Document: JOY-3-59-7

The role of nitrogen dynamics in modeling historical and projected carbon balance of mature black spruce ecosystems across North America: Comparisons with CO₂ fluxes measured in the Boreal Ecosystem Atmosphere Study (BOREAS).

CLEIN, J.* ¹, ADAVEMCGUIRE ¹, X.ZHANG ¹, D.KICKLIGHTER ², J.MELILLO ², S.WOFSY ³ and P.JARVIS ⁴

University of Alaska Fairbanks, Fairbanks, AK 99775, USA ¹
Marine Biological Laboratory, Woods Hole, MA, USA ²
Harvard University, Cambridge MA, USA ³
University of Edinburgh, Scotland ⁴

Abstract:
We used the Terrestrial Ecosystem Model (TEM) to evaluate the role N dynamics play in influencing the spatial and temporal responses of C dynamics in black spruce ecosystems to historical and projected climate variability in North America. First, we developed black spruce parameterizations for versions of the model in which C and N dynamics were coupled and uncoupled. We evaluated the performance of these parameterizations at two eddy correlation tower sites in black spruce stands by comparing field-based C fluxes to simulated C fluxes. Monthly estimates of GPP and RESP for the coupled and uncoupled simulations were highly correlated with the field-based estimates (R² = 0.90). The correlations between field-based and simulated monthly NEP were lower for both tower sites as compared to the correlations for GPP and RESP (R² between 0.3 and 0.6). Next, we used a global-based climatology to simulate historical and projected C dynamics from 1900 to 2100 with both parameterizations of TEM at one of the tower sites. There was greater inter-annual variability in NPP simulated by the uncoupled parameterization, which led to substantial differences in inter-annual variability in NEP between the parameterizations. Additioanlly, heterotrophic respiration increased at a faster rate in the uncoupled simulation during the projected period, causing more C storage. These responses were the result of different responses of the parameterizations to changes in CO₂ and climate. Across the range of North American black spruce ecosystems, the range of simulated decadal changes in C storage was greater for the uncoupled parameterization than for the coupled parameterization. Analysis of the spatial variability in decadal responses of C dynamics revealed that C fluxes simulated by both parameterizations have different sensitivities to climate and that the climate sensitivities of the fluxes change with time. The results of this study suggest that uncertainties can be reduced through studies focused on elucidating the role of C and N interactions in the response of black spruce ecosystems to manipulations of atmospheric CO₂ and climate and establishment of a network of continuous, long-term measurements of C dynamics across the range of mature black spruce ecosystems in North America.

Keywords: black spruce, C storage, ecological modeling

This abstract is being presented at: 11:15 AM in session:
Oral Session #25: Organic Matter Dynamics in Ecosystems.