Ecological sites serve as frameworks for developing state-and-transition models (STM) which help guide management decisions on rangelands. Understanding how management and environmental factors impact plant demography can help develop STMs for different ecological sites. A project near Mahto, SD, USA evaluated the impact of herbivory level on two ecological sites, a Loamy (Ly) and a Thin Claypan (Tcp). Several 10-year or older prairie dog (Cynomys ludovicianus) colonies were located on the study site which was also grazed by cattle (Bos taurus). Study sites (25 x 25 m), both with and without prairie dogs, were identified on both ecological sites, and cattle grazing was regulated using exclosures. Treatments were 1) grazed with both cattle and prairie dogs (CD); 2) grazed with cattle but no prairie dogs (CND); 3) grazed with prairie dogs but not cattle (NCD) and 4) not grazed by either cattle or prairie dogs (NCND). Each treatment was replicated three times on each of the two ecological sites. Within each study site, all western wheatgrass tillers [Pascopyrum smithii (Rydb. Á. Löve)] within four randomly located sub-plots (15 x 15 cm) were counted and new tillers were marked with unique colored wires at each census period. Census periods were in the spring and fall of 2014, 2015 and 2016 and dead and lost along with newly recruited daughter tillers were recorded. Tiller tiller-1 recruitment ratio was determined for each census period by dividing the number of new tillers by existing live tillers. A tiller tiller-1 ratio > 1 indicates a growing tiller population.
Results/Conclusions
Tiller tiller-1 ratios had year X treatment and ecological site X treatment interactions. Tiller tiller-1 ratios differed between treatments in 2014 and 2016 but not 2015. Only the CND and NCND treatments in 2014 and the CD treatment in 2016 had tiller tiller-1 ratios > 1; the Ly ecological site never had recruitment ratios > 1. There was an ecological site X grazing intensity interaction for tiller survivorship, with greater western wheatgrass tiller survival on the Tcp when no grazing was present. Mean survivorship was greatest for NCD tillers on the Tcp ecological site (358 days) and least for NCND tillers on the Ly ecological site (278 days). These data suggest tiller recruitment is environmentally controlled but mortality is impacted by grazing. Longer survival in stressful conditions may be a persistence strategy for western wheatgrass.