Interactive impacts of CO2 and nutrients on forest production

Background/Question/Methods Results from the ORNL FACE experiment suggest that an observed initial increase in plant production on elevated CO₂ plots is reduced after 10 years due to increasing nutrient stress, while results from the Duke FACE show increased plant production on elevated CO₂ plots throughout the 15 year experimental record. The NCEAS Elevated CO₂ Model Comparison Project was proposed to compare the impacts of nutrients on ecosystem dynamics represented in ecosystem models and to assess model efficacy in simulating observed patterns of plant production under elevated CO₂ at the ORNL and Duke FACE sites. The participant models allow soil nitrogen availability to influence soil and plant C:N ratios, plant photosynthetic rates, carbon allocation to the different plant parts, and overall amount of plant growth. Some of the models also include the impact of elevated CO₂ levels on soil N fixation and nitrogen losses (as N₂O, NO_x, and NO₃).

Results/Conclusions

Model results suggest that allowing the plant and soil C:N ratios  to increase with decreasing nitrogen availability reduces nutrient down regulation of elevated CO₂ plant production increases. This paper focuses on soil nutrient availability impacts on ecosystem models and how well these models represent the impacts of elevated CO₂ at the Duke and ORNL FACE sites. The models with a dynamic carbon allocation scheme and variable plant and soil C:N ratios were more effective at replicating observed responses of the ORNL and Duke FACE sites to elevated CO₂ levels.