Sutherland Pond Mansell Pond

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FIGURE 8 Representative pollen diagrams from the Northeast: Sutherland Pond (Maenza-Gmelch, 1997) and Mansell Pond (Almquist-Jacobson and Sanger, 1995).

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FIGURE 8 Representative pollen diagrams from the Northeast: Sutherland Pond (Maenza-Gmelch, 1997) and Mansell Pond (Almquist-Jacobson and Sanger, 1995).

limit soon after arriving at ca. 10,200 B.P., indicating that the climate was already warmer by that time than it is now (Davis et al., 1980). Picea had almost reached its modern altitudinal limit by 11,500 B.P. Tsuga grew 300-400 m above its modern limit soon after arriving at 7800 B.P. Tsuga began to retreat downslope by ca. 5700 B.P., but occurred above its present limit until the LIA. After Tsuga declined at 5600 B.P., it reestablished at lower altitudes by ca. 3200 B.P., but not at higher altitudes. Picea was rare throughout most of the Holocene until it expanded ca. 2000 years ago. These data indicate a warmer and drier early to middle Holocene, with the warmest period from 10,200 to at least 5600 B.P., when Tsuga disappeared from the higher sites. The last 2000 years, corresponding with the expansion of Picea, were the coldest part of the Holocene (Gajewski, 1987, 1988; Russell et al., 1993).

Pinus strobus not only reached its maximal altitudinal limits in the early Holocene, it also reached its maximum latitudinal limits (Jacobson and Dieffenbacher-Krall, 1995), essentially attaining its modern limits by 10,200 B.P. From 7800-4400 B.P., it had large populations north of its present limit. The timing of the northward expansion of P. strobus suggests that the period of maximal warmth occurred later than in the White Mountains.

Webb et al. (1993) used pollen response surface techniques to reconstruct soil moisture and mean annual precipitation at 3000 radiocarbon-year intervals for the Northeast. This study indicates that the climate was drier than it is today during times when Pinus domi nated at 10,200 B.P. (9000 14C B.P.) with a return to moist conditions at 6800 B.P. (6000 14C B.P.). Lithostrati-graphic studies by Webb (1990) indicate low water levels during the early Holocene Pinus maximum.

Pollen, lake-level, and charcoal studies at Mansell Pond in Maine (Fig. 8) provide a multiproxy record of climate and vegetation change (Almquist-Jacobson and Sanger, 1995). Pinus strobus was high between 10,200 and 6500 B.P. However, in this record, this P. strobus interval is different from many other records in New England in that a Tsuga phase is intercalated from 82007300 B.P. The lake level was at least 5.9 m below the modern level at 8000 B.P. Increased charcoal during the P. strobus phase indicates greater burning. Other studies also indicate a greater frequency of forest fires in the early Holocene P. strobus forest than in the hardwood forests that succeeded it (Anderson et al., 1992; Clark et al., 1996). At Mansell Pond, the lake level rose at 6700 B.P., and Tsuga increased again at 6500 B.P. The lake level increased again at 3400 B.P., but fell slightly with the increase of Picea 2000 years ago, indicating perhaps a cooler, but drier, late Holocene climate.

Water-level studies at Owasco Lake in New York indicate a low stand and dry climate at 10,200 B.P., in agreement with other studies in New England (Dwyer et al., 1996). A high stand occurred at 7700 B.P., during the time of the early Tsuga phase at Mansell Pond, providing additional evidence of a dry-wet-dry climate cycle during the early Holocene Pinus strobus phase.

In summary, the climate in the Northeast was warmest and driest during the early Holocene from

10,200 to ca. 6000 B.P., with the most extreme conditions early at ca. 10,200 B.P. The climate became moister and cooler after the Pinus strobus phase, with the establishment of hemlock-northern hardwoods forest, with abundant Tsuga, Fagus, and Betula. Distinctly cooler, but perhaps somewhat drier, conditions commenced ca. 2000 years ago with the expansion of Picea.

19.2.9. Northeastern Canada

In the northeastern part of Canada, which lies north of the Tsuga-Pinus-northern hardwoods forest, the vegetation grades northward from boreal forest to forest-tundra to tundra. Dominant trees in the boreal forest are Picea glauca, P. mariana, Pinus banksiana, Betula papyrifera, and in the southeastern part of the region Abies balsamea. B. papyrifera is an important postfire species. This part of North America was the last to be deglaciated, with much of the area still ice covered in the early Holocene.

In southeastern Labrador, ice had retreated by 12,500 B.P., and tundra persisted until ca. 10,200 B.P. Lamb (1980, 1984) recognized four regional zones: a Betula-Salix-Cyperaceae zone representing tundra from 12,500 -10,200 B.P., an Alnus-Betula zone representing shrub-tundra from 10,200-6800 B.P., an Abies-Picea glauca zone from 6800-5700 B.P., and a Picea mariana zone from 5700 B.P. to the present. Moraine Lake (Fig. 9) (Engstrom and Hansen, 1985) is representative of northeast Canada. The development of shrub-tundra at 10,200 B.P. indicates a warming trend, but in contrast to the northwestern Arctic, the early Holocene was cooler than the middle and late Holocene. The Abies peak from 6800-5700 B.P. may represent the warmest part of the Holocene based on the greater abundance of Abies farther south. Widespread paludification after 5700 B.P. favored Picea mariana, which almost completely replaced P. glauca by 4400 B.P. A cooler, wetter climate may have favored the development of peat, but pollen-influx data suggest that the warmest time was at ca. 4400 B.P., with cooling after 2700 B.P. (Lamb, 1980, 1984).

Sites in northern Quebec were not deglaciated until 7400-6400 B.P. (Richard, 1979; Richard et al., 1982; Gajewski et al., 1993). A short-lived herb zone, possibly representing successional vegetation, followed ice retreat, but all the important boreal trees and shrubs were present almost immediately. At some sites, for example, Lake GB2 (Gajewski et al., 1993) and Kanaaupscow (Richard, 1979) (Fig. 9), a peak of Populus, which may also have been successional, followed the herb zone. At Lake GB2, a peak of Larix accompanied the Populus peak. Picea, Alnus, and Betula predominated thereafter. At Kanaaupscow, forest-tundra with Picea mariana, Alnus viridis, and Betula glandulosa dominated the rest of the Holocene. Forest density was highest from 68002800 B.P., after which Pinus banksiana became more prominent. At Lake GB2 in the northern boreal forest, Alnus was most abundant until 4200 B.P., after which Pinea dominated. In forest-tundra sites, the tundra component increased after 3000 B.P., indicating progressive cooling. At Delorme 2 (Fig. 9), dense Picea mariana forest existed from 6300-5000 B.P., with forest opening after 5000 B.P. (Richard et al., 1982). Betula pa-pyrifera was more abundant from 6600-5500 B.P. The greater prevalence of Betula in the boreal forest in the middle Holocene is evident in sites throughout the eastern boreal forest and may indicate a greater incidence of fire owing to a warmer, drier climate (Jacob-son et al., 1987).

Although sites in the northern boreal forest and forest-tundra show evidence of decreasing cover and expansion of tundra in the late Holocene from 5000-3000 B.P., no evidence exists that Picea ever occurred north of its present limit or that any latitudinal retreat of the forest limit occurred (Gajewski et al., 1993; Gajewski and Garralla, 1992; Lamb, 1985). However, in the mountainous terrain of northern Labrador, tree line fell ca. 40 m between 3200 and 1000 B.P. and another 30 m from 1000-250 B.P., although a latitudinal shift did not occur.

Whereas much of the continent was warmest in the early Holocene, including Alaska and northwest Canada, northeastern Canada was still ice covered. The warmest temperatures followed deglaciation from ca. 6000-3000 B.P., and progressive cooling has occurred since 3000 B.P. or earlier. Considerable vegetation change is associated with extensive paludification, which a cooler, wetter climate may have initiated.

19.2.10. Southeast

With the exception of Florida, sites with good Holocene records are few in the southeastern United States, and the vegetation history is not as well known as those in the Northeast and Midwest. Most sites are small sinkholes, and the Holocene sedimentary record is often confined to the top 1 or 2 m. At many sites, a significant local swamp or wetland component consisting of taxa such as Nyssa, Cephalanthus, and Alnus obscures the upland signal. The Holocene sediments in many of these sinkholes are low in organic matter, and radiocarbon dates of bulk sediment are suspect. Florida, however, has a good Holocene record. The lakes are sinkholes, but are much larger and deeper than elsewhere in the Southeast. Many of these lakes were dry in the late glacial period and did not fill with water until sea level rose in the early Holocene.

Lake Tulane (Grimm et al., 1993) (Fig. 10), Lake An-

Lake GB2

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