Plant competition, abiotic, and long- and short-term effects of large herbivores on demography of opportunistic species in a semiarid grassland.

The emergence and subsequent survival and growth of five opportunistic "weeds" were monitored after seed additions to long-term grazing treatments with or without current-year grazing, long-term ungrazed treatments, and removal treatments designed to eliminate plant competition from existing perennials while either leaving vegetation and soil structure unaltered or disturbed. The treatments were applied on both uplands and lowlands to assess the relative influence of macroabiotic environment versus plant competition. The long-term effects of large herbivores on the initial emergence of seedlings were greater than the effects of removing competition. Very few individuals emerged on the long-term grazed treatments that were either grazed or ungrazed during the experiment. Numbers of individuals emerging on the long-term ungrazed treatments were greater or equal to those emerging on the no-competition-undisturbed treatments, but numbers were greatest on no-competition-disturbed treatments. None of the seeded individuals on the long-term grazed, currently grazed treatments survived to the end of the growing season. There was a slightly greater end-of-season biomass of seeded species and percentage of the total population reaching reproductive status on the long-term ungrazed compared with grazed-nondefoliated treatments, and very high survival, biomass, and proportions of reproductives on both no-competition treatments. Cover types in the immediate vicinity of seedlings influenced both germination and survival, but the effects differed between species and treatments. Equal compensation to current-year herbivory occurred on long-term heavily grazed treatments even though above-ground production was much greater on long-term protected sites. Productivity varied with topography, but very few topographic main effects or interactions occurred with demographic variables of seeded species, suggesting that macroabiotic effects were of minor importance compared with grazing and plant competition.