Introduction

Vegetation is one of the most important terrestrial proxies for evaluating climate change. In this chapter, we document vegetation changes during the Holocene in the Americas and discuss the climate signal that vegetation changes imply. The primary data are pollen and macrofossils from lake and peatland sediments. Pollen is a particularly valuable climate proxy because it typically forms a continuous time series and registers the timing and rates of climate change. Macrofossils do not occur as regularly nor do they sample the vegetation as completely as pollen, but because they are not so widely dispersed, they have major value in that they indicate local occurrence. Macrofossils have been particularly valuable for tracing fluctuations in tree line, which is particularly sensitive to climate.

A great diversity in physical geography and vegetation occurs throughout the Americas from the polar tundras to the tropics. In this chapter, we cannot exhaustively treat every vegetation type and region; that would require an entire book. Several recent volumes have described the major vegetation-climate trends in the Americas (e.g., Wright, 1983; Ruddiman and Wright, 1987; Wright et al., 1993), and our aim is not to duplicate these efforts. We will emphasize sites that have been published since these volumes were published. We consider major geographic regions and describe representative sites (Fig. 1 and Tables 1-3) that illustrate the regional climate trends during the Holo-cene. For the most part, these sites are in the Global Pollen Database available from the World Data Center-A for Paleoclimatology. We use calendar years throughout this chapter, except where otherwise noted. We consider the beginning of the Holocene to be ca. 11,500 B.P. By this time, the continental glaciers had retreated from most of North America, although ice did not completely disappear from northeastern Canada until ca. 6000 B.P. (Dyke and Prest, 1987).

19.2. NORTH AMERICA NORTH OF MÉXICO

19.2.1. Northern Alaska and Northwest Canada

The vegetation of Alaska and adjacent northwest Canada is boreal forest and tundra today. The dominant trees of the boreal forest are Picea, Larix, Betula, and Populus. Pinus contorta occurs in central Yukon. Tundra occurs in the far north, in westernmost Alaska, and at higher altitudes in the many mountain ranges. Shrub tundra dominated by Betula glandulosa/B. nana and Al-nus viridis is widespread.

Betula, Alnus, and Picea dominate the Holocene of northern Alaska and northwestern Canada. Livingstone (1955) first identified the two-zone sequence typical of the Holocene across the region: an early Holo-cene Betula zone and a middle to late Holocene Alnus zone. The pollen in these zones derives from predominantly shrubby Betula and Alnus. The pollen diagram from Screaming Yellowlegs Pond (Edwards et al., 1985) is representative of northern Alaska (Fig. 2). The radiocarbon dates of the Betula/Alnus transition vary from ca. 10,300-8200 B.P., with much geographic variability but perhaps a west-to-east trend toward a younger age (Anderson, 1985). However, the radiocarbon dates are almost all on bulk sediment with low organic matter and typically slow sedimentation rates, and in many cases, the dates are not exactly on the transition from Betula to Alnus. Therefore, the existence or nonexistence of synchrony in this zone boundary is uncertain (Bru-baker et al., 1983).

Picea shows more regional variability. It immigrated very rapidly from the east and south, increasing before Alnus in the east but after Alnus in the west. At sites

FIGURE 1 North America with sites discussed in text for which summary pollen diagrams are shown. BA, Balsam Meadow; BI, Billys Lake; BG, Battle Ground Lake; CH, Lake Chalco; CL, Clear Pond; CO, Como Lake; DE, Delorme 2; DI, Divide Lake; DV, Devils Lake; GB, Lake GB2; KA, Kanaaupscow; KI, Kirk Lake; LI, Lily Lake; MA, Mansell Pond; MO, Moon Lake; MR, Moraine Lake; MU, Munday Creek; MY, Myrtle Lake; MZ, Montezuma Well; PR, Pretty Lake; SC, Slough Creek Pond; SL, Sleet Lake; SU, Sutherland Pond; SY, Screaming Yellowlegs Pond; TE, Telegraph Creek; TO, Toboggan Lake; TU, Lake Tulane; WO, Wolsfeld Lake; ZA, Lake Zacapu.

FIGURE 1 North America with sites discussed in text for which summary pollen diagrams are shown. BA, Balsam Meadow; BI, Billys Lake; BG, Battle Ground Lake; CH, Lake Chalco; CL, Clear Pond; CO, Como Lake; DE, Delorme 2; DI, Divide Lake; DV, Devils Lake; GB, Lake GB2; KA, Kanaaupscow; KI, Kirk Lake; LI, Lily Lake; MA, Mansell Pond; MO, Moon Lake; MR, Moraine Lake; MU, Munday Creek; MY, Myrtle Lake; MZ, Montezuma Well; PR, Pretty Lake; SC, Slough Creek Pond; SL, Sleet Lake; SU, Sutherland Pond; SY, Screaming Yellowlegs Pond; TE, Telegraph Creek; TO, Toboggan Lake; TU, Lake Tulane; WO, Wolsfeld Lake; ZA, Lake Zacapu.

where the pollen analyst has separated the two Picea species, P. glauca increased first, followed by P. mariana, after which at many sites P. glauca subsequently decreased (Anderson et al., 1988; Brubaker et al., 1983; Cwynar and Spear, 1995; Spear, 1993).

Sites in northern Alaska and the Yukon had a number of taxa that were north of their present range in the early Holocene, including Populus, Myrica, Typha, and Juniperus, which indicate warmer conditions than today (Ritchie et al., 1983). Total pollen influx of trees and shrubs was also typically highest in the early Holocene (Ritchie, 1984). The pollen data indicate that conditions warmer than today existed from ca. 13,000-8000 B.P., with maximum warmth at ca. 11,500 B.P., and that the warmest conditions existed before Picea migrated into the region. Soon after Picea arrived, it became established north of its present range and at higher elevations than today. In the Tuktoyaktuk Peninsula, northern Yukon, Picea occurred north of its present range in the early Holocene (Ritchie and Hare, 1971; Spear, 1993), for example, at Sleet Lake (Spear, 1993) (Fig. 2); and in the southern Ogilvie Ranges, central Yukon, Picea occurred at higher elevations than today at sites that are now in shrub tundra (Cwynar and Spear, 1995). Cooling is evident in the pollen data by ca. 9000 B.P., but temperatures remained warmer than modern until ca. 5000 B.P. (Ritchie, 1984). The late Holocene expansion of Alnus indicates wetter summers (Anderson, 1985). In general, vegetation changes in the Yukon indicate cooler and wetter growing sea-

TABLE 1 Sites in North America North of Mexico

Site name

Region

Coordinates

Elevation (m)

Ref.

Sleet Lake

Northern Alaska

69°17' N, 133°35' W

0 0

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