Livestock grazing has been shown to benefit low-stature grassland plants, as grazing can keep tall exotic annual grass cover and thatch low, thereby reducing competition for light at the soil surface. As a result, short-term studies suggest that cattle grazing can help to conserve native annual forbs, which comprise much of the native richness in northern California coastal prairies. Recent field and lab research by others has indicated that native annual forbs in California grasslands may be particularly sensitive to drought and increasing aridity, and that this group has declined in some locations over the last 15 years. There have been few longitudinal studies examining the long-term, interactive effects of grazing management and precipitation on California coastal prairies. This study is a longitudinal follow-up on a 2000-2001 comparison of native annuals forbs in grazed and ungrazed northern coastal prairies. We resampled sites from Monterey through Sonoma County in 2016 and compared results to the earlier surveys to determine whether the effects of grazing on native annual forb species richness and cover has remained consistent over time.
Shrub cover has increased substantially (21.8%) in ungrazed grasslands and a small amount in grazed grasslands (4.2%) between 2001 and 2016. Although there continued to be significantly greater native annual species richness in grazed than ungrazed grasslands in 2016, there was a marginally significant treatment × time interaction. Average species richness declined from 9.1 ± 2.2 to 6.0 ± 1.4 species in grazed prairies, and remained nearly constant in ungrazed prairies (4.1 ± 1.7 to 4.3 ± 1.5 species). Native annual forb cover varied greatly across sites, and there was no significant effect of grazing management or time on native annual forb cover. Overall, it appears that differences in native annual forb richness between grazed and ungrazed prairies may be shrinking. Though this study does not identify the causes of this trend, reduced grass cover associated with the severe drought that California experienced from 2012-2014 and increased small mammal seed herbivory associated with greater shrub cover are potential explanations. Our results suggest that cattle grazing helps maintain native annual forb diversity and reduce shrub cover, but that the cover and richness of native annual forbs is strongly affected by other factors, including variability in annual precipitation and localized site conditions.