PS 6-55 - Assessing temporal stability of plant community composition in grazed and ungrazed grasslands

Background/Question/Methods

The tallgrass prairie is a highly productive and diverse grassland in the eastern Great Plains frequently used for grazing by large herbivores such as cattle and bison. In tallgrass prairie, plant community composition is dependent on the presence or absence of grazers. Grazed tallgrass has higher species richness and an abundance of forbs, and ungrazed tallgrass has lower species richness and is dominated by C4 grasses. While grazing is known to lead to more diverse plant communities, the temporal stability of community composition is less well-studied. In order to investigate whether grazed or ungrazed plant communities are more stable over time, this project analyzes nine years of plant community composition data from a long-term grazing exclosure experiment at the Konza Prairie Biological Station, a native tallgrass prairie preserve in Kansas. Three strings of 7 blocks each were established in 2005, with each block consisting of two plots (1 large-herbivore exclosure, 1 paired plot open to grazing). Plant community composition was collected each year from 2006 to 2014. I used multivariate techniques (PERMANOVA and non-metric multidimensional scaling (NMDS)) to investigate whether the community composition of grazed or ungrazed plots was more stable in multivariate space over time.

Results/Conclusions

As expected, the ungrazed plots changed rapidly immediately after the fence was set up while the grazed plots remained more stable. This is due to the plant communities within the grazing exclosures transitioning from a diverse grazed community to a less diverse ungrazed community during the first few years after grazers were excluded. However, after ~4 years, ungrazed plant communities were more stable than grazed communities. Around that time, the ungrazed communities were strongly dominated by C4 grasses, contained few forbs, and experienced very little species turnover. Interestingly, a summer drought occurred in 2012 naturally, and we saw larger community shifts in the ungrazed plots. So, while the ungrazed plots appear to be more temporally stable under nominal climate conditions, ungrazed grasslands may be more sensitive to drought.