Holocene variations in summer temperature and effective moisture: Understanding the causes of vegetational dynamics in Alaska

Arctic tree-line response to 21st century warming is an important positive feedback mechanisms that can facilitate further tree line advance and amplify changes to the global heat budget through changes in the planetary albedo and effects on soil carbon pools. However, responses of the forest-tundra ecotone to the modern warming trend are spatially heterogeneous because controls other than temperature limit tree growth and seedling establishment near tree line. Paleo-investigations can address the sensitivity of the forest-tundra ecotone to external drivers by identifying factors that led to the establishment and persistence of modern vegetation composition. In contrast to many circumpolar regions that experienced advanced positions of forests associated with an early Holocene maximum in solar irradiation, closed boreal forests in Alaska only developed ∼6000 years ago after a prolonged forest-tundra phase. The contrasting history of forest development in Alaska compared to circumpolar trends has intrigued paleoecologists for over five decades, and provides a unique opportunity to test alternative hypotheses of ecological and climatic drivers of forest-tundra ecotone dynamics. Progress in understanding Holocene vegetation patterns in Alaska has been hampered by a lack of vegetation-independent climatic and environmental reconstructions. This dissertation project takes advantage of state-of-the-art tools for paleoclimatic reconstruction, including stable isotope analysis of carbonates and midge assemblage analysis, to test the role of orbital forcing and moisture balance on interior Alaskan vegetation assemblages. The results presented here provide some of the first high-resolution, vegetation-independent climate records from Alaska that span the entire Holocene and highlight heterogeneous climatic and vegetation responses to spatially uniform radiative forcing via complex atmospheric feedbacks. The results complement recent neo-ecological work in Alaska that indicates the importance of spring moisture limitation to forest-tundra ecotones and suggests that moisture played an important role in determining the position of the Alaskan forest-tundra ecotone throughout the Holocene.