FIGURE 14 Variations in the ratio of forest to open-ground beetle taxa and species richness of fossil beetle assemblages in southern Chile since the last glaciation.
the Northern Hemisphere because of both the greater tilt of the Earth's axis then and the occurrence of perihelion in the Northern Hemisphere summer (Kutzbach et al. 1993). In the Pacific Northwest, insolation was 8.5% greater than it is today in summer and 10% less in winter in the early Holocene, with greatest seasonality occurring between 11 and 10 cal ka. In the Southern Hemisphere, the amplitude of the seasonal cycle and summer insolation were less than they are now. In southern midlatitudes, early Holocene insolation was about 6% less in summer and 4% greater in winter than it is today. These variations likely had a direct effect on temperature and effective precipitation and an indirect effect in terms of changes in the strength of the subtropical high and monsoonal circulation.
The CCM1 simulation for 11 cal ka suggests a hierarchy of responses to these controls:
• In the Northern Hemisphere, as well as globally, the impact of the ice sheet on temperature was much diminished relative to the LGM (Fig. 15 [see color insert]).
• The large impact of the Laurentide ice sheet on circulation was also greatly diminished, the anomalies of upper-level wind speeds were reduced in magnitude, and the areas of the largest anomalies were reduced in size (see also Fig. 6).
• The greater-than-present insolation in the northern summer (Figs. 3 and 4) led to warmer-than-present temperatures over the Northern Hemisphere continents, which increased the land-ocean temperature contrast in the Northern Hemisphere, producing lower pressure over the continents, higher pressure over the oceans, and, consequently, greater onshore flow into summer-wet regions.
• In both hemispheres, owing to the mechanisms described earlier, growing degree days increased sharply relative to those earlier (Fig. 11), and while they remained lower than they are now in the Southern Hemisphere, most of the LGM-to-present increase in both hemispheres occurred between the 14 and 11 cal ka simulations.
In individual regions,
• Greater summer insolation also strengthened the east Pacific subtropical high-pressure system (Fig. 15), which further intensified summer drought (Thompson et al., 1993; Heusser et al., 1985) and produced lower-than-present AE/PE ratios (Fig. 11).
• In subtropical and tropical South America, there is again a small indication of weaker summer monsoons (slightly higher-than-present SLP over the continent in January accompanied by lower-than-present SLP over the oceans, Fig. 15), with drier-than present conditions resulting (Fig. 15).
The number of paleoecologic records from the early Holocene is quite extensive compared to the LGM. The effects of higher-than-present summer insolation are well registered along the northwestern coast of North
America by a shift toward more xerothermic communities (Fig. 16) (Mock and Brunelle-Daines, 1999). The vegetational changes are a response to the direct effects of greater summer insolation—namely, increased temperatures and decreased effective moisture—as well as to the indirect effects of a strengthening of the northeast Pacific subtropical high. In central and coastal Alaska, greater summer insolation is registered by warmer-than-present temperatures with little change in precipitation (Hansen and Engstrom, 1996; Peteet, 1986; Heusser, 1985). Tundra was replaced by alder thickets in south-central Alaska (Peteet, 1986) and by Pinus contorta, T. mertensiana, and Alnus in southwestern Alaska (Hansen and Engstrom, 1996) by 10.5 cal ka. Forests of Picea sitchensis, Alnus, and T. heterophylla developed in southwestern Alaska and the Queen Charlotte Islands between 10.5 and 7.8 cal ka, and the tree line lay above its present elevation (Pellatt and Mathewes, 1997). Pseudotsuga, A. rubra, and Pteridium were widespread in the seasonal rain forests of southern British Columbia, Washington, and Oregon (Hebda, 1995; Cwynar, 1987; Worona and Whitlock, 1995; Heusser, 1985), and Quercus garryana, an indicator of xeric conditions, shifted its range both northward and to higher elevations (Barnosky, 1985; Sea and Whitlock, 1995). Sites where charcoal analysis has been undertaken suggest that fires were more frequent than they are today (see Long et al., 1998; Cwynar, 1987; Mohr et al., 2000). Similarly, on the east side of the Cascades, the forest-steppe border shifted northward by 100 km and to higher elevations (Mack et al., 1978; Whitlock and Bartlein, 1997). In the northern Rocky Mountains, grasslands were more widespread at low elevations, and Pinus contorta was a forest dominant in response to dry conditions and more fires (Whitlock, 1993; Millspaugh et al., 2000). The tree line lay above its present elevation in both the Coast Range and the northern Rockies (Kearney and Luck-man, 1983; Clague and Mathewes, 1989).
In the American Southwest, greater-than-present summer insolation apparently increased the onshore flow of moisture from the Gulf of California and Gulf of México, increasing the strength of summer monsoons. The abundance of succulents, which depend on summer precipitation, has been attributed to increased precipitation in the Mojave Desert at this time (Spauld-ing and Graumlich, 1986), and pollen records from the Colorado Plateau indicate a rise in the upper tree line in the early Holocene that is attributed to increased summer precipitation (Weng and Jackson, 1999). At the subcontinental scale, the trade-off in precipitation regimes is evident between the Pacific Northwest affected by the stronger-than-present subtropical high-pressure system and the American Southwest with stronger monsoons. The pattern of these two precipitation regimes becomes spatially more complex, however, at smaller spatial scales in the Rocky Mountain region, where topography strongly influences air mass patterns. Records from the Yellowstone region indicate that stronger monsoonal circulation in the summer-wet regions led to wetter conditions in the early Holocene, while an expansion of the subtropical high in the adjacent regions caused drier conditions there (Whitlock and Bartlein, 1993; Whitlock et al., 1995). The boundary between summer-wet and summer-dry areas did not shift from its present location, however, because it was and is constrained by topography.
Across most of the Pacific Northwest and into coastal Alaska, the onset of increased moisture preceded the cooling, leading to the definition of a middle Holocene warm, moist interval (the mesothermic period) between 7.7 and 5 cal ka (Hebda, 1995). This period probably represents the changing importance of the direct and indirect effects of greater-than-present insolation through the Holocene. Summer insolation was greater than present levels in the middle Holocene, but lower than that of the early Holocene; this intermediate condition would have maintained high summer temperatures (the direct effects), but perhaps weakened the east Pacific subtropical high. This meso-thermal period is characterized by the northward expansion of Picea and Alnus forest along coastal Alaska (Peteet, 1986) and the inception of peatland develop ment in many regions. Subalpine parkland expanded at high elevations as the upper tree line retreated downslope. In southern British Columbia and northern Washington, Thuja plicata, Tsuga heterophylla, and Picea replaced Pseudotsuga and Alnus, and fires became less frequent (Hebda and Whitlock, 1997). Sequoia forests were established in the warm, temperate rain forest region of northern California, and areas of grassland and oak woodland shrank on both sides of the Cascade Range. Modern forest associations and the position of the forest-steppe border were established in the last 3000-5000 years as summer insolation approached present values and the climate became cool and humid. In the interior region of British Columbia, Pseudotsuga, Betula, and T. heterophylla extended their range at the expense of grassland and pine forest. In the northern Rocky Mountains, mesophytic taxa, such as Picea, Abies, and Pinus albicaulis, became major constituents of high- and middle-elevation forests after 4.5 cal ka, and P. contorta was confined to xeric and disturbed habitats (Thompson et al., 1993).
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