Ben Clegg

Ph.D. student
Program in Ecology and Evolutionary Biology

I am interested in climatic and edaphic controls on the establishment of modern boreal communities in interior Alaska. My main focus is on potential causes of the rapid expansion of Picea mariana (black spruce), the dominant species in Alaska's boreal lowland sites since the mid-Holocene. Today this species commonly occurs on 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. My research tests these alternatives by establishing paleoclimate and permafrost records from lake sediment and peat cores spanning the latitudinal range limit of P. mariana along the southern edge of the Brooks Range. I am using a multiproxy approach to constrain various climatic variables as well as some aspects of edaphic changes in the study area. These include a combination of stable carbonate isotopes and chironomid (non-biting midge) assemblages to constrain effective moisture (precipitation minus evaporation) and mean July air temperature, respectively, as well as bryophyte, amoebal tests, and humic/fulvic acid assays on peat cores to infer permafrost histories within the study region. Results to date 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.

updated 02/22/11