Impacts of Elevated Atmospheric CO\(_{2}\) on Temperate Forest Successional Dynamics: A Case Study of Duke’s Free-air CO\(_{2}\) Enrichment Site

Abstract

This paper investigates the impacts of elevated atmospheric CO\(_{2}\) on simulated ecosystem functioning, structure, and composition. Our efforts are focused on confirming real-life experimental work via model as well as answering questions that were previously unexplored at our test site. The Duke Free-air CO\(_{2}\) Enrichment (FACE) site was selected as the case study site for model simulations. Fine root and nitrogen cycling dynamics were investigated, and a new model for stomatal conductance derived from H\(_{2}\)O optimization theory is implemented into the Ecosystem Demography model version 2 (ED2). Results show that increased atmospheric CO\(_{2}\) increases the growth rate of both the loblolly pine and American sweetgum. Use of the stomatal conductance model also results in simulations of basal area growth rate that are more similar to observations than when a traditional, empirical stomatal conductance model is used. Also consistent with observations, the model simulated an increased standing fine root crop under elevated atmospheric CO\(_{2}\). The hypothesis that different allocations of carbon between roots and leaves might be more optimal at elevated atmospheric CO\(_{2}\) is rejected. Finally, an examination of nitrogen limitation shows that the simulated system is not increasingly nitrogen limited as a result of increased tree growth from elevated atmospheric CO\(_{2}\), rejecting the progressive nitrogen limitation hypothesis for the system. Still, nitrogen limitation is shown for one species, the American sweetgum. Overall, this work confirms previous experimental work at Duke FACE via improved simulation and successfully answers some previously unanswered questions from FACE scientists.