For over three decades there has been debate about whether grazing increases productivity by stimulating plants and allowing more light to reach the soil surface, or whether grazing reduces productivity as plants are unable to compensate for lost photosynthetic tissue and resources such as nitrogen and water. However, because a large amount of ecological research has been done in the absence of native grazers that have been locally extirpated across much of their historic range, we still lack an understanding of the role of large grazers in many ecosystems, especially their impacts on productivity. Here, we ask whether grazing by bison in Minnesota oak savanna increases or decreases productivity, and whether other factors such as fire frequency and the dominant plant cover type (shrub or grass) interact with grazing to affect productivity.
In the summer of 2018, 32 bison were released into an oak savanna at Cedar Creek Ecosystem Science Reserve (East Bethel, MN). The bison were allowed to freely graze the savanna for the entire growing season. To measure productivity in grazed and ungrazed plots we compared peak aboveground biomass in permanent ungrazed plots to plant biomass measured inside and outside moveable exclosures that were moved every three weeks to capture repeated grazing and regrowth.
Results/Conclusions
We found that aboveground productivity was higher in grazed plots than ungrazed plots by 49% on average (p=0.05). This is likely because grazing by bison allows more light to reach the ground stimulating growth when adequate nutrients and water are available to plants. However, contrary to our hypotheses, neither dominant cover type nor fire frequency modified the relationship between grazing and productivity. Our findings illustrate the importance of considering grazing in natural ecosystems as large herbivores may influence important processes such as productivity, altering the flow and stocks of carbon and nitrogen in these ecosystems. Our work here can help inform grazing management decisions and further our understanding of which conditions might promote or constrain productivity in grazed systems.