Search

The number of flowering stems for 82 species on a transect 2 x 400 m was counted twice weekly during 1978 at Tucker Prairie, Callaway County, Missouri, USA, a tall-grass prairie remnant. Phenological curves (number of flowering stems vs. day of the year) are narrower (smaller standard deviations) for wind-pollinated species than for insect-pollinated species. Symmetry (g ) does not differ significantly for wind- and insect-pollinated species. The curves are largely either symmetric or begin abruptly and end gradually (right-skewed). Overlap, measured as the fraction of the area under the curves occupied by two or more species, does not differ significantly for wind- and insect-pollinated species, among 20 groups of five randomly drawn species. Dates of peak flow- ering are distributed randomly over the season (May to November 1978) and their distributions do not differ for wind- and insect-pollinated species. Dates of peak flowering are also randomly distrib- uted for grasses, legumes, and composites considered separately. Thus, although species with differ- ent pollination modes show different shapes for phonological curves, the species aggregated into the "community" do not have ensemble patterns of temporal dispersion or overlap and cannot be dis- tinguished from a random collection.

Sparse species have chronically small local population sizes, even though they occur in several habitats over a wide geographic range. Greenhouse de Wit replacement series with seven species of sparse and common perennial grasses of tallgrass prairie were performed with seedlings and tiller fragments for 5, 10, and 15, mo. As younger and older seedlings, sparse grasses overyielded and were advantaged by the interaction with common grasses. The common grasses underyielded and were disadvantaged in mixture with sparse grasses. As tillers, the interaction was less antagonistic, and both common and sparse grasses either overyielded or were unaffected by the interaction. Seedlings of sparse species were largest when planted in low proportion, surrounded by individuals of a common grass. Because the sparse species are not disadvantaged by interactions with their common neighbors, their competitive abilities are not implicated as a cause of their local rarity. Rather, the good competitive abilities of these spar...

In even-aged monocultures, distributions of seedling mass shift from symmetric to right-skewed as plants grow. One explanation for the shift is that numerous small individuals and few large ones reveal underlying dominance and suppression caused by depletion of resources. A second explanation is that the asymmetry represents a shift from normal to lognormal mass distri- butions because of variance in exponential growth rate. As a consequence of the first mechanism but not the second, the presence of resource depletion should increase both the skewness and the variance of distributions of individual plant mass. Even-aged populations of the prairie grass Festuca paradoxa were grown in the greenhouse for 44 d with and without competition (dense flats or one individual to a container), and with and without augmented nutrients. At each harvest, plants grown densely were smaller than isolated plants, dem- onstrating that resource depletion occurred. Distributions of mass of isolated plants skewed first and remained more highly skewed throughout the experiment. At a given mean mass, the standard de- viation of mass for crowded and isolated plants did not differ. At a given mean mass, the skewness of mass for crowded plants was not greater than for isolated ones. The second explanation is sup- ported, and shifts in distribution shape can be attributed to growth alone, in the absence of dominance and suppression. Competition in this case did not promote skewing but actually retarded its appear- ance. Consistency of these results with previous findings is discussed.