Our primary objective is to provide an improved understanding of the biological and climatic controls over carbon (C) and energy cycles during and after a successional shift from a mature aspen to a young mixed confer/deciduous forest ecosystem that will be widely distributed across the upper Great Lakes region in coming decades. In Spring 2008, we implemented the Forest Accelerated Succession ExperimenT (FASET) by stem girdling all aspen and birch (gt;6,700 trees, ~35% canopy LAI) within a 39 ha area. A suite of ongoing ecological and meteorological measurements conducted in treatment and control stands before (2007) and after (2008 onwards) the succession treatment are used to quantify effects of climate, species composition, and canopy structure on the forest C cycle. We have established paired treatment and control plots, and surveyed a 25 m grid system within our treatment plots, begun operation of a eddy-covariance tower within the 33 ha treatment plot, conducted intercomparisons of carbon exchange and N allocation between treatment and control plots, remote sensed forest canopy structure, and begun or continued collaborative projects with investigators utilizing this project as a platform for further studies. Our overarching hypothesis is that forest NEP across much of the upper Great Lakes region will increase following transition from aspen dominated ecosystems to those of later-successional species with biologically and structurally more complex canopies. Specific hypotheses: a) Tree mortality will prompt a short-term reduction in NEP. A rapid recovery and stabilization of NEP above that of the control forest will be linked to the magnitude of N leaching losses and the pattern of redistribution of available N. Stands with more species and structurally diverse canopies and greater allocation of N to photosynthetic tissues will have higher NEP. b) Successional change will increase spatial variation in microclimate and nutrient distribution, both of which constrain landscape-level variability in C storage.