Postglacial history of subalpine forests, Olympic Peninsula, Washington, USA

To investigate subalpine vegetation history on the Olympic Peninsula, Washington (USA), two pollen, macrofossil and charcoal records were studied in climatically distinct zones: Martins Lake (1415 m) in the moist Tsuga mertensiana zone and Moose Lake (1508 m) in the drier Abies lasiocarpa zone. The interpretation of fossil pollen assemblages was aided by comparisons with 308 modern assemblages from the Olympic Peninsula and western North America. Both pollen records show a cold/dry period following deglaciation (>10000 radiocarbon years BP) with sparse tundra and little similarity to any modern pollen assemblage. In the early Holocene, when summer temperatures are thought to have been higher than present, high percentages of Alnus sinuata-type pollen at both lakes suggest increased avalanche activity. At Martins Lake warmer summers were not accompanied by forest establishment, possibly because persistent spring snow pack, snow avalanches, and/or edaphic constraints limited tree establishment at this site. The Martins Lake record shows a steplike shift in vegetation to modern Tsuga mertensiana/Abies amabilis parkland across the Mazama tephra (6730 BP). In contrast to Martins Lake, Abies lasiocarpa forest quickly established at Moose Lake in the early Holocene, though forests were probably initially open and fires may have been frequent. From 7800 to 5100 BP forests near Moose Lake shifted gradually to cooler and moister species composition, with the addition of Chamaecyparis nootkatensis, Tsuga mertensiana and Pinus, though Abies lasiocarpa remained dominant. Forest cover was probably greatest during this transition, with parklike conditions at Moose Lake beginning at c. 5100 BP. The major differences in the records between the two sites may be due to differences in the local expression of regional climatic change and/or differences in soil development and stabilization.