14. Ten Years of Vegetation Observations on Formerly Grazed Oklahoma Grassland

In summer 2003, we established a series of 40 adjacent, 10310-m blocks in formerly grazed grassland in southcentral Oklahoma. The blocks were allowed to rest and received no grazing, mowing, or burning. We tracked changes in the vegetation of the site over time between 2005 and 2015. Frequency sampling of prairie vegetation was performed at irregular intervals. In spring 2006 we seeded half the site with a Texas/Oklahoma prairie forb mix. We found no significant trends of change in species richness or diversity over time. However, there were subtle changes in abundance of individual species. Across fall sampling periods, Shannon diversity ranged from 1.1 to 1.3, and species richness ranged from 29 to 41, with higher richness and diversity in the sole spring sample. The percentage of nonnative species present at the site ranged from 13 to 18%. The earliest samples showed that the dominant grass species were little bluestem (Schizachyrium scoparium (Michx.) Nash), three-awn (Aristida oligantha Michx.), and Scribner’s panic grass (Dichanthelium oligosanthes (Schult.) Gould). The dominant forb present was ragweed (Ambrosia artemesiifolia L.). By the end of the 10-year time period, little bluestem had declined in relative frequency and Scribner’s panic grass and three-awn had increased in frequency. Among forbs, sumpweed (Iva annua L.) had entered the site and become a dominant forb, heath aster (Symphyotricum ericoides (L.) G.L. Nesom) increased in frequency, and ragweed abundance fluctuated over time. The most commonly encountered nonnative species were Bermuda grass (Cynodon dactylon (L.) Pers.), singletary-pea (Lathyrus hirsutus L.), and sericea lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don.), but none of these species increased in abundance over time. In addition, there was anecdotal evidence of woody species encroachment on the site, mostly winged elm (Ulmus alata Michx.), wild plum (Prunus sp.), and red cedar (Juniperus virginiana L.). Establishment of the prairie mix appears to have been unsuccessful. In this study, cessation of grazing (‘‘resting’’ a site) alone did not allow for recovery of prairie vegetation and may have permitted invasion of undesirable herbaceous and woody species.