The rapid warming of the Arctic is expected to cause major changes to northern forests. While warming may lead to increased growth in forests limited by short, cold growing seasons, climate change could also lead to surprises. One example is a decoupling between temperature variability and annual growth rates in boreal forest trees that has been detected over the past few decades. This ?divergence problem? suggests that tree growth in some arctic forest systems is no longer limited by temperature. The causes are not well understood and can be hard to test due to other factors impacting growth, such as drought due to the warming climate. This project will test for divergence across the treeline forests of northern North America, especially in rapidly warming regions of Alaska and northwestern Canada. The investigators will use tree-ring records obtained from the wood of ancient trees, a combination of mathematical methods and modeling, and wood anatomy analysis, to better understand forest growth patterns related to divergence. Broader Impacts: New measurements are critical if we are to understand how boreal forests interact with the atmosphere and feedback with other features of the global environment. This project will contribute to this understanding by estimating relationships between climate and forest growth using tree rings at sites spanning many northern locations. The project provides funding for students, Native American citizens, and for teachers to take part in Arctic research. The rapid warming of the Arctic is expected to impact profoundly northern forest systems. While warming may lead to greater productivity and growth in forests typically limited by short, cold growing seasons, the speed and magnitude of climate change could also lead to surprises. One example is a decoupling between temperature variability and annual growth rates in boreal forest trees that has been detected at forest sites since the middle of the twentieth century. This phenomenon, known as the ?divergence problem?, suggests that tree growth in some Arctic forest systems is no longer primarily limited by temperature. The causes are not well understood and have been difficult to test due to the co-varying biological, physiological, and environmental factors potentially impacting recent tree growth. One hypothesis is that once warming has surpassed a physiological threshold, drought stress limits growth. Another hypothesis is ?global dimming?, in which increased atmospheric aerosols decrease the amount of solar radiation available for photosynthesis. This project will assess current divergence hypotheses across the treeline forests of northern North America, emphasizing rapidly warming regions of Alaska and northwestern Canada, which appear to be key areas for divergence-type effects. The project will leverage a new, large compilation of tree-ring data (already in hand) and update key locations that have shown early evidence of unusual growth decline despite recent warming. Detection of recent changes in growth parameters will use a novel, integrated combination of statistical techniques, quantitative wood anatomy methods, and tree growth modeling to develop a detailed assessment of the extent, causes, and carbon cycle implications of divergence. Broader Impacts: New measurements of boreal forest dynamics are critical for understanding how boreal systems interact with the changing atmosphere and the feedbacks between coupled earth system domains. This project will contribute to this understanding by developing quantitative estimates of climate/growth interactions at daily to centennial time scales and spatial scales from cellular to continental, yielding valuable, novel data for modeling of environmental change and productivity in northern forests. The project provides three years of support for a graduate student, funding for high school curricular development and courses in native educational settings, and fieldwork-based opportunities for a teacher to take part in hands-on Arctic research. 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.