OCR Text |
Show Northeastern Great Basin. Paleovegetation records dating to the pre- Bonneville lake cycle ( early and middle Wisconsin, > 26 ka) are scarce, but suggest elevational and latitudinal variation in vegetation. Upland areas were dominated by woodlands consisting mainly of bristlecone pine, limber pine, spruce, and Utah juniper. Bristlecone pine and Utah juniper were present south of about 40 degrees north latitude; north of this mark, limber pine dominated, bristlecone pine dropped out, and Utah juniper occurs only in the earliest samples (> 50 ka). Sagebrush steppe covered foothills and upper valley margins, and some valley floors may have been covered by greasewood or shadscale scrub. Temporal fluctuations are indicated but not detailed in the available record. The pre- full glacial ( 26- 20 ka) vegetation record south of 40 degrees latitude appears relatively similar to the previous period; north of 40 degrees, spruce increased and limber pine was reduced in abundance. Full- glacial ( 20- 17 ka) vegetation indicates further significant cooling, with increases in spruce, fir, subalpine pines, and montane shrubs in the southern part; to the north a nearly treeless steppe tundra was established at relatively low elevations in places on the western margin of the Bonneville Basin. This period coincides with the maximal extent of glaciation in the Wasatch Range and other ranges in the eastern Great Basin. By the time of the maximum extent of Lake Bonneville ( ca. 16- 15 ka) the steppe- tundra in the north had given way to increased spruce and subalpine/ montane shrubs. This period is poorly represented in the southern part of the eastern Basin, but midden samples bracketing the period show similar abundances of subalpine conifers and shrubs. Terminal Wisconsin records ( 14- 10 ka) show dramatic changes in vegetation around the Bonneville Basin. To the southwest, subalpine pines grew with woodland and montane shrubs on the lower elevations of mountain ranges. Sagebrush- grass steppe similar to that of upper elevations today dominated valley margins. To the north, a similar sagebrush grass- steppe combined with montane shrubs colonized lands exposed by the dessication of Lake Bonneville. Beginning about 13 ka and lasting to at least 11 ka, a limber pine forest or forest/ steppe mosaic spread widely over much of the lower elevations around the northwestern Bonneville Basin; limber pine and spruce forests appear to have occurred at higher altitudes. Modem ecological tolerances of limber pine put the terrestrial vegetation record at odds with reconstructions of a dramatic decline of Lake Bonneville to levels at or below that of the present Great Salt Lake ( Rhode and Madsen 1995). By 12 ka, the area exposed by Lake Bonneville began to be colonized by chenopods, with various tolerances to saline soils. It is during this time or slightly later that Atriplex may have undergone an explosive adaptive radiation. Evidence for vegetation changes during the Gilbert lake cycle ( 11- 10 ka) are presently rare in the northern part of the region, while to the south vegetation appears to be stable from ca. 13- 10 ka. During the early Holocene ( post- Bonneville cycle, 10- 7 ka), desert scrub vegetation including shadscale, greasewood, and horsebrush dominated valley bottom settings around the Bonneville Basin. Upper valley margins contained vegetation similar to modem- day upper sagebrush- grass vegetation; in the mountains a mosaic of limber pine and montane shrub communities dominated. Rocky Mountain juniper was apparently common, and Utah juniper was present below about 40 degrees latitude. Singleleaf pinon moved northward along the western margin of the Bonneville Basin to at least 41 degrees latitude by ca. 7 ka, a migration more rapid than elsewhere in the Basin. Modem day vegetation associations were largely in place in the central eastern Basin by the middle Holocene ( ca. 7 ka), but further north important taxa such as Utah juniper and ephedra may not have arrived until after ca. 4 ka. Evidence exists for slight elevational range shifts and relative abundance of certain taxa in response to middle and late Holocene climatic fluctuations, apparently coincident with Holocene lake histories in the Bonneville Basin, Ruby Valley, and elsewhere in the Great Basin. |