POSTGLACIAL VEGETATION AND CLIMATE OF GRAND-TETON AND SOUTHERN-YELLOWSTONE-NATIONAL-PARKS

Pollen records from northern Grand Teton National Park, the Pinyon Pea k Highlands, and southern Yellowstone National Park were examined to s tudy the pattern of reforestation and climatic change following late-P inedale Glaciation. The vegetational reconstruction was aided by analy ses of associated plant macrofossils and the modem pollen rain of the region. Radiocarbon-age determinations and tephrochronology provided a chronologic framework to help correlate pollen records among sites. T he fossil records indicate that alpine meadow communities, with Betula and Juniperus, were present between almost-equal-to 14 000 and almost -equal-to 11 500 yr BP. This early assemblage implies a lowering of mo dem upper treeline by at least 600 m and a climate that was almost-equ al-to 5-6-degrees-C colder than present. Between 11 500 and 10 500 yr BP, increased temperature and winter precipitation allowed first Picea , and then Abies and Pinus cf. albicaulis to spread into areas underla in by andesite and nonvolcanic bedrock. By 10 500 yr BP, the fossil re cord in these areas resembled modem spectra from subalpine forest. In contrast, the Central Plateau of Yellowstone, which is underlain by in fertile rhyolitic soils, was treeless prior to almost-equal-to 10 000 yr BP. The absence of late-glacial subalpine parkland in this area is attributed to the same edaphic factors that limit Picea, Abies, and Pi nus albicaulis from the rhyolite plateau today. Between 10 000 and 950 0 yr BP, Pinus contorta forest developed throughout the region in resp onse to further warming. Pseudotsuga and Populus were present between 9500 and 5000 yr BP, suggesting a warmer, drier climate than today and more frequent fires. In the last 5000 yr BP mixed forests of Picea, P inus, and Abies have developed on andesitic and nonvolcanic terrain, a nd closed forests of Pinus contorta have persisted on rhyolitic substr ates. The vegetational patterns are attributed to a combination of cli matic and nonclimatic controls operating on different spatial and temp oral scales. Climatic changes brought about by the retreat of Laurenti de ice and the amplification of the seasonal cycle of radiation explai n the broad patterns of vegetational change on millennial time scales. On shorter time scales and smaller spatial scales, substrate differen ces and migration history shaped the vegetational variability.