As a result of landuse, 50% or more of forests in the Northern Hemisphere mid-latitudes are in hilly to mountainous terrain, accounting for half or more of the mid-latitude Gross Primary Productivity. The mid-latitude sink observed in the atmosphere may reflect carbon dynamics occurring in complex terrain.  This is challenging: these regions are inherently highly heterogeneous and currently reflect complex land use histories, and atmospheric techniques for estimating spatially integrated carbon fluxes don’t work well in sloping terrain. Consequently, the impacts of climate, harvesting regimes, disturbances and fire/pest management on carbon exchange are poorly constrained in mountains. While mountains are heterogeneous, the orientation of slopes to incident radiation and gravitational flows of air and water result in organization of the variability that can be exploited. Analysis using model-data fusion techniques of long-term eddy covariance data showed 1) mid-aged Rocky Mountain forests are sinks, 2) most of the net uptake occurs in the spring when melting snow provides moisture for photosynthesis but low soil temperatures inhibit respiration and 3) interannual variability is mainly due to GEE and is largely driven by spring temperature and precipitation, which both determine spring fluxes and set the stage for mid-summer soil moisture conditions.