PS 28-90 - CANCELLED - The influence of herbivory and ecological site on the persistence of a native perennial cool-season grass: An exploration from the axillary bud to the community level.

Grasslands in the Great Plains of the US and Canada have evolved under a history of grazing. However, managing grazing in the context of both livestock and wildlife herbivory is challenging. In addition, Great Plains grasslands are composed of several ecological sites which contain different soils, soil depth and topographic position further complicating the response of perennial grasses to herbivory. We evaluated how Pascopyrum smithii [(Rydb.) Á. Löve] (PS), a native perennial C₃ grass present throughout our research site, responded to herbivory by either black-tailed prairie dog (Cynomys ludovicianus), a burrowing, herbivorous, keystone wildlife species, cattle (Bos taurus), a domesticated large herbivore, or both on loamy (Ly) and thin claypan (Tcp) ecological sites. By placing 30x30 m exclosures, on- and off- prairie dog colonies, we created 4 different treatment combinations on each ecological site. Treatments were 1) no cattle or prairie dog grazing (CON); 2) prairie dog grazing but no cattle grazing (PDNC); 3) cattle grazing but no prairie dog grazing (CNPD) and 4) both cattle and prairie dog grazing (PDC). PS response variables included 1) axillary bud counts, 2) tiller demography, and 3) contribution to species composition. The data were collected from 2013 to 2016.

Results/Conclusions

There were no differences between either the different grazing treatments or ecological sites on the numbers of axillary buds per PS tiller. New tillers and rhizomes produced per tiller were adequate to ensure population persistence (0.99, 1.14, 1.17, and 1.2 for PDC, PDNC, CNPD and control; respectively, and 1.07 and 1.17 per tiller for the Tcp and Ly ecological sites; respectively). However, tiller replacement ratios (TRR) or number of new tillers recruited at the plant population level never rose above replacement levels ( < 1) for the Ly ecological sites and the PDNC grazing treatment during the study period. In 2015, TRR for all treatments and ecological sites was > 0.5. The low tiller recruitment in 2015 was reflected in the number of tillers per m² which exhibited a sharp decline for most herbivory treatments on both ecological sites. The TRR decline may also have contributed to the 3 to 5% reduction in PS as part of the relative species composition. While TRR suggests that environmental conditions impact recruitment of new PS tillers, the PS survivorship curves and median tiller lifespan indicate longer survivorship allows these tillers to persist in unfavorable conditions.