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Assessing earth system model predictions of C4 grass cover in North America: From the glacial era to the end of this century

Still, Christopher J., Cotton, Jennifer M., Griffith, Daniel M.
Global ecology and biogeography 2019 v.28 no.2 pp. 145-157
C3 plants, C4 plants, animals, biogeography, carbon dioxide, ecosystems, global change, grasses, grasslands, models, nitrogen, soil carbon, temperature, North America
AIM: C₄ grasses are distinct from C₃ grasses, because C₄ grasses respond in a different manner to light, temperature, CO₂ and nitrogen and often have higher resource‐use efficiencies. C₃ and C₄ grasses are typically represented in earth system models (ESMs) by different plant functional types (PFTs). The ability of ESMs to capture C₄ grass biogeography and ecology across differing time periods is important to assess, given the crucial role they play in ecosystems and their divergent responses to global change. LOCATION: North America. TIME PERIODS: Last Glacial Maximum (LGM), historical modern period (ca. 1850) and end of this century. MAJOR TAXA STUDIED: C₄ grasses. METHODS: Proxy data representing relative cover and productivity of C₄ grasses were collated, including carbon isotope ratios of soil carbon and animal grazer tissue, and vegetation plot data in undisturbed grasslands. We selected available model predictions of C₄ PFT percentage cover. Models were compared against one another and assessed against proxy data at key time points: the LGM, the historical modern period before widespread grassland conversion to agriculture, and the end of this century. RESULTS: We highlight large differences among model predictions of percentage C₄ grass cover across North America: all pairwise combinations have correlations < .5, and most are < .2. Models also do not capture spatial patterns of the percentage C₄ grass cover from proxy data, during either the LGM or the historical modern period. Models generally under‐predict percentage C₄ grass cover, particularly during the historical modern period. MAIN CONCLUSIONS: Earth system models do not accurately represent the biogeography of C₄ grasses across a range of time‐scales, and their outputs do not agree with one another. We suggest model improvements to represent this crucial functional type better, including more collection and greater integration of C₃ and C₄ grass trait data, explicit representations of tree–grass competition for water, and a greater focus on disturbance ecology.