View to the northwest across Summit Lake

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 late 1980s from and 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 ago.

• 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.

At the current rate of global warming destruction of the major portion of Great Basin and Mojave Desert woodlands and forests can be expected within the next 100 to 150 years.

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