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

Authors

Bharat Sharma¹* (sharmabd@ornl.gov), Anthony Walker¹, Ryan Knox², Charlie Koven², Daniel Ricciuto¹, Xinyuan Wei¹, Xiaojuan Yang¹, Richard J. Norby¹, Ram Oren³

Institutions

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

URLs

• https://facedata.ornl.gov/facemds

Abstract

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 eCO_2.