| OCR Text |
Show downward translocation are centered at - 12- 13 ka, 14- 16 ka, 21- 26.5 ka and - 32- 35 ka. The wettest periods, based upon the exclusion of limber pine, occurred between 21- 23 ka and 33- 35 ka. These appear to coincide with the onset and demise of the late glacial maximum and an earlier cold/ wet period. Such episodes suggest that Pacific air masses played a greater role in the climates of southern Nevada at that time. Limber pine appears in the record from - 11- 13 ka, 14- 21 ka, 23- 26.5 ka and 30- 33 ka. Periods of cold, dry continental climates ( continental polar and arctic air mass predominance) occur from 16- 21 ka and 29- 32 ka based on the absence of white fir in communities with abundant limber pine. This is corroborated by the pollen record from Tule Springs which records the regional dominance of pine in the late Pleistocene and at least two expansions of fir. Modern analogues of mean annual precipitation ( MAP) requirements of key plant species suggest that Pleistocene changes in plant species distribution reflect rapid shifts toward century- to millennia- long wet- winter precipitation patterns, and suggest a Pleistocene MAP of at least 380 to 635 mm. This is an increase of roughly 50 to - 90% at elevations between 1,500 and 2,000 m. The difference between modern xeric shrub dominance and dominance of Utah juniper during the Pleistocene at elevations from 1,500 to - 700 m indicates precipitation increases of approximately 33% to 66%. Although punctuated by more mesic periods, most of the Pleistocene between 35 and 12 ka was dominated by more continental conditions characterized by cold- dry climate about 33% wetter than today. These increases suggest that precipitation gradients increased significantly during the Pleistocene due to dramatically enhanced orographic precipitation effect. Elevations above 2,500 m may have been relatively dry because much of the precipitation had been winnowed out. Pollen from Tule Springs and woodrat midden records indicate that the Pleistocene/ early Holocene vegetation community transition occurred - 12 ka. Although white fir and limber pine retreated to higher elevations by 11.7 ka., Utah juniper lingered at lower elevations until about 9.5 ka. Pifion which had been restricted below 1,500 m south and east of the Spring Range during much of the Pleistocene expanded upward in elevation and northward into the central Great Basin where it reached its approximate current limits around 5.5 ka. However, it remained less abundant than juniper until the late Holocene. Middle Holocene xericity is evidenced by expansion of desert shrub vegetation and the appearance in the south of summer- rainfall- loving species such as creosote bush and white bursage. This trend was terminated ca. 5.5 ka by a sudden increase in precipitation evidenced in the dramatic appearance of aquatic peats in the previously dry playa/ spring records of Lower Pahranagat Lake, Ash Meadows and Little Lake. This event can also be seen in the drowning of forests in Lake Tahoe and the shift from shadscale to sagebrush shrub communities in southern Oregon and eastern Washington, and heralds a trend toward mesic conditions that climaxed during the Neoglacial. Pollen and woodrat midden records indicate several range changes and shifts in dominance of major woodland taxa in response to climatic trends during the Late Holocene. Juniper- Pinon dominated semi- arid woodland reached its late Holocene maximum areal extent during the Neoglacial between 2 to 4 ka, consistent with evidence from further north. The extent of semi- arid woodland contracted after the Neoglacial about 1.9 ka. Desert shrub community expansion coupled with increased precariousness of wetland areas in the southern Great Basin between 1.9 and 1.5 ka coincides with shrinking wetlands in the west- central and northern Great Basin. A shift in dominance from juniper to pifion in southern Nevada semi- arid woodlands at ca. 1.3 ka signaled the end of post- Neoglacial drought conditions in central and southern Nevada. Vegetation suffered from renewed severe droughts between .5 to .6 ka. Effectively wetter conditions during the Little Ice Age resulted in re- expansion first of juniper and then of pifion in the semi- arid woodland ca. .35 ka. |
