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White spruce forests on the most elevated surfaces of the Mackenzie Delta are dying out and are being replaced by open spruce/lichen-heath or spruce/bog woodlands. Soil profiles indicate that these woodlands have not received flood deposited sediments for many years. The active layer is only 20 to 50 cm deep by mid-summer, and the organic soils are colder and more acidic than soils under white spruce forests flooded during spring ice breakup in 1961, 1972, and 1982. Spruce regeneration is limited to those stands that are flooded periodically, have moderately-closed canopies, and have a ground cover of herbs rather than feathermosses and lichens. It is proposed that a decrease in flood frequency is primarily responsible for the poor regeneration of white spruce on the most elevated delta surfaces. Spruce woodlands on the delta could succeed to tundra vegetation if present fluvial regimes continue. [ABSTRACT FROM AUTHOR]

The treeline in Alaska is usually mapped as the limit of white spruce Picea glauca (Moench) Voss along the south slope of the Brooks Range and in western Alaska and as the limit of Sitka spruce Picea sitchensis (Bong.) Can. on Kodiak Island and in the Alaska Peninsula. In some localities black spruce Picea mariana (Mill.) B.S.P. and paper birch Bemia papyrifera Marsh, fonn components of the treeline community with white spruce. Of special interest are the groves of balsam poplar Populus balsamifera L. occurring on the north slope of the Brooks Range and to the west and southwest of the spruce treeline. There is some question as to whether trees survived in unglaciated portions of Alaska during the last glacial period. Spruce trees became prevalent about 10000 yr ago and may still be expanding to their climatic limits. In some areas of Alaska the treeline was more expanded during the Hypsithermal period than it is at present, but in other regions evidence for such an expansion is lacking. Treetine in some areas seems to have been stable for the last several centuries, but is has been expanding rapidly for the last 40 yr in entral and western Alaska and for a longer period on Kodiak Island. [ABSTRACT FROM AUTHOR]