, Washington

Abstract

A 10.6-m section of mostly calcareous sediments at Waits Lake, Washington, reveals a multizoned history of vegetation change since recession of Pinedale (P-2) glacial ice from the Colville River Valley. The oldest unit with high percentages of Artemisia, Gramineae, and Shepherdia canadensis-type pollen characterizes vegetation in which trees were not major components and the climate was cooler and moister than today. Pollen Zone II (ca. 10,000-6,700 yr before present [BP]) records a warmer period in which diploxylon pine was prominent. Concomitant with the Mazama ash fall (6,700 yr BP), Artemisia became particularly prominent (Pollen Zone II); but by about 5,000 yr BP, diploxylon pine was conspicuous. This comparatively short-term vegetation change probably indicates the advance of drought-tolerant Artemisia-dominated steppe northward in the Colville River Valley. The modern climax vegetation in the vicinity of Waits Lake (Pseudotsuga-dominated forest) appears to have emerged around 2,300 yr BP, although more detailed documentation of this event is hampered by the low pollen production of Pseudotsuga menziesii. The 14C dates from these calcareous sediments were corrected through use of the Mazama and Glacier Peak ash layers as time-stratigraphic marker horizons.