Modeling distributions of C3 and C4 plants during glacial/interglacial periods

Jed O. Kaplan

Max Planck Institute for Biogeochemistry, Jena, Germany

jed.kaplan@bgc-jena.mpg.de

The distributions of C3 and C4 plants have changed greatly over glacial-interglacial cycles, with implications for the global carbon cycle, biophysical feedbacks, and consequent effects on animals and humans. Modeling C4 plant distributions has evolved from simple correlations with a small number of climate variables to fully mechanistic representations of both C3 and C4 plant growth and their competitive dynamics. In this study, C3 and C4 plants are parameterized at the plant functional type level and implemented in two global vegetation models. An equilibrium vegetation model

(BIOME4) represents C4 plants as three functional types: tropical grass, temperate grass, and woody shrub. I present simulations with BIOME4 at the present and 21,000 yr. BP (the Last Glacial Maximum). At the LGM the cover of C4-dominated vegetation was expanded compared to the present, especially in the dry tropics and the mid-latitude continental deserts. A second suite of simulations was performed using the LPJ dynamic vegetation model. This model has a single C4 plant functional type, a generic grass. The model simulated vegetation distribution continuously from 21,000 yr. BP to the present, driven by a series of GCM climate simulations. In the dynamic simulations, C4 vegetation was not as widely distributed at the LGM as in the equilibrium runs, but the C4 grass type does tend to expand northward during the deglaciation in both North America and

Eurasia. The area of C4 vegetation may be underestimated in these simulations because of the simple parameterization in the model. Both models indicate that the distribution and importance of C4

vegetation at the glacial compared to the present resulted in lower terrestrial carbon storage, more enriched isotopic composition of the biosphere, an earth surface dominated by grass and shrublands, and with habitat more suitable for grazing animals and the humans that depended on them.