Investigating the Carbon Dioxide Response of Secondary-Succession Forests at Duke and Oak Ridge FACE Experiments Simulated with ELM-FATES-CNP


Bharat Sharma1* ([email protected]), Anthony Walker1, Ryan Knox2, Charlie Koven2, Daniel Ricciuto1, Xinyuan Wei1, Xiaojuan Yang1, Richard J. Norby1, Ram Oren3


1Oak Ridge National Laboratory, Oak Ridge, TN; 2Lawrence Berkeley National Laboratory, Berkeley, CA; 3Nicholas School of the Environment and Pratt School of Engineering, Duke University, Durham, NC



Anthropogenic activities have greatly impacted Earth’s ecosystems via changes in land use, land cover, and changes in climate due to increased atmospheric carbon dioxide (CO2) concentration. Rising atmospheric CO2 drives carbon (C) fertilization which can increase vegetation biomass production and soil C. Gains in biomass and soil C slows atmospheric CO2 growth.

Experiments have demonstrated significant gains in net primary productivity (NPP) and biomass, suggesting that interventions such as increased fertilization, CO2 enrichment, and improved nutrient management can have positive impacts on plant growth and C sequestration in terrestrial ecosystems. Many terrestrial biosphere models have also suggested that elevated atmospheric CO2 (eCO2) has caused a large fraction of land C sequestration during recent decades and predict that this sequestration will continue to increase in the future. Thereby continuing to slow the pace of climate change. Another significant component of global change has been the conversion of natural forests to secondary forests and plantations. First-generation Free Air Carbon Enrichment (FACE) experiments were conducted primarily in secondary forests and plantations, such as Duke and Oak Ridge, and provide information on eCO2, nitrogen (N), and decade-long demographic process interactions. FACE experiments indicated that the variability in the eCO2 response is related to stand structure, nutrient limitation, and progressive nitrogen limitation (PNL). The experiments at Oak Ridge National Laboratory (ORNL) demonstrated the evidence of PNL of the net primary production response to eCO2.

N cycling processes are still poorly understood and represented differently among models. Understanding the interactions of availability of N for plant uptake and growth is necessary to improve predictive capabilities of models to simulate ecosystem C storage in response to eCO2. Researchers use ELM-FATES-CNP (nutrient-enabled and size-structured vegetation demography model with C and nutrient cycling) to simulate the Duke and Oak Ridge FACE experiments and investigate the influence of forest size structure on eCO2 responses and their nitrogen constraints during FACE experiments in early and late successional secondary forests. The team used a C-only version of the model alongside two soil nutrient cycling hypotheses or conceptualizations that currently exist in ELM—relative demand and equilibrium chemistry approximation—modified to represent a dynamic allocation scheme that is more consistent with the data. Using the data collected at ORNL and Duke FACE experiments, the team will evaluate the various ELM-FATES simulations and investigate the coupling of nutrient dynamics and stand structure development and their influence on eCO2.