Daniel H. Mann

A long-standing paradigm in the ecology of the Alaskan taiga states that black spruce (Picea mariana [Mill.] BSP) replaces white spruce (Picea glauca [Moench] Voss) after several centuries of primary succession on floodplains. According to this Drury Hypothesis, autogenic thickening of organic horizons and shrinking of the active layer interact with the species’ different physiological tolerances to cause black spruce dominance. We test the Drury Hypothesis on > 200-year-old portions of the Tanana River floodplain near Fairbanks, Alaska, and reject it. In the meander belt portion of the study area, white spruce mixed with black spruce persists on geomorphic surfaces approximately 3 000 years old. Predictions of the Drury Hypothesis regarding active-layer and organic-horizon thicknesses are not substantiated. Neither of these variables correlates with the abundances of the different spruce species. Forest communities in the study area are distributed along geologically based environmental gradients and are shaped by secondary succession following fires and probably floods. Black spruce dominates in the poorly drained, permafrost-rich, and fire-prone backswamp and white spruce in the oppositely characterized meander belt. Although geological chronosequences can be identified along avulsion-prone rivers like the study reach of the Tanana River, superposition of a meander belt-backswamp plan and frequent fire and flood disturbances may negate any vegetation chronosequences older than several centuries.

Late-glacial vegetational, tephra, and climatic history of western Kodiak Island is developed based upon detailed palynological, macrofossil, and AMS ¹⁴C stratigraphy. A series of distinctive Purple, White, and Grey tephras allows correlation amongst several sections along the southwestern Kodiak coastline. Changes in loss-on-ignition within these sections support our interpretation of climatic change since deglaciation. The macrofossil component of the sedimentary record provides insight for late-glacial plant diversity. Earliest vegetation after glacial recession 14 000 years BP is composed of a diverse array of moist herbaceous species, along with a dominance of Empetrum nigrum (heath). As climate became warmer and moister after 13 000 BP, Polypodiaceae (ferns) spread over the landscape. A dramatic reversal in vegetation and climate to colder, drier conditions is indicated from 10 800 to 10 000 years BP, which we term the “Fern Gap”. We interpret this oscillation as a high-latitude Pacific expression of the Younger Dryas event, well-known from Greenland and continental regions surrounding the North Atlantic. The subsequent Holocene warming at 10 000 years BP is quite abrupt, as shown by the section lithostratigraphy, palynological and macrofossil records, and percent loss on ignition. This late-glacial series of events indicates a high correlation between paleoclimate on Kodiak Island and the North Atlantic region, suggesting comparable climatic forcing in both regions.