| Summary: | 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. Our 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. Here, we propose to dramatically expand the spatial extent of our previous work and further examine the causes and consequences of divergence in the Brooks Range. Specifically, we aim to 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. We hypothesize 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.
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