VEGETATION DEVELOPMENT ON HECETA ISLAND, SOUTHEASTERN ALASKA DURING THE LATE GLACIAL AND HOLOCENE

Pollen analysis and AMS radiocarbon dating of a lowland lake sediment core (14 RC ages) and a peat core from an upland raised bog (10 RC ages) from Heceta Island, in the Alexander Archipelago, provide the longest late Quaternary vegetation history obtained so far from southeast Alaska. Heceta Island is located within the western limit of late Wisconsin glacial ice. Local deglaciation began before 14,000 yr B.P. (all ages in radiocarbon years). Between 13,800 and 12,500 yr B.P. pollen accumulating in the lowland lake site represents sedges, willows, heaths, herbs and some pines. The presence of pine pollen in otherwise tundra-like assemblages suggests that pine trees may have survived in refugia in the western Alexander Archipelago and on subaerially exposed parts of the continental shelf during the late Wisconsin. By about 12,000 yr B.P., pine woodland with ferns and sedges replaced tundra. Pine populations declined while alder rapidly expanded after about 11,300 yr B.P. By 9200 yr B.P., spruce (presumably Sitka spruce) began to colonize the island. By 8500 yr B.P., western hemlock arrived on the island and became the dominant tree species by about 7000 yr B.P. Mountain hemlock became an important component of the island's upland forest vegetation after 8500 yr B.P. During the middle and late Holocene, a more diverse coastal rainforest developed, with western hemlock, Sitka spruce, red cedar, yellow cedar and mountain hemlock, while sedge-heath-Sphagnum bogs with pines also expanded regionally. These vegetation changes appear to have been a response to the onset of wetter and cooler climate during the late Holocene.