Collaborative Research: Soil temperature, mycorrhizal association and tree nutrition as determinants of divergent changes in tree growth and abundance in arctic Alaska.

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Vegetation change has been widespread in the Arctic in recent decades, with implications for carbon storage, surface energy exchange, and local wildlife habitat. Most studies of patterns and processes responsible for vegetation change have focused on deciduous shrubs, which have shown increases in abundance throughout much of the Arctic. Fewer studies have examined changes in tree abundance at the interface between arctic tundra and the boreal forest. This project will take advantage of historical aerial images collected throughout the Brooks Range during the 1950s and 1970s to provide a comprehensive census of changes in tree abundance in recent decades in northern Alaska. Investigators will also test hypotheses about whether differences in nutrition, colder soils, and more widespread permafrost explain why trees in the western Brooks Range have increased their growth in response to recent warming, while those in the eastern Brooks Range have not. In addition, the investigators plan extensive education and outreach activities in rural and urban Alaska. Rural efforts will include visiting classrooms in Kotzebue High School to share research results, providing opportunities for Kotzebue High School students to join the research team in the field, arrainging to be interviewed by local public radio stations, and delivering public lectures at National Park Service headquarters in several local villages. In Anchorage, the investigators plan to equip a public high school in Anchorage with supples needed to conduct tree-ring research, train students in standard methods, and help them design a study to link white spruce tree ring data in their backyards to long term climate records. The relative abundance of forest and tundra has important implications for surface energy exchange, carbon cycling, wildlife habitat and the availability of subsistence resources at high latitudes. The northern limit of the boreal forest in Alaska is formed within the Brooks Range. The eastern Brooks Range is an area well known to dendrochronologists as an epicenter of divergent tree growth responses to climate warming. Divergence refers to the deterioration of historically strong positive correlations between temperature and tree growth. The recognition that divergence has been widespread in the circumboreal has undermined confidence in paleoclimate reconstructions and created uncertainty in projections of vegetation-climate feedbacks. While divergence is a well-known phenomenon, its implications for changes in tree abundance and shifts in treeline position remain unknown. Recent findings in four watersheds along a west to east gradient in the Brooks Range suggest colder, more permafrost-affected soils limit tree access to soil nutrients and may be the cause of divergence in the eastern Brooks Range. In this study, investigators will expand the spatial extent of this previous work to examine the causes and consequences of divergence in the Brooks Range. They will combine repeat aerial photography with tree-ring analysis and detailed measurements of tree microclimates, mycorrhizal associations, nutrient relations, and reproductive effort in 25 Brooks Range watersheds to yield broad-scale mechanistic insights into controls on tree growth and changes in tree abundance in a changing climate. They will test the hypothesis that positive growth responses to warming and increased tree abundance will prevail in the western Brooks Range and in habitats with warmer soils, while neutral growth responses to warming and stagnant treelines will be common in the eastern Brooks Range and in areas with cold soils.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.