Background/Question/Methods In many areas of North America, the Holocene establishment of boreal forests is characterized by the rapid expansion of Picea mariana (black spruce) as the dominant species. Today this species commonly occurs in lowland sites characterized by cold, wet, and nutrient-poor soils. Previous paleoecological studies suggested that P. mariana forests developed as a result of (1) autogenic ecosystem processes leading to waterlogged soil conditions, or (2) climatic cooling and/or moistening. A rigorous evaluation of these alternative hypotheses has not been possible because of the lack of pollen-independent climatic records with suitable spatial and temporal resolutions. Here we present paleoecological and stable-isotope results from Alaska to test these hypotheses.  We analyzed sediment cores from Takahula Lake (67°21’7”N, 153°39’53”W) and Malamute Lake (67°7’2”N, 153°8’42”W) for the oxygen and carbon isotopes of authigenic carbonate to infer effective moisture and for the assemblages of subfossil chironomid (non-biting midge) larvae to estimate mean July air temperature.

Results/Conclusions

Results reveal large fluctuations in effective moisture and temperature over the past 8,000 years. Strikingly, the rapid expansion of P. mariana occurred during a period of severe moisture deficit (4500 – 4100 yrs BP), thus refuting the hypothesis that this vegetational change was caused by an increase in effective moisture. The invasion of P. mariana coincided with a transient climatic cooling that lasted ~900 years and centered at 4500 yr BP. P. mariana remained dominant in the regional forests throughout the following 4000 years despite marked fluctuations in effective moisture and temperature. Hence the establishment of P. mariana as a dominant species appears to have been triggered by summer cooling but was buffered from subsequent climatic variation. This apparent climatic insensitivity of P. mariana may be attributed to changes in soil thermal properties following P. mariana establishment, allowing the persistence of permafrost through late-Holocene climatic fluctuations, and hence leading to the widespread occurrence of moist to waterlogged soils. These results imply that local loss of permafrost in response to future warming will probably be a key factor to accelerate boreal-forest community changes.