The effects of fire frequency and grazing on tallgrass prairie plant composition and productivity are mediated though bud bank demography

Periodic fire, grazing, and variable climate are key drivers of tallgrass prairie ecosystems, having large impacts on the component species, ecosystem structure and function. Though the patterns of change in composition and productivity in response to fire and grazing have been well described for tallgrass prairie, the underlying mechanisms are largely unexplored. Using long-term experiments at Konza Prairie, we tested the hypothesis that the effects of fire and grazing on plant species composition and ANPP are mediated principally through demographic effects on bud banks (belowground meristems associated with rhizomes). Our 3-year data set indicates that plants are responding to fire and grazing with altered rates of belowground bud natality, bud emergence, and both short-term (fire cycle) and long-term changes in bud bank density. Our prediction that grass bud banks will increase with increasing fire frequency while forb bud banks will decrease was supported in the absence of grazers. Grazing increased the rate of emergence from the grass bud bank, resulting in decreased grass bud banks compared to ungrazed prairie. By contrast, grazing increased bud density of forbs in annually burned prairie, but grazing had no effect on forb bud density in the four-year burn treatment. Fire and grazing strongly interact in their regulation of belowground bud bank dynamics. The size of the bud bank is an excellent predictor of long-term ANPP (P = 0.006, r² = 0.99), supporting our hypothesis that ANPP is strongly regulated by belowground demographic processes. Meristem limitation due to management practices such as fire or grazing may constrain tallgrass prairie responses to inter-annual changes in resource availability. An important consequence is that prairie with a large bud bank may be the most responsive to future climatic change or other phenomena such as nutrient enrichment, and may be most resistant to phenomena such as exotic species invasions.