PS 45-192 - What makes a grassland? A synthesis of 252 grassland sites determining grass and forb contribution to biomass and biodiversity in grazed and ungrazed grassland

Abstract: In grasslands a frequent assertion is that grasses make up the majority of plant abundance (i.e., biomass, cover), while the herbaceous flowering non-grass plants (forbs) are responsible for grassland biodiversity. However, no study to date quantitatively tests this commonly held belief at a global scale. Additionally, this statement oversimplifies dynamic plant community differences between grazed and ungrazed grasslands. I hypothesize that, in ungrazed grasslands, grasses make up the majority of biomass while grassland plant biodiversity is due to the many forb species present (in other words, that this assumption is correct). However, in grazed grassland, I hypothesize this assumption may not hold true. Grazing in grasslands that are dominated by palatable grasses leads to a reduction in grass biomass and an increased abundance of forbs. To test this hypothesis in both grazed and ungrazed grasslands from across the globe, I synthesized data from large-grazer-exclusion experiments in 252 grassland sites and determined the respective contributions of grasses and forbs to relative aboveground cover and species richness.

These 252 sites span a wide range of locations (6 continents) as well as large gradients in mean annual precipitation (MAP; 45 to 1511 mm), production (ANPP; 13 to1204 g/m2), and site level regional richness (species number; 3 to 179 species). In ungrazed grasslands, my hypothesis is supported: on average and for the vast majority of sites, grasses account for the majority of aboveground plant cover, while forbs are responsible for species richness. However, this pattern is not consistently seen in grazed grasslands, possibly because whether biomass and diversity are driven by grasses or forbs depends on the palatability of the dominant species. Overall, this work shows that a common assumption regarding the makeup of grassland plant communities is true in ungrazed grasslands but may not accurately describe grazed grasslands where forb species may contribute significantly to both diversity and production. This is an important distinction, as grazing is a critical process in grassland systems, and many grasslands are grazed by either domestic or wild large grazers.