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FIGURE 3 Representative pollen diagrams from southeast Alaska: Lily Lake (Cwynar, 1990) and Munday Creek (Peteet, 1986).

B.P. for T. heterophylla. In the southern part of Alaska, Pi-nus contorta reappeared in the middle to late Holocene, following Picea and the two Tsuga species. Increased moisture and paludification probably led to the increase in Pinus contorta ssp. contorta, which grows on muskegs (Hansen and Engstrom, 1996).

Most investigators (e.g., Peteet, 1986; Hansen and Engstrom, 1996; Heusser, 1983) ascribe the development of coastal forest to increasing precipitation and to increasing temperature in the early Holocene in the southern part of the area (Hansen and Engstrom, 1996). The development of coastal forest at Prince William Sound at the time of neoglacial cooling suggests that precipitation rather than temperature was the critical factor. The progressive cooling and increasing moisture during the Holocene suggest a weakening of the Pacific Subtropical High and an intensification of the Aleutian Low and associated cyclonic storms (Heusser et al., 1985).

Investigators have debated the relative importance of climate change versus migrational lag to explain the migration of conifers along the coast. However, macro-fossil evidence indicates the presence of species long before pollen percentages increase beyond background values. A cone of Picea sitchensis indicates its presence in the Glacier Bay region 2500 years before pollen indicates certain presence (Ager, 1983). Pinus contorta macrofos-sils occur throughout a core from near Yakutat, Alaska, from which pollen data suggest a recent arrival (Peteet, 1993). Thus, the macrofossil data support the climate change hypothesis. In summary, the pollen and macro-fossil data suggest that the early Holocene was warmer and drier than it is today and that the trend throughout the Holocene has been toward a cooler and moister climate.

19.2.3. Pacific Northwest and the Sierra Nevada

The modern vegetation of the Pacific Northwest is conifer forest with Tsuga heterophylla, Thuja plicata, and Pseudotsuga menziesii. Alnus rubra is an important post-fire species. The lowlands between the Coastal Ranges and the Cascade Range are drier; Quercus garryana occurs in the Willamette Valley. A north-south transect of sites have a consistent vegetation history: Pinecrest Lake (Mathewes and Rouse, 1975), Marion Lake (Math-ewes, 1973), Kirk Lake (Cwynar, 1987), Davis Lake (Barnosky, 1981), Battle Ground Lake (Barnosky, 1985), and Little Lake (Worona and Whitlock, 1995).

High percentages of Alnus, Pseudotsuga, and Pteridi-um characterize the early Holocene, indicating drier conditions and high fire frequencies. Charcoal studies at Kirk Lake (Cwynar, 1987) and Little Lake (Long et al., 1998) confirm higher fire frequencies. The Alnus-Pseudotsuga-Pteridium zone began before 12,000 B.P. at most sites and ended between 7400-5000 B.P. The pollen diagram from Kirk Lake (Fig. 4) is representative of the Pacific Northwest. At Battle Ground Lake (Fig. 4) in the southern Puget Trough in southwest Washington, Pseudotsuga-Quercus savanna occupied the area from 12,000-5000 B.P. (Barnosky, 1985). Northward advance of Pseudotsuga and higher tree lines indicate warmer temperatures. Between 9800 and 7800 B.P., Pseudotsuga

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