Washoe Lake south of Reno, Nevada

THE INTERMOUNTAIN WEST:

35,000 Years of Paleoecology and Paleoclimate:

Investigations into the ecological history of the Intermountain West of North America.

or

What fossil pollen and ancient woodrat nests tell us about past vegetation history and climate.

INTERMOUNTAIN WEST

Tule Peak area northeast of Reno, Nevada.

LATE QUATERNARY PALEOECOLOGY AND PALEOCLIMATOLOGY

The Great Basin is a land of high valleys and north/south trending mountain ranges that lies between the Rocky Mountains on the east and the Sierra Nevada Mountains on the west. Most of the region lies just below or just above a mile high. In just a few miles one can travel from valley floors around 4,500 feet to mountain tops over 11,000 feet and higher. This diversity of topography has given rise to a mosaic of habitats that provide both home and refuge to a great variety of plant species. The rain-shadow created by the Sierra Nevada is felt throughout the area except on the tops of the highest mountain ranges. Influenced by Pacific storms in the winter and spring, monsoonal rains in the middle to late summer, and occasional cold, Arctic outbreaks in the depths of winter a climate of great extremes has come to characterize the region. This harsh climate has bred a very hardy vegetation type that can survive both its hot, dry summers and its cold, dry winters. Great Basin vegetation runs the range from valley floor salt desert shrub communities to subalpine and alpine grasslands. Between these two extremes lie semi-arid and subalpine, and occasionally, montañe woodlands all with a diverse and often beautiful understory filled with wild flowers. In the wetter regions the shrub steppes are grassy, in the drier areas they are not. Marshes often fill the basins below the tallest and wettest of the Basin mountain ranges. At low elevations native grasses and other plants have succumbed to invading cheat grass and other Eurasian plant species that follow in the wake of cattle. At higher elevations native plants still stave-off the invader.

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LATE QUATERNARY PALEOECOLOGY AND PALEOCLIMATOLOGY

By studying plant remains from ancient packrat middens; pollen and charcoal accumulations from lake, marsh and playa sediment cores, and from cave deposits and archaeological sites, and the isotopic content of these plant macrofossils, Peter Wigand and his colleagues and students have been shedding vital, new light on the problem of biotic response to climate change during the late Quaternary (~ the last 250,000 years of earth history). Paleoenvironmental studies of the forests of the central Sierra Nevada during the last ten years have been directed toward: 1) reconstruction of forest history to derive understandings of the relationships between climate and vegetation response - in particular, frequencies, rates and magnitudes of climate change and resulting forest response have been investigated; and 2) the relationship of climate, vegetation change and fire regime dynamics has been investigated.

To clarify these relationships a series of sites on the east slope of the Cascade/Sierra mountain chains have been cored for pollen ranging from the Summer Lake area in south-central Oregon through Little Valley just east of the Lake Tahoe Basin. Records spanning about 230,000 years (Summer Lake), 45,000 years (Eagle Lake), 9,000 years (Bicycle Pond) and 6,300 years (Little Valley) have been collected, analyzed, and are now in the process of being reported. Additional records within the area include a 2,300-year pollen record from the Stillwater Marshes that can be compared with the Little Valley record. In addition, almost 200 dated woodrat midden strata from east of the Sierra Nevada crest east, north and south of the Lake Tahoe area are being used as additional proxies to confirm regional climate changes against which forest response and changing fire regimes can be measured. These can be compared and contrasted with a detailed pollen record from Diamond Pond in the Steens Mountain area of south-central Oregon and from Lower Pahranagat Lake at the northern edge of the Mojave Desert in southeastern Nevada.

These understandings of how ecosystems work will help planners, managers and modelers administer today's resources and plan for the future especially as global change becomes more of an issue. 

Below are brief descriptions of the research, and of the preliminary conclusions that have been generated. 

AT THE CURRENT RATE OF GLOBAL WARMING,DESTRUCTION OF THE MAJOR PORTION OF INTERMOUNTAIN WEST WOODLANDS CAN BE EXPECTED WITHIN THE NEXT 100 TO 150 YEARS.

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MIDDLE THROUGH LATE HOLOCENE CLIMATES OF THE GREAT BASIN

There is enough evidence accumulating from pollen and woodrat midden records in the Great Basin for regional synthesis with the high-resolution pollen records from Lower Pahranagat Lake and Diamond Pond, south-central Oregon respectively providing the southern and northern lynch-pins for comparisons and contrasts. Pollen records from Lead Lake and Little Valley provide intervening details. Drought indexes generated at each of these sites indicate some similarity though the differences are more striking. It is clear that the "long drought" that characterized the region met a sudden end throughout the region with a dramatic, though brief, episode of wet climate between 5,600 and 5,400 years ago. This event renewed spring discharge in the northern and the southern Great Basin, and drowned forests at Lake Tahoe. Later it appears that the northern Great Basin (Diamond Pond record) was more strongly effected by the cooling and greater effective moisture that characterized the "Neoglacial" period between 2,000 and 4,000 years ago. In the southern Great Basin and the northern Mojave Desert it is only the more pronounced episodes during this period that caused a response in the vegetation. On the other hand, the cooler, moister climate of the "Little Ice Age" episode between 350 and 175 years ago seems to have affected the entire Great Basin and northern Mojave Desert equally.

Another period which was characterized by increased rainfall occurred between ~1,700 and 900 years ago. However, unlike other episodes this period was characterized by increased late spring to summer rainfall not increased winter rainfall. This episode is reflected from pollen records from Diamond Pond in southern Oregon, Summit Lake in northern Nevada, Lead Lake in the Carson Sink of western Nevada, and in Lower Pahranagat Lake in the northern Mojave Desert of southern Nevada. This period of climate, as has been suggested earlier with the Lead Lake record, has had significant impacts throughout the Great Basin. In the northern Great Basin it is associated with the increase in grasses in the sagebrush steppe and the expansion of bison and the people that hunted them (dots are radiocarbon dates on bison from Native American sites in the northern Great Basin and Plateau of eastern Washington). In the west it appears to coincide with the final appearance of piñon pine and a significant change in Native economic and settlement patterns. In the east it allowed expansion of the Fremont Culture and it summer-rainfall dependent maize horticulture. In the south greater spring and summer rainfall resulted in the replacement of sedge and cat-tail marshes by emergent and floating aquatic plant dominated shallow ponds. It was also characterized by the in-filling of juniper woodlands with piñon. It is during this period that piñon pine finally became a co-dominant in the Piñon-Juniper woodland.

The global nature of this event is seen Nile River flow levels recorded at the Roda Nilometer in Cairo. Correspondence between wet/dry events seen in the Lower Pahranagat Lake drought index can also be seen in the low annual Nile River levels which reflect the White Nile contribution to downstream flow of the Nile. The Blue Nile contribution shows only general similarity to the Lower Paharanagat River drought index. There is also correspondence between the Lower Pahranagat Lake drought index and the record of both sediments and olive pollen in a core obtained from the Dead Sea. Higher organic content in Dead Sea sediments between 2,200 and 1,100 radiocarbon years ago combined with high olive (Olea) and grape (Vitis) pollen counts may reflect increased summer precipitation. Summer precipitation would freshening the Dead Sea when salts would normally be precipitating in the lake. Although a general expansion of olive groves near the Dead Sea during the Roman and early Islamic period is probably a cultural phenomena, the great similarity between the timing of increases and decreases in olive pollen in the Dead Sea record and the cycles of wet and dry in the Lower Pahranagat Lake index indicates that there is a strong climatic signal as well.

LATE PLEISTOCENE RESEARCH IN THE INTERMOUNTAIN WEST

Anna River at the northern end of the Summer Lake Basin, south-central Oregon, USA.

SUMMER LAKE BASIN, SOUTH-CENTRAL OREGON, U.S.A.

The Summer Lake Basin of south-central Oregon has been a source of scientific interest since the early part of the last century. In the middle 1980s a new episode of research began that still continues. In the middle 1980s the late Dr. Jonathan Davis began investigating the many volcanic ashes in the exposures along the Ana River at the north end of the basin. At his instigation a group of scientists from California State University, Bakersfield (paleomagnetist, Robert Negrini), University of Arizona (ostracode specialist, Andrew Cohen), and the Desert Research Institute (paleoecologists, Lonnie Pippin and Peter Wigand) sampled the Ana River exposures and started a program of coring that followed up on some that had been conducted by Dr. Dave Adam of the U.S.Geological Survey at the southern end of the basin.

In addition, to samples collected along the Ana River which reveal strong correspondences (especially prior to the last interglacial) between local vegetation history and the oceanic O18 record (a reflection of global temperature), cores were taken at three other localities. Two of these, Wetlands Levee and the Bed & Breakfast localities, have now been analyzed and will shortly appear in publication. The one hundred-foot core obtained from Wetlands Levee in the west-central part of Summer Lake, south-central Oregon, covers the last 94,000 years. Its record is providing a unique comparison of the response of lake chemistry, and aquatic and terrestrial vegetation to climates since the last interglacial. Ratios of pollen, spores and algae from the Wetlands Levee core and the Bed and Breakfast core reveal not only periods of colder climate associated with glacial advances, but also episodes of significant drought such as that around 30,000 years ago. The information recovered here mirrors that recovered 250 km to the southeast where much larger lakes existed.

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THE "MOTHER MIDDEN" AT PYRAMID LAKE, NORTHWESTERN NEVADA, U.S.A.

Forty-one radiocarbon-dated strata from a large indurated woodrat midden west of Pyramid Lake in west-central Nevada is providing a detailed 35,000-year record of terrestrial vegetation change and climatic stress in relationship to lake history during the last glacial cycle. Plant macrofossils including those of the trees Utah Juniper (Juniperus osteosperma) and White-bark Pine (Pinus albicaulis) are providing a unique view of the vegetation that covered the slopes of pluvial Lake Lahontan (the large "Ice Age" lake that once covered a large portion of northwestern Nevada). Pollen and abundant twigs, and seeds from the midden layers indicates that around the nest Utah Juniper was quite abundant where today mixed sagebrush steppe and saltbush shrub communities are found. In particular, the plant remains indicate that around 24,000 and 12,000 years ago White-bark Pine actually grew on the shores of pluvial Lake Lahontan. Today White-bark Pine is found over 24 km (14.5 miles) away and at least 1,000 m (3,300 ft) higher in elevation. Today areas where White-bark Pine grow receive around 560 mm (22 inches) of rainfall annually and are about 7°C (12.6°F) cooler.

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VEGETATION AND CLIMATE HISTORY FROM ANCIENT WOODRAT MIDDENS OF THE NORTHERN MOJAVE DESERT

During the 1990s a group of scientists from the Desert Research Institute working with Dr. Wigand have sampled a series of several hundred woodrat midden strata in the southern Great Basin and northern Mojave Desert. This research is both difficult and dangerous. However, the reward has been a much clearer view of vegetation change in southern Nevada during the last 35,000 years. Our current understandings are a cumulative effort building upon the pioneering research in the 1960s and 1970s of Philip V. Wells, Peter J. Mehringer, Jr., Clive Jorgensen, and C. Wes Ferguson, and brought to its current standard of excellence by W. Geoff Spaulding and Robert S. Thompson in the 1980s and 1990s. 

It is clear that subalpine woodland species had migrated down the mountain slopes of southern Nevada by at least 1,000 meters (over 3,600 feet) several times between 35,000 and 12,000 years ago, usually without displacing the ever presentUtah Juniper. Species such as Limber Pine and White Fir that are currently associated with high elevation woodlands in the Great Basin reached elevations well below their current distributions indicating that mean annual temperatures in southern Nevada were at least 7°C (12.6°F) below their current level and that, at times, mean annual rainfall may have exceeded 2.5 times its current amount. Therefore, mean annual rainfall of between 560 to 635mm (22 and 25 in) at elevations of 1,600 m (5,250ft) characterized both the onset (24,000 to 21,000 years ago) and decline (16,000 to 13,000 years ago) of the Glacial Maximum. At elevations below 1,525 m (5,000 ft) conditions were only slightly wetter than today with mean annual rainfall estimates ranging between 130 to 190 % of current values. These estimates are confirmed by recently published  modeling conducted by Pat Bartlein, Bob Thompson and Kathy Anderson.

EARLY TO LATE HOLOCENE

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BICYCLE POND, SOUTH-CENTRAL OREGON, U.S.A.

A pollen record spanning 9,000 years from Bicycle Pond in the Warner Valley of south-central Oregon, reflects changes in local spring discharge during the Holocene, as well as of continuous changes in terrestrial vegetation since before the fall of Mazama Ash. Evidence of Holocene forest expansion and contraction from here can be compared with other pollen records in the area such as that obtained from the 46,000-year core from McCoy Flat on Pine Creek west of Eagle Lake, California. The pollen  record here indicates both early and late Holocene expansions of juniper woodland. Thin volcanic ashes found in deeper probes in Bicycle Pond indicate that a late Pleistocene record may be found here as well.

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LITTLE VALLEY, WEST-CENTRAL NEVADA, U.S.A.

A 6,300-year core from Little Valley, Nevada on the east slope of the Sierra Nevada Mountains east of Lake Tahoe provides evidence of the relationship of climate change, vegetation dynamics and fire history near the lower forest boundary. The recordreveals that forests more characteristic of moister climates began moving down the east slope of the Sierra Nevada Mountains into the Little Valley area around 5,500 years ago. Fir - most likely White Fir, though Red Fir also grows in the area today - characterized this more mesic mixed conifer forest. Increased abundance of willow, birch and alder around Little Valley itself also occurred. These trends reached a climax between 2,000 and 4,000 years ago during the "Neoglacial" Period. Since then, with the exception of the "Little Ice Age" event, conditions have generally been drier in the Little Valley area. Little Valley provide a fire history that is in concert with regional climate. Increased fire frequency reflects both more abundant fuels generated by wetter climates and the droughts the punctuate these periods. About 6,000 years ago as Middle Holocene drought conditions were being alleviated, local forest fire severity seems to have increased significantly, perhaps reflecting greater fuel availability. It decreased slightly about 5,500 years ago during a Great Basin-wide episode of significantly wetter climate. During the major part of the "Neoglacial", after about 3,000 years ago it become relatively insignificant. During the last 600 years it began increasing again.

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LEAD LAKE, CARSON SINK, WEST-CENTRAL NEVADA, U.S.A.

A 2,300-year core from Lead Lake in the Carson Sink east of Reno, Nevada provides evidence of the regional expansion of Single-needled Pine (Pinus monophylla) into the northwestern part of the Great Basin. The pollen record in combination with microfossil remains of piñon pine in woodrat nests documenting the appearance of piñon pine in the area indicates that the piñon-juniper woodland that characterizes much of Nevada occurred within the last 1,600 years in the northwestern corner of the state. This would suggest that the Washoe and Paiute peoples made their major shift to piñon nut collection and use during the last 1,400 years. A sudden increase in the frequency of charcoal layers in the sediments of Horse Creek on the west slope of the Clan Alpine Mountains after about 1,400 years ago seems to reflect a change in the local fire regime. Prior to 1,400 years ago a more open, juniper woodland seems to have characterized the region. With the incursion of piñon, woodlands became more closely packed and the opportunity for flames to hop from one tree crown to the next resulted in more wide-spread and devastating fires in the mountain ranges of northwestern Nevada. Pollen of marsh plants reflects great variability rather than stability over the last 2,000 years. Deep water pond weeds alternated with shallow water sedges and cat-tails. At times cat-tails, reflecting fresher water inputs into the Carson Sink, predominated while sedges became less dominant.

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LOWER PAHRANAGAT LAKE, NORTHERN MOJAVE DESERT, NEVADA, U.S.A.

Sediments in cores from Lower Pahranagat Lake, southern Nevada, contain proxy data to reconstruct a hydrologic, vegetation, and climate history for the past 6,000 years. Pollen, plant remains, and mollusks within the core are used to interpret past environments. Radiocarbon dated plant remains and whole sediment provide a chronologic framework. Close interval pollen samples from rapidly deposited sediments presents a series of snapshots of the local and regional environment approximately every 13 years. The record  reveals decade-long climatic cycles superimposed upon much broader cycles lasting several hundred years.

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The 5,600-year record (3,800 years of which have been analyzed thus far) of pollen, algae, ostracode, and mollusk history from Lower Pahranagat Lake provides a high- resolution (about 14-years between samples) record of middle and late Holocene climate and vegetation change in southern Nevada. The lake is located about 97 km north northeast of Las Vegas, Nevada, just within the northern boundary of the Mojave Desert. It lies on the floor of the White River Valley in the midst of creosotebush (Larrea tridentata) dominated desert shrub vegetation, and is surrounded by mountains dominated by piñon-juniper woodland. Higher elevation mountains to the north have mixed conifer woodlands at their highest elevations. A drought index from Lower Pahranagat Lake indicates that over 75% of the last 4,000 years has been drier than currently. Vegetation response on the order of decades may mirror changes in El Niño cycles. The drought record is correlatable  to the Methuselah Walk bristle-cone pine tree-ring record from the White Mountains on the Nevada/California border. Cool, moist events between 3,800 and 2,600 radiocarbon years ago correspond to neopluvial events seen in records from the northern Great Basin (e.g., Diamond Pond). Their magnitude is clearly less pronounced than those seen in the Diamond Pond record. A strong winter wet event which occurred about 2,000 radiocarbon years ago can be correlated to other records from Carp Lake in southern Washington State to pollen records from the Los Angeles Basin.

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CAVE ROCK, LAKE TAHOE, NEVADA WOODRAT NESTS

Changing forest composition of the forests around Lake Tahoe during the last 1,300 years is hinted at by preliminary studies of prehistoric woodrat middens preserved in the cave of Cave Rock on the east shore of the lake. Collection of one woodrat midden  near the mouth of the cave yielded 13 layers spanning the last 1,300 years. The plant remains are dominated by Incense Cedar and Mountain Mahogany. These suggest that conditions during the last 1,300 years have often been warmer and drier than today when plants that prefer slightly cooler and drier conditions predominate in the area.

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GRAYS LAKE NATIONAL WILDLIFE REFUGE, SOUTHEASTERN IDAHO, U.S.A.

The marshes that cover the floor of a former large Pleistocene lake comprise the Grays Lake National Wildlife Refuge, one of the largest in the United States and lying just outside the northeastern margin of the Great Basin. This stop along the flyway is rich in resources that serve the needs of its many visitors. The role of fire in enhancing the marsh ecosystem is an issue that has repercussions for the management of the marsh. A set of cores spanning the last 11,000 years provides the potential for a very detailed Holocene vegetation and climate record. Organic carbon percentage of samples spanning the last 1,000 years reflects productivity in the marsh, that is, greater organic content equals greater marsh productivity. Although short-term responses (increased productivity) of the marsh to fire might have occurred, the only clear relationship is that between lowered water table and fire when the marsh dried out in response to drought. Cat-tail and rush charcoal was used as evidence that fires were occurring in the marsh and not just in the area surrounding the marsh. Appearance of distinctive plant leaf-hairs and changes in pollen types from plants of deeper or fresher (e.g.,Ceratophyllum leaf-hairs) to shallower or more brackish (e.g., cat-tail and sedge pollen) water plant species was used as evidence of changes in water depth in the marsh.

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Ongoing research at the Grays Lake National Wildlife Refuge has recently included analyses of the pollen, spores and algae from the cores taken at locality 2. A deposition rate cure displays the ages assigned to the samples from the core. For these analyses samples from the last 4,200 years were counted. These pollen counts fill-in a gap in the original pollen work conducted by Jane Beiswenger (1991), and both double the resolution and the precision of the samples that spanned the late Holocene. The record of terrestrial and aquatic plants and algae  from these cores reflect the location of Grays Lake between the subalpine and montañe forests to the east and the sagebrush steppe to the west. Although changes in the pollen record are relatively subtle there are significant shifts in the regional abundance of juniper and fir. If ratios of indicator plant species are generated to reveal both of precipitation in the Grays Lake pollen record significant changes are clear. Pulses of wetter climate are evident in the Neogene Glacial (4,000 to 2,000 years ago) is clear as is the "Little Ice Age" between 350 and 175 years ago. In addition, at least one other significant event of wetter climate occurred around 1,100 years ago. 

There have been particularly revealing changes in the marsh both in the dominance of sedges, cat-tails, submerged and emergent aquatic plants, grasses and algae. Perhaps the most significant relationships that have emerged from the current research are details in the sequence of events leading into and out of fire cycles in the marsh. Each cycle begins with a sedge peat dominated marsh characterized by high organic production (high organic weight percent). Then there is a decline sedge peat dominance (and organic weight percent) as areas of open water appear in the marsh. Increased abundance of emergent and submerged aquatic plant species (lower organic weight percent) correspond with increased areas of open water. Open water may or may not be related to higher water table in response to moister climate. In some cases they appear to correspond, and in other cases they do not. The carbonate content of the sediments deposited during these episodes increases as aquatic plants begin to predominate. This may be the result of 1) increased evaporation rates because of more open water surface, 2) aquatic plant chemistry, or 3) greater contribution of carbonate containing dust from drying playas and lakes upwind of Grays Lake in the Snake River Plain or the Bonneville Basin. Thus, lowest sediment organic weight percent, highest sediment carbonate weight, and highest proportion of aquatic vs. littoral species mark the climax of open water episodes. A return to greater sedge predominance (increasing organic weight percent), decreasing carbonate weight percent, and decreasing aquatic plant abundance with respect to littoral plant species immediately precede the advent of fire! Fires seem to occur just before the peak of organic percentage weight is reached and are marked by significant influxes of charcoal into the marsh. These episodes seem to be related to drought events, but closer interval sampling of the pollen record is needed to confirm this. The morphology of many of the charcoal fragments indicates that cat-tails and sedges were among the plants being burned, that is, the fires are marsh fires and not simply fires in the area surrounding the marsh. The most startling impact of the charcoal influx into the marsh is a shift in the water chemistry marked by a change in algae dominance from eutrophic, Botryococcus, to oligotrophic, Pediastrum spp. Pediastrum dominance lasts only as long as charcoal counts remain high. Then they decline and the entire cycle just described is repeated. This cycle repeated three times during the period from 1,700 to 1,100 years ago, a period characterized in the pollen records from the region between the Rocky Mountains and the Sierra/Cascade mountain ranges by increased May/June rainfall and reduced winter precipitation. Apparently each cycle lasted about three hundred years though higher interval sampling may reveal more frequent cycles. However, two similar cycles fire beginning about 350 years ago in a portion of the pollen record that has been much more closely sampled suggests that these cycles may actually be ~300 years apart. The most recent fire cycles do have some significant differences from the older fire cycles. This may reflect the differences in the underlying climatic regimes of the two periods. In summary, it is clear that over the span of several hundred years changes in the composition of marsh plant communities seem to be related to fluctuations of water table, perhaps in response to climate. Trends to apparently drier conditions appear to result in fire, which in turn results in significant changes in marsh water chemistry. Such changes may also have had significant impacts upon renewal in the marsh and eventually in productivity.

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Airport Playa on the Southern Margin of the Coso Range, Southern Owens Valley, California, U.S.A.

Recent investigation of the plant macrofossils recovered from ancient woodrat middens from Airport Playa has revealed a late Holocene history of Mojave Desert shrub response to changes in both precipitation amount and seasonality. Airport Playa lies close to the northwestern corner of the Mojave Desert about 30 km due north of Ridgecrest, California. It encompasses roughly the southern third of the Coso Basin. The Coso Basin lies within a 10-km diameter amphitheater-like depression opening to the southeast and enclosed on the northwest, north and northeast by the Coso Range. Elevations range from just under 900 meters above sea level on the floor of Airport Playa to just under 2500 meters on Coso Peak. The region lies just east of the crest of the Sierra Nevada Mountains and therefore is effected to some extent by its rainshadow. However, the Coso Range to the north generates orographic rainfall. Models of regional moisture input generated from digital elevation model (DEM) data indicate that the driest areas lie around Airport Playa and to the south while the wettest are to the north. The digital download of regional hydrology indicates that these areas are also major sources of surface water. Most of the surface water input for Airport Playa comes from the north and northeast in the eastern portion of the Coso Range. Although there are small drainages entering Airport Playa from the south the main ones are across the broad bajadas lying to the north.

The radiocarbon dated  macrofossil record from woodrat middens recovered from Airport Playa reflects an environment dominated by creosote bush shrub. The pollen record extracted from the middens indicates dramatic shifts in climate including: 1) seasonal dominance of rainfall as well as 2) strong drought and wet episodes. Winter rainfall dominated Neoglacial climates 2,500 B.P. gave way to increased summer rainfall after 1950. Two wet episodes during the very dry last 1,000 years are evidenced in the midden record as not as wet as around 1950, but still with a strong summer rainfall component. The last 300 years are characterized by dramatic shifts in annual rainfall amount and seasonal rainfall contribution. Aeolian activity centered between 910 and 310 B.P. may reflect the combined effects of wet episodes and the general drought of the last millennium which both loosened playa surface sediment and removed vegetation as an impediment to sediment transport. Additional macrofossil evidence from a human coprolite indicates Native American seed resource use with chia (Salvia columbariae) being an important component. Its sudden abundance may reflect increasingly sandy soils during the last 1,000 years rather than a more straight-forward change in climate.

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Summit Lake, North-central Nevada, U.S.A.

Summit Lake is a narrow lake that fills a former river channel that was probably dammed by a large landslide that occurred during the early Holocene before the fall of the Mazama volcanic ash prior to 6,800 rcyr BP. In places around its margin it supports both littoral and emergent aquatic plant communities. The terrestrial vegetation is dominated by sagebrush steppe. Riparian communities along streams and around springs is dominated by quaking aspen (Populus tremuloides). Cores were taken during the mid-1980s from an arm of Summit Lake at its southeastern edge. A series of samples removed from radiocarbon-dated sediment cores recovered from Summit Lake, Nevada, were analyzed to assess the potential for reconstructing a record of vegetation and climate history, and its impact on fire recurrence and intensity for the late Holocene (~the last 3,000 to 5,000 years). These analyses were based upon the microbotanical fossils and charcoal the samples contained. This research included radiocarbon dating, and analysis of samples originally taken from the core, and extracted in the late 1980s. Microfossil analysis of the cores from Summit Lake indicates that a history of both terrestrial and aquatic vegetation change, and an algal record are present. From these an inferred climate history can be generated through ratios generated from the major climate indicator pollen types. Fire history in both terrestrial and marsh habitats (there are fragments of burned rush and cat-tail leaves) is evidenced by the occurrence of abundant charcoal in these cores though a much closer sampling and radiocarbon dating is required to derive a more detailed indication of frequence and magnitude of changes through time. The results indicate several points:

• Fire has been a very active agent of ecological change and regeneration in both terrestrial and aquatic environments around Summit Lake during the last 2,500 years.

• Increased fire frequency and intensity is restricted to time spans that are clearly characterized as periods of transitional climates (usually from wetter to drier climates).

• Occurrence of fire in the adjacent terrestrial environment seems to have had short-term, but immediate impacts upon terrestrial plant community productivity, and longer-term, but delayed impacts upon productivity in the adjacent aquatic system.

• Regionally, fire seems to have been a much more important component of the ecosystem between ~3,000 and 1,700 years ago than it was during the middle Holocene prior to 3,000 rcyr BP or during the last 1,700 years.

• The ratios generated form the Summit Lake pollen record appears to offer a good, correlatable proxy (where preservation is favorable) for the reconstruction of vegetation and climate history. The aquatic pollen and algae records highlight a period of significantly increased productivity during the last 1,400 years centered around 730 radiocarbon years.

• Finally, given what is known about past Holocene climate, vegetation, and fire history, it can be predicted that with global warming there will be large-scale destruction of woodlands and forests in the Intermountain West. It is expected, given past analogues, that they will be replace by, at the minimum, the next drier vegetation type within the region today, or perhaps by even drier vegetation types if global temperatures continue to increase.