FITNESS COMPONENTS OF GRAZING-INDUCED POPULATION REDUCTION IN A DOMINANT ANNUAL, TRITICUM-DICOCCOIDES (WILD WHEAT)

1 Intensive cattle grazing reduces population density of the dominant annual Triticum dicoccoides (wild wheat) in Mediterranean grasslands i n northern Israel. An experiment was designed to quantify the impact o f grazing on various components of plant fitness in the growing season that may contribute to this reduction. Height, tiller number, surviva l and reproduction of marked plants were monitored throughout two grow ing seasons in paired grazed and protected plots and in recently defol iated and undefoliated plants in grazed plots. 2 Plant survival in the vegetative stage was similarly high in grazed and protected plots in 1990-91, but in 1991-92 it was 14% higher in protected plots. Plants i n protected plots were 60-172% taller than those in grazed plots, but plants in grazed plots had 44-107% more live tillers at the end of the vegetative stage. Tiller initiation was enhanced in undefoliated as w ell as defoliated plants within grazed plots, suggesting that the incr ease may have been a response to the modified radiation environment in the grazed community. 3 The proportion of plants that produced mature inflorescences was 51-59% greater in protected than in grazed plots, and within grazed plots it was much lower among plants that had been g razed in the last month of the season. The number of mature infloresce nces per fertile plant was not consistently different between treatmen ts, but the number of immature inflorescences was greater in grazed pl ots. Apparently, following removal of inflorescences late in the growi ng season, insufficient resources remained for maturation of secondary inflorescences. Spikelet number per inflorescence was 21-42% greater in protected than in grazed plots, which may reflect effects of grazin g in the vegetative stage on inflorescence development. Seed number an d weight per spikelet were not significantly affected by grazing. 4 Th e number of mature spikelets or seeds produced per seedling (an estima te of fitness throughout the growing season) was reduced by about 50% in grazed compared to protected plots in both years. The largest consi stent components of reduction were due to removal of maturing inflores cences in the late reproductive stage. 5 Deferment of grazing during t he reproductive stage may be sufficient to maintain abundant populatio ns of wild wheat, while grazing during the remainder of the year will limit their suppressive effects on other annual species.