I. Noymeir et Dd. Briske, FITNESS COMPONENTS OF GRAZING-INDUCED POPULATION REDUCTION IN A DOMINANT ANNUAL, TRITICUM-DICOCCOIDES (WILD WHEAT), Journal of Ecology, 84(3), 1996, pp. 439-448
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.