Fitness components of grazing-induced population reduction in a dominant annual, Triticum dicoccoides (wild wheat)

Intensive cattle grazing reduces population density of the dominant annual Triticum dicoccoides (wild wheat) in Mediterranean grasslands in northern Israel. An experiment was designed to quantify the impact of grazing on various components of plant fitness in the growing season that may contribute to this reduction. Height, tiller number, survival and reproduction of marked plants were monitored throughout two growing seasons in paired grazed and protected plots and in recently defoliated and undefoliated plants in grazed plots. 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 in 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 well as defoliated plants within grazed plots, suggesting that the increase may have been a response to the modified radiation environment in the grazed community. 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 grazed in the last month of the season. The number of mature inflorescences per fertile plant was not consistently different between treatments, but the number of immature inflorescences was greater in grazed plots. Apparently, following removal of inflorescences late in the growing 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 grazing in the vegetative stage on inflorescence development. Seed number and weight per spikelet were not significantly affected by grazing. The number of mature spikelets or seeds produced per seedling (an estimate of fitness throughout the growing season) was reduced by about 50% in grazed compared to protected plots in both years. The largest consistent components of reduction were due to removal of maturing inflorescences in the late reproductive stage. Deferment of grazing during the reproductive stage may be sufficient to maintain abundant populations of wild wheat, while grazing during the remainder of the year will limit their suppressive effects on other annual species.