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

To investigate subalpine vegetation history on the Olympic Peninsula, Washi ngton (USA), two pollen, macrofossil and charcoal records were studied in c limatically distinct zones: Martins Lake (1415 m) in the moist Tsuga merten siana zone and Moose Lake (1508 m) in the drier Abies lasiocarpa zone. The interpretation of fossil pollen assemblages was aided by comparisons with 3 08 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 moder n pollen assemblage. In the early Holocene, when summer temperatures are th ought to have been higher than present, high percentages of Alnus sinuata-t ype pollen at both lakes suggest increased avalanche activity. At Martins L ake warmer summers were not accompanied by forest establishment, possibly b ecause persistent spring snow pack, snow avalanches, and/or edaphic constra ints 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, A bies lasiocarpa forest quickly established at Moose Lake in the early Holoc ene, though forests were probably initially open and fires may have been fr equent. From 7800 to 5100 BP forests near Moose Lake shifted gradually to c ooler and moister species composition, with the addition of Chamaecyparis n ootkatensis, Tsuga mertensiana and Pinus, though Abies lasiocarpa remained dominant. Forest cover was probably greatest during this transition, with p arklike conditions at Moose Lake beginning at c. 5100 BP. The major differe nces 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 dev elopment and stabilization.