Process-based vegetation models often represent functional diversity in natural grasslands with two plant functional types (FPTs), based on key differences between grasses using the C3 or C4 photosynthetic pathways. C4 grasses dominate in (sub)tropical grasslands and savannas while C3 grasses dominate in temperate and cold climates. Grasslands and savannas cover some 40 % of the Earth’s terrestrial surface and dominance by C4 versus C3 grasses has major consequences for gross primary productivity and ecosystem structure and function. Recent global-scale studies highlight that functional trait variation within biomes arises from evolutionary histories that vary biogeographically, leading to species with differing ecological behavior. Only around 600 of the roughly 11,000 grass species are ecologically dominant and these species cluster phylogenetically into two C4 lineages and one C3 lineage. Beyond their photosynthetic types, these lineages have key differences such that they occupy different climates, and are unlikely to have a uniform response to future climate. We argue for greater consideration of evolutionary history due to the phylogenetic constraints on traits within lineages and where different lineages dominate in different regions. We assessed grass PFTs in a selection of three modern vegetation models by comparing fore/hind-casts of C4 grass distributions to vegetation plot data and isotope proxies. Then, we used a global dataset of grassland botanical records to delineate the distributions of each lineage. Finally, we developed a set of parameters, from the literature, describing the functional attributes of each grass lineage.
The global vegetation models compared in this study differed substantially in their predictions of C3 and C4 grass dominated vegetation as well as from the distribution of modern vegetation. We found that across a spectra of physiological, structural, biochemical, optical, as well as disturbance related (e.g., fire) and phenology related traits there was wide variation among lineages. Indeed, lineage explained more trait variation among grass species than did photosynthetic type categories. We relate the functional parameters of each lineage to their niche and found that the addition of functional variation among groups of species with shared evolutionary histories will be an important advance in Earth System modeling of grass-dominated biomes. We discuss these findings in the context of functional trait ecology and provide suggestions for implementing our approach in other PFTs and biomes.