FIG. C1. Pollen record of the major arboreal taxa from Cooley Lake, British Columbia. Thin lines show 5× exaggeration. Symbols indicate plant macrofossils (needles, cone scales, and seeds).
The Cooley Lake pollen and macrofossil record of the past 7500 years (calibrated years before present, cal. yr BP) showed no major variation in the dominant tree taxa (Fig. C1). Macrofossil evidence suggests that forests near Cooley Lake were dominated by Picea engelmannii and Abies lasiocarpa throughout much of the record, with Pinus contorta, P. monticola, Pseudotsuga menziesii, Larix occidentalis, and Alnus as minor components. The abundance of Pinus is typical of the Holocene pollen spectra from the region, reflecting the wide dispersal range and productivity of pollen from this genus. We base this interpretation on examination of modern pollen assemblages from a network of small lakes in the same region (D. G. Gavin, unpublished data) and from northeast Washington State (Mack et al. 1978).
Despite little variation in the dominant taxa, the pollen and macrofossil diagram displays two notable changes (Fig. C1). The first is at the beginning of the record, after deposition of the Mazama tephra at 7490 cal. yr BP, Alnus pollen spikes to 20% and then decreases to < 1% in association with an increase of Pinus pollen from 36% to 76% and a slower increase in P. engelmannii pollen. This suggests a successional sequence perhaps connected to addition of a thick tephra to the soils.
T. heterophylla pollen occurs rarely (<1%) but consistently after 3800 cal. yr BP, and it increases to >2% at 3100 cal. yr BP. Cupressaceae pollen (which is often underrepresented and most likely represents T. plicata rather than Juniperus spp. at this site) increased to ~1% by 2300 cal. yr BP. These taxa probably arrived near the lake during a period of climatic cooling and increased moisture availability in the region (Luckman et al. 1993; Rosenberg et al. 2003). Because Cooley Lake is today at the upper elevation limits of T. heterophylla and T. plicata, increases in their pollen likely represent their progressive expansion at lower elevations prior to local arrival near the lake. T. heterophylla trees probably arrived at the site by 2600 cal. yr BP, as indicated by a pollen increase to 5%, and a needle in the sediment confirms its local presence at 2400 cal. yr BP. The local presence of T. plicata at 2500 cal. yr BP is confirmed by a seed.
| |
FIG. C1. Pollen record of the major arboreal taxa from Cooley Lake, British Columbia. Thin lines show 5× exaggeration. Symbols indicate plant macrofossils (needles, cone scales, and seeds). |
LITERATURE CITED
Luckman, B. H., G. Holdsworth, and G. D. Osborn. 1993. Neoglacial glacier fluctuations in the Canadian Rockies. Quaternary Research 39:144–153.
Mack, R. N., V. M. Bryant, and W. Fell. 1978. Modern forest pollen spectra from eastern Washington and northern Idaho. Botanical Gazette 139:249–255.
Rosenberg, S. M., I. R. Walker, and R. W. Mathewes. 2003. Postglacial spread of hemlock (Tsuga) and vegetation history in Mount Revelstroke National Park, British Columbia, Canada. Canadian Journal of Botany 81:139–151.