Early evidence from free-air carbon dioxide (CO₂) enrichment (FACE) experiments suggested that stimulation of net primary productivity (NPP) was a general property of temperate forests. More recently however, a discrepancy in NPP responses has emerged: while some forests show a sustained enhancement of NPP for up to a decade, others show only a transient response. These findings challenge results from global land models, which commonly predict persistent increases in NPP with rising CO₂. This discrepancy raises the critical question of whether and how forest response depends upon secondary site factors such as nutrient availability and historical land use. Here we explore how land-use history, successional stage, atmospheric nitrogen (N) deposition, and climatic conditions interact to generate the observed forest NPP responses to elevated CO₂. We compare a series of CO₂ enrichment and N addition experiments using the Princeton-Geophysical Fluid Dynamics Laboratory (GFDL) global carbon-nitrogen (C-N) model LM3V-N to understand the general rules that shape the response of NPP to elevated CO₂.  

Results/Conclusions

Results from one of the forest FACE sites show a stronger stimulation of modeled NPP in a very young stand on former agricultural land compared to an old growth forest. Our findings indicate that land-use history, successional stage, climate and N availability can significantly affect the NPP response to a step increase in atmospheric CO₂ (550 ppm). In line with experimental evidence, we found that response in NPP declined over time due to progressively greater N limitation as stand age increased. At five FACE locations (Duke, ORNL, web, POP-EURO, and Aspen-FACE), our model predicts a variable response to CO₂, due to interactions between N and water availability within the context of land-use history. In C-only models, the stimulation in NPP was driven strongly by the physiological response of trees to rising CO₂. We demonstrate here that inclusion of feedbacks between the C and the N cycle allows to account for potential biogeochemical constraints. We suggest  that these interactions are critical for capturing spatial and temporal variations in the response of NPP to elevated CO₂.