Deborah Rabinowitz

Factors Affecting Frequency Distributions of Plant Mass: The Absence of Dominance and Suppression in Competing Monocultures of Festuca Paradoxa

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.

Seed rain in a North American tall grass prairie.

(1) In order to measure total annual seed rain for a native grassland in North America, fifty 9 cm diameter sticky traps were exposed to collect dispersing propagules for 26 weeks in 1978. (2) 19 700 seeds m-2 were trapped (per trap for ln-transformed data, x = 126, x s = 91, x + s = 173). This figure is much higher (by two orders of magnitude) than previous reports for non-grassland habitats, but is an underestimate because vegetation partially obscured the trap surfaces and because dispersal continued into the winter when the traps became snow-covered. (3) There were two peaks of dispersal activity: one in early summer for graminoid species and another in early autumn for both dicots (mostly Solidago spp. and Aster spp.) and warm season grasses. The number of seeds falling per week is weakly correlated with the strength of wind for the previous week. (4) Thirty taxa were identified to species, and two taxa were lumped, probably containing an additional nine species. The distribution of the number of seeds caught among species appears uniform for logarithmic classes (strongly right skewed for arithmetic classes). (5) The seed rain was spatially patchy, and this result may strongly influence subsequent seedling interactions. (6) The species composition of the seed rain qualitatively resembled the flowering community much more closely than it resembled the seed pool in the soil. There is a 71 % reduction of seed density from the rain to the soil pool.

Competitive Abilities of Sparse Grass Species: Means of Persistence or Cause of Abundance

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...