Grazing in grasslands creates changes in plant community structure.  The magnitude of these changes depends on the productivity and the intensity of grazing.  Low productivity grasslands coupled with high grazing intensity may lead to shrub encroachment in some arid land ecosystems.  We examined the effects of cattle grazing in arid grassland at the Sevilleta LTER site in central New Mexico USA where cattle were removed in 1973 and an area just north of the Sevilleta LTER where grazing by cattle still occurs.  At each site we measured plant species composition and cover in permanent plots in the spring and fall from 2004 to 2007. Quadrats were clipped annually to quantify aboveground standing crop to answer the following questions: 1) Does cattle grazing affect plant species composition and biomass? 2) Does grazing alter community response to inter-annual climate variability? and 3) how does grazing impact the abundance of native grasses. 

Results/Conclusions

Our results showed that: 1) grass, forb and total cover were lower in grazed area for all years.  Total species and forb species richness were higher in un-grazed plots in all years while grass species richness was higher.  Total aboveground biomass was significantly lower on grazed as compared to un-grazed plots.   2) On grazed plots annual precipitation was correlated with lower plant biomass while un-grazed plots did not vary in total plant biomass from year to year. Seasonal precipitation increased grass and forb abundance suggesting positive or neutral interactions between functional types.  3) Black grama (Bouteloua eriopoda) was the dominant grass species in both grazed and un-grazed plots.  Abundance and coverage of B. eriopoda was significantly lower in grazed compared to un-grazed sites.  We conclude that cattle’s grazing reduces grass cover and decreases plant species diversity, and that precipitation interacts with grazing to modulate total biomass in this arid land ecosystem.