Grazing by large herbivores influences grasslands through herbivory, physical impacts, and deposition. These pathways can manifest at local or ecosystem levels through changes in vegetation, soil, and nutrient cycling. Planted pastures, common in the southeastern United States, are frequently established with a single forage species, primarily for forage production. This study aimed to understand how long-term grazing exclusion might change the composition and structure of plant communities and soil characteristics in these highly managed pastures. Research was conducted at the MacArthur Agro-ecology Research Center at Buck Island Ranch near Lake Placid, Florida. The site has an average annual rainfall of 130 centimeters, a mean annual temperature of 22° C, and poorly drained sandy soils. Data was collected from grazing exclosures installed in 2002 in four bahiagrass pastures, designed as 10 x 10 meter fenced plots with adjacent paired plots open to grazing. In 2014 canopy cover, height, and light availability were quantified in randomly selected subplots within each plot. Soil sores were collected to a depth of 30 centimeters, separated into four depth increments, and analyzed for bulk density, and concentrations of carbon and nitrogen.
Results/Conclusions
Grazing exclusion caused significant shifts in the relative contributions of different plant functional groups to plot composition. In grazed plots, the contribution of bahiagrass and other graminoid species to total canopy cover was constant between 2002 and 2014 (>95%), while fenced plots shifted from graminoid dominance to a mixture of graminoids (56%) and shrubs and vines (33%). Differences in aboveground functional composition coincided with differences in belowground allocation, with almost twice as much biomass in the top 5 centimeters of grazed (6.13 ±0.34 g cm2) compared to fenced (3.84 ±0.36 g cm2) plots. Grazing exclusion and soil depth significantly affected soil carbon stocks. Carbon stocks were highest in the top 5 cm in both the grazed (3.15±0.16 kg m2) and fenced (2.36 ±0.13 kg m2) plots, but were 25% higher in the grazed plots. Thus, shifts in functional composition following grazing exclusion altered the structure of the plant communities above and below ground. Most importantly, changes in belowground productivity in fenced plots coincided with a significant loss of soil carbon. These findings document a positive relationship between grazing, plant community structure, and soil carbon storage in bahiagrass pastures, demonstrating the potential for highly managed grasslands to provide multiple ecosystem services.