PS 18-249 - The influence of herbivory and ecological site on the persistence of a native perennial cool-season grass. An exploration from seed and axillary bud to the community level.

Grasslands in the Great Plains of the US and Canada have evolved under a history of grazing. However, these grasslands are composed of several ecological sites which contain different soils and topographic position which complicate interpreting how perennial grasses respond to herbivory. Therefore, we evaluated how Pascopyrum smithii [(Rydb.) Á. Löve] (PS), a native perennial C₃ grass present throughout our research site in South Dakota, USA (45.741975^o N; 100.667446^o W), 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. We placed three exclosures (30 x 30 m) and 3 paired non-exclosed sites on each combination of on- and off- prairie dog colonies and the two ecological sites, creating four different treatment combinations that were replicated three times. 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). The PS response variables included 1) soil seed bank cores 2) axillary bud counts, 3) tiller demography, and 4) contribution to species composition. Data were collected from 2013 to 2016.

There were no differences in axillary bud number per PS tiller between different grazing treatments or ecological sites. Seed bank data suggested differences in seedling contribution between ecological sites. Mean tiller lifespan was 368, 368, 375 and 354 days for PDC, PDNC, CNPD and CON; respectively, and 366 and 369 days for the Tcp and Ly ecological sites; respectively. Tiller replacement ratios (TRR) or number of new tillers recruited at the plant population level were low and 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 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 increased mean PS tiller lifespan under grazing indicate longer survivorship allows these tillers to persist in unfavorable conditions.