The climate sensitivity of plant seasonal life cycles, or phenology, may impart significant carbon cycle feedbacks on climatic change. Analysis of interannual ecosystem carbon exchange provides one way to assess this climate sensitivity. Multiyear eddy covariance carbon dioxide flux observations from five different ecosystems (deciduous forest, northern hardwood mixed forest, old-growth forest, shrub wetland, and mixed wetland-forest) in the Upper Great Lakes, United States, located within 400 km of each other and exhibiting coherent interannual variability, were used to parameterize a simple ecosystem model. The model, when properly constrained with an interannual sensitive cost function, was able to explain a significant proportion of the interannual variation of carbon fluxes in all ecosystems except the old-growth forest. The results reveal that spring or autumn climate thresholds impact annual carbon uptake, though the magnitude and strength varied by site. When the model was forced to maintain the same climate-phenology relationship across the five sites, most of the interannual variability could still be explained except at the old-growth forest and the forest farthest in distance from the others. These results suggest that at least for this region, coarse spatial resolution carbon-climate models could likely specify general climate-phenological relationships at grid scales on order of 100 km without appreciably sacrificing ability to model interannual carbon cycling.