May 24, 2017

Can Models Predict Grassland Responses to Environment?

Challenging terrestrial biosphere models with data from the long-term multifactor prairie heating and CO2 enrichment experiment.

The Science

Researchers challenged ten carbon cycle models, often used to simulate ecosystem responses to environmental change, to simulate a grassland in Wyoming subjected to experimental carbon dioxide (CO₂) enrichment and increased temperature.

The Impact

Carbon cycle models used for regional or global simulations are known to perform poorly when used to simulate a specific site. Researchers identified a number of areas for carbon cycle model improvement. Model development to improve the accuracy of grassland simulations should focus on improving the realism of the controls of water availability on growth and soil nitrogen in these nonforested ecosystems.

Summary

Multifactor experiments are often advocated as important for advancing terrestrial biosphere models, but this claim is rarely tested. As part of the U.S. Department of Energy–supported Free Air CO₂ Enrichment Model Data Synthesis (FACE-MDS) project, researchers aimed to investigate how a CO₂ enrichment and warming experiment can be used to identify a road map for carbon cycle model improvement. Researchers found that the ten models tested simulated a wide spread in annual aboveground growth in current environmental conditions (i.e., not experimentally manipulated conditions). Comparison with data highlighted that the reasons for these model shortcomings were poor representation of: carbon allocation, seasonality of growth, impact of water stress on the seasonality of growth, sensitivity to water stress, and soil nitrogen availability. In response to the experimentally manipulated conditions, models generally overestimated the effect of warming on leaf onset and were lacking the mechanism to allow CO₂-induced water savings to extend the growing season. However, when both CO₂ and warming were increased, the observed effects of the experimental increase in CO₂ and temperature on plant growth were subtle and contingent on water stress, phenology, and species composition. Since the models did not correctly represent these processes under ambient and single-factor conditions, little extra information was gained by comparing model predictions against interactive responses. The study outlines a series of key areas in which this and future experiments could be used to improve model predictions of grassland responses to global change.

Principal Investigator

Anthony Walker
Oak Ridge National Laboratory
walkerap@ornl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

Free Air CO₂ Enrichment Model Data Synthesis (FACE-MDS) project of the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.

References

De Kauwe, M. G., B. E. Medlyn, A. P. Walker, and S. Zaehle, et al. "Challenging terrestrial biosphere models with data from the long-term multifactor Prairie Heating and CO2 Enrichment experiment." Global Change Biology 23 (9), 3623–3645 (2017). https://doi.org/10.1111/gcb.13643.