U.S. Department of Energy Office of Biological and Environmental Research

PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Global model improved by incorporating new hypothesis for vegetation nutrient limitation, supported by field experiment

Peter E. Thornton
Oak Ridge National Laboratory

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The chambers in operation at the study site (photograph courtesy of M. Blackburn). Used by permission.

 

Comparison of default and revised model estimates of key local and global parameters. (a) The relative difference between fertilized and unfertilized study plots in terms of NEE400 and the slope of the change in NEEi with ambient CO2 concentrations according to observations, the default (without C storage) and the modi?ed (with C storage) CLM 4.5CN model. Slope and NEE400 refers to the ?t between modeled and observed data between the slope of change in NEEi with CO2 and NEEi at the CO2 concentration of 400 ppm on each plot, respectively. The bars and errors represent plot means and standard errors (n = 8). (b) The seasonal anomaly in Northern Hemisphere atmospheric CO2 concentrations according to observations, the default (without C storage) and the modi?ed (with C storage) CLM 4.5CN model incorporated into the CESM1 ESM. The seasonal atmospheric CO2 cycle is presented as an anomaly from the 1992 to 1999 detrended signal. Observations come from marine boundary layer data from NOAA ESRL. Used by permission.

09/21/2016
Low-cost experiment provides first robust test of alternative hypotheses regarding short-term vegetation response to chronic nutrient limitation.

The Science
An innovative and low-cost field experiment provided new results regarding the fundamental process of photosynthetic carbon uptake in the face of varying levels of nutrient limitation. Experimental results refute current modeling approach for instantaneous downregulation of carbon uptake, and support new hypothesis for long-term storage and release of excess carbon.

The Impact
This new hypothesis has significant impact on seasonal cycle of atmospheric CO2, an important performance metric for global carbon cycle models. The fate of excess carbon can have significant impact on other ecosystem processes.

Summary
Models predicting ecosystem carbon dioxide (CO2) exchange under future climate change rely on relatively few real-world tests of their assumptions and outputs. This work demonstrated a rapid and cost-effective method to estimate CO2 exchange from intact vegetation patches under varying atmospheric CO2 concentrations. We found that net ecosystem CO2 uptake (NEE) in a boreal forest rose linearly by 4.7 ± 0.2% of the current ambient rate for every 10 ppm CO2 increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clear short-term NEE response to fertilization in such an N-limited system is inconsistent with the instantaneous downregulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with considerable empirical support - diversion of excess carbon to storage compounds - into an existing earth system model brings the model output into closer agreement with our field measurements. A global simulation incorporating this modified model reduced a long-standing mismatch between the modeled and observed seasonal amplitude of atmospheric CO2. Wider application of this chamber approach would provide critical data needed to further improve modeled projections of biosphere-atmosphere CO2 exchange in a changing climate.

Contacts (BER PM)
Dorothy Koch, Daniel Stover and Jared DeForest
Dorothy.Koch@science.doe.gov (301-903-0105), Daniel.Stover@science.doe.gov (301-903-0289), and Jared.DeForest@science.doe.gov (301-903-1678)
PI Contact
Peter E. Thornton: Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, thorntonpe@ornl.gov (865-241-3742)

Funding
This work was supported by DOE Office of Science, Biological and Environmental Research, Earth System Modeling (ACME project) and Terrestrial Ecosystem Science (ORNL TES SFA)

Publications
Metcalfe, D. B., D. Ricciuto, S. Palmroth, C. Campbell, V. Hurry, J. Mao, S. G. Keel, S. Linder, X. Shi, T. Näsholm, K. E. A. Ohlsson, M. Blackburn, P. E. Thornton and R. Oren 2016. Informing climate models with rapid chamber measurements of forest carbon uptake. Global Change Biology DOI: 10.1111/gcb.13451.

ORNL TES SFA, ACME, OLCF

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