Overall responses of tree productivity to recent increases in atmospheric [CO₂], shifts in the frequency of disturbances, as well as regional variation in the magnitude of global warming and Nitrogen deposition have not been adequately characterized. Tracking changes in tree productivity at regional to hemispheric scales should better inform near term predictions of the global carbon budget. However, the few types of data available to do this are either limited in temporal scope to a few decades (remote sensing) or spatial extent (long-term forest inventories). In view of these shortcomings we asked how tree-ring data can best be used to track productivity from the late Holocene into the Anthropocene. We used a simple but powerful dendroecological approach to track tree productivity across the extra-tropical northern hemisphere over the last millennium. The 29 species analyzed thus far belong to the genera Pinus, Quercus or Picea and were obtained largely from the International Tree-ring Databank.

Results/Conclusions:

Our analyses of greater than 7.5 million ring-widths revealed that average tree productivity has increased 40% across the Anthropocene. Larger increases, mostly in young trees, were found for maximum rather than minimum growth rates. Increasing tree productivity trends across North America and Eurasia have been remarkably uniform across the last 200 years. Regionally, the largest productivity increases were found across eastern North America. This signal may have included N deposition and other global change phenomena as recent influences, though the timing of the response strongly suggests that the well documented increases in disturbance levels have driven much of the increased productivity. When all low N deposition sites (wet deposition < 4 kg ha^-1 yr^-1) were compared to the high N deposition sites across Europe (wet deposition > 4 kg ha^-1 yr^-1) the trends in tree productivity were similar until the most recent 60 years, when productivity on high N deposition sites started increasing at a greater rate. Tree age and N deposition also interacted; the most rapid increases in productivity were found for younger trees on high N deposition sites, whereas no differences in productivity associated with N deposition were apparent among older trees.