FIGURE 4 Representative pollen diagrams from the Pacific Northwest and the Sierra Nevada: Kirk Lake (Cwynar, 1987), Battle Ground Lake (Barnosky, 1985), and Balsam Meadow (Davis et al., 1985).
occurred north of its present range in northeast Vancouver Island (Hebda, 1983). Fossil wood above modern timberline dated to 10,200-9200 B.P. in the southeastern Coast Mountains of British Columbia show that timberline was at least 60 m higher and perhaps as much as 130 m higher in the early Holocene than it is today (Clague and Mathewes, 1989).
Between 8000 and 5000 B.P., Thuja plicata and Tsuga heterophylla expanded, indicating cooler, moister conditions; and lower charcoal indicates a reduction in fire frequency (Cwynar, 1987; Long et al., 1998). The Thuja-Tsuga expansion began at ca. 7400 B.P. at northern sites (Pinecrest Lake, Marion Lake, and Kirk Lake) and ca. 1000 years later at southern sites (Davis Lake and Little Lake). At Battle Ground Lake, which is in a somewhat drier area, Thuja-Tsuga forest did not develop until ca. 5000 B.P. In the Olympic Peninsula, Picea sitchensis and Tsuga heterophylla forest developed after ca. 6800 B.P., and Thuja plicata expanded after 3200 B.P. (Heusser, 1974, 1977). All of the sites from Pinecrest Lake to Little
Lake show an increase in Pseudotsuga and Alnus sometime after 2000 B.P., indicating a return to somewhat drier conditions and increased fire frequency (Worona and Whitlock, 1995).
Sites in the Sierra Nevada range in California also show evidence of early Holocene aridity. At high-altitude sites in the central Sierra Nevada, an open Pinus contorta forest prevailed. After ca. 6800 B.P., populations of Tsuga mertensiana and Abies magnifica increased, and after 2500 B.P., these two species retreated downslope (Anderson, 1990). At lower elevations, Artemisia dominated from 13,000-7800 B.P., and conditions were much drier than they are today. Pinus forest prevailed from 7800-3200 B.P., and Abies increased after 3200 B.P., indicating a cooler and wetter climate (Davis et al., 1985). The pollen diagram from Balsam Meadow (Anderson et al., 1985; Davis et al., 1985) illustrates these trends (Fig. 4). Thus, the climate trends in the Sierra Nevada parallel those reconstructed from pollen records farther north, with the warmest and driest conditions during the early Holocene.
The modern vegetation of the northern Rocky Mountains is conifer forest with grassland or steppe in the intermontane valleys. The vegetation history varies geographically and altitudinally. Unfortunately, many of these sites are not in the pollen database.
Similar to the Pacific Northwest, the mountainous regions in northeastern Washington, northern Idaho, and northwestern Montana were warmer and drier in the early Holocene than they are today. Three sites in northeastern Washington, which are now in Pinus-Pseudotsuga/Larix forest, were in either Artemisia steppe or open Pinus savanna. This drier, warmer period was 11,500-5600 B.P. at Bonaparte Meadows (Mack et al., 1979) and Waits Lake (Mack et al., 1978d). The mire at Simpsons Flats (Mack et al., 1978a) was dry from 10,200-7600 B.P., after which open Pinus savanna occurred until 4400 B.P. Similar to sites in more mesic areas to the east, Simpsons Flats shows evidence of a cooler period from 4400-2800 B.P., when Abies and Picea were more abundant. Sites that are now in more mesic Tsuga heterophylla forest were open Pinus forest or savanna in the early Holocene. This period of more open vegetation was 11,200-3500 B.P. at Big Meadow (Mack et al., 1978b), 9300-3200 B.P. at Hager Pond (Mack et al., 1978c), and 11,500-4400 B.P. at Tepee Lake (Mack et al., 1983). At all three of these sites, a period cooler and moister than today followed with increased Picea and Abies. Tsuga heterophylla increased in the late Holocene: 2800 B.P. at Tepee Lake, 2400 B.P. at Big Meadow, and 1400 B.P. at Hager Pond.
At high-altitude sites in southwestern Montana and in the Yellowstone region, Pseudotsuga was present at higher altitudes in the early to middle Holocene than it is today. At Telegraph Creek (Fig. 5), Pseudotsuga was present from 11,500-4400 B.P., indicating early Holo-cene warming and cooling after 4400 B.P. (Brant, 1980). At Lost Trail Pass Bog (Mehringer et al., 1977), Pseudotsuga was present from 7800-4400 B.P., and at Divide Lake (Whitlock and Bartlein, 1993) (Fig. 5) it was present from 10,200-5700 B.P. Thus, the signal is somewhat mixed. Telegraph Creek and Divide Lake suggest early to middle Holocene warmth; whereas at Lost Trail Pass Bog, the warmer period is confined to the middle Holocene, more similar to the Great Plains to the east. At Toboggan Lake (Fig. 5) on the east slope of the Rockies in Alberta, Canada, Picea was less abundant in the middle Holocene from 8100-7300 B.P. (MacDonald, 1989), similar to the Great Plains records of middle Holocene warmth. On the other hand, the record from Rapid Lake (Fall et al., 1995), a high-altitude lake at tree line in the Wind River Range south of the Yellowstone region, indicates early Holocene warmth. Tree line was 100-150 m higher than it is today from 12,600-3200 B.P., and the alpine trees Pinus albicaulis, Picea, and Abies were more abundant than they are today. By 12,600 B.P., climate was warmer than it is today, and warmer conditions persisted until 3200 B.P. when tree line retreated.
In contrast to the precipitation minimum in the early Holocene at Divide Lake, Whitlock and Bartlein (1993) interpret increased Pseudotsuga in the late Holocene at Slough Creek Pond (Fig. 5) in the northern Yellowstone region to indicate a late Holocene precipitation minimum. They relate the temporal variability in the timing of maximum Holocene drought to the spatial variability in precipitation. Sites having an early Holocene warm/dry maximum are in areas having maximum precipitation in winter, whereas sites with a later Holo-cene warm/dry maximum are in areas having maximum precipitation in summer. Over a larger area, increased spatial and temporal variability in the timing of maximum drought may be owing to the precipitation source—either the Gulf of México or the Gulf of California. Based on patterns in the Midwest and Southwest, the Gulf of México source was minimal during the early and middle Holocene and more important during the late Holocene, whereas the Gulf of California source was the reverse. The possibility of both sources further complicates the pattern. A greater spatial array of multiproxy climate records from various elevations will be necessary to better understand the complicated spatial and temporal patterns of climate history in the northern Rocky Mountains of the United States.
In summary, most sites in the northern Rocky Mountains were warmer in the early Holocene than they are now, and these conditions persisted until the late Holocene (see also Chapter 21). Many sites show evidence of cooling sometime after 4400 B.P., and some sites were cooler after this time then they are today, with the modern climate and vegetation regime becoming established sometime after 2800 B.P. At a few sites, the palynological evidence suggests that warmer conditions were delayed until the middle Holocene, more similar to the pattern on the Great Plains.
19.2.5. Southern Rocky Mountains and Southwest Deserts
In the southern Rocky Mountains and Southwest deserts today, conifer forests dominate at higher altitudes in the mountains, whereas desert scrub occurs at low altitudes. Maximum precipitation is in July and August, in contrast to May and June in the northern Rocky Mountains (Thompson et al., 1993). Factors complicating proxy climate data are altitudinal variability and the importance of summer versus winter precipitation. In general, the southern Rockies and Southwest were wetter and warmer in the early Holo-cene than they are today and became drier and cooler after 6500 B.P.
A number of studies have focused on tree line as a climate indicator. The upper tree line is particularly sensitive to temperature, whereas the lower tree line is responsive primarily to precipitation (Fall, 1997b). In the San Juan Mountains of southwest Colorado, evidence from pollen, fossil insects, and fossil wood at Lake Emma indicates that tree line was 80-140 m higher than modern levels in the early Holocene from ca. 11,000-4000 B.P. (Carrara et al., 1991; Elias et al., 1991). Isotopic analysis of fossil wood also indicates increased summer precipitation (Friedman et al., 1988). At Como Lake (Fig. 6), which lies 100 m below tree line in the Sangre de Cristo Mountains of southern Colorado, the upper tree line was highest in the earliest Holocene, 11,500-10,200 B.P., as much as 200 m higher than it is today (Jodry et al., 1989; Shafer, 1989).
In the Crested Butte area of central Colorado, the upper tree line was higher and the lower tree line was lower from 11,500 to 4400 B.P., also indicating a warmer and wetter climate during the early to middle Holocene than today (Markgraf and Scott, 1981). Fall (1997b) studied a number of sites at various altitudes in the Colorado Front Range. From 10,200-4400 B.P., the upper timberline was as much as 270 m higher than it is today, whereas the lower tree line descended, indicating a warmer and wetter climate than today. At Keystone Ironbog, fire-sensitive Picea-Abies forest predominated from 9000-2800 B.P.; climate then became drier, fires became more frequent, and Pinus contorta forest has prevailed since (Fall, 1997a).
In contrast to these reconstructions of warmer climate in the early Holocene, Thompson (1990) argued for a cooler climate in the Great Basin from 11,300-7800 B.P., when upper montane plants occurred 400-500 m below their modern limits. However, if moisture controls these lower limits instead of temperature, the climate signal may be of increased precipitation rather than decreased temperature, an interpretation more in agreement with the Southwest. During the middle Holocene, 7800-4400 B.P., the upper tree line was 100 m higher than it is today, indicating warmer temperatures.
Packrat midden data from the deserts indicate that the early Holocene was wetter than today, although the details vary. Van Devender (1990a,b) argued for a wetter climate from 13,000-4400 B.P. in the Sonoran and Chihuahuan Deserts. In the Grand Canyon, Cole (1990) interpreted the data to indicate a warm, wet climate from 13,000-9500 B.P., becoming drier after 9500 B.P. Packrat midden and pollen data from the Sierra Bacha in Sonora, México, indicate greater precipitation in the early Holocene from 11,300-10,400 B.P. (Anderson and Van Devender, 1995). Agap in the record exists between 10,400 B.P. and 6100 B.P., by which time essentially modern conditions existed. The pollen data from Mon-tezuma Well (Fig. 6) on the northern edge of the Sonoran Desert indicate Juniperus woodland from 13,0009400 B.P., with a long-term decline in precipitation and the development of desert scrub from 9400-5500 B.P. (Davis and Shafer, 1992).
Water levels in desert lakes were higher in the early to middle Holocene. Lake Cochise in southeastern Arizona had deep phases between 10,000 and 3500 B.P. (Waters, 1989). Apermanent lake existed on the San Augustin Plains in western New Mexico in the early Holocene from 11,300-5700 B.P. After 5700 B.P., the modern playa and salt flats developed (Markgraf et al., 1984). However, shallow, high-altitude lakes on the Mogollon Rim in northern Arizona and in the Chuska Mountains in northern New Mexico were frequently dry during the early and middle Holocene (Anderson, 1993; Jacobs, 1983, 1985; Wright et al., 1973), resulting in truncated Holocene records. Potato Lake on the Mogollon Rim was dry from 11,200-3000 B.P., and lake levels rose after 3000 B.P. (Anderson, 1993). Thus, these lake-level records conflict with the evidence from Colorado and the low-altitude deserts for a wetter early to middle Holocene. Seasonality of precipitation may be the explanation, inasmuch as the vegetation and water levels of desert lakes respond primarily to summer monsoonal precipitation, whereas water levels in the
high-altitude lakes of northern Arizona and New Mexico are dependent on winter snow for replenishment. If this interpretation is correct, then the data indicate an early Holocene maximum in summer monsoonal precipitation, but with decreased winter precipitation. In the late Holocene, summer precipitation was less, but winter precipitation, especially at high altitudes, was greater.
The Midwest and northern Great Plains region, extending from the Dakotas to Ohio, includes the northern Great Plains, the western Great Lakes region, and the Prairie Peninsula. The vegetation varies from mixed-grass prairie in the northern Great Plains to tall-grass prairie in the Prairie Peninsula, a wedge of grassland projecting eastward from the Great Plains into the Midwest from western Iowa to Indiana. Before European settlement, deciduous forest covered most of Indiana and Ohio, although islands of prairie extended into Ohio. Bordering the northern side of the Prairie Peninsula were fire-tolerant oak savannas and woodlands. More fire-sensitive deciduous forest (Big Woods) dominated by Ulmus americana, Acer saccharum, and Tilia americana, together with Quercus, occurred behind effective firebreaks. Northwards, these forests graded into northern hardwood-pine forests with Betula, Pop-ulus, and Pinus in addition to mesic hardwoods. Picea, Abies, Larix, and Thuja are important in northern Minnesota, Wisconsin, and Michigan where the Great Lakes forest grades into the boreal forest.
Picea forest covered the area at the end of the Pleistocene and declined rapidly ca. 11,500 years ago. The
Representative pollen diagrams from the Midwest and northern Great Plains: Moon Lake (Laird et al., 1996), Myrtle Lake (Janssen, 1968), Billys Lake (Jacobson and Grimm, 1986), Wolsfeld Lake (Grimm, 1983); Devils Lake (Baker et al., 1992), and Pretty Lake (Williams, 1974).
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