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We observed colonization and mortality of plants in small (26 m2) artificial soil disturbances and undisturbed controls in a tall-grass prairie from 1977-1980. This paper examines the dynamics of colonists based on mode of origin (seedlings and shoots) and broad taxonomic affinity (forbs and graminoids). Few seedlings colonized either disturbances or controls the first year, probably due to a severe drought in 1976 when few seeds were produced. Graminoid shoots were more abundant in disturbed sites than in controls the first year, probably due to the sSedge effect" of cutting roots and rhizomes and stimulating new growth. For each year following the first, the number of current residents peaked early in the season and then declined, with the decline more precipitous for seedlings than for shoots. The population fluctuations of seedlings were very predictable, and separate years resembled each other. In contrast to seedling populations, graminoid shoot populations generally were accumulating throughout the study. Populations of forb shoots rose and fell, but the years did not resemble each other. Neither seedlings nor forb shoots showed a response to the new uncolonized soil resource of the artificial disturbances compared to controls, but graminoid shoots were more common on disturbed sites. These results suggest a nonequilibrium dynamic among colonizers of small-scale microsites.

Seven grass species on an abundance continuum from common to sparse on a Missouri tallgrass prairie were examined for two diaspore traits (morphology and weight), for three dispersal traits (terminal velocity, and mean and maximum lateral movement propagules released in still air from a height of 1 m), and for two parental traits (culm biomass at maturity and culm height at dispersal). Dispersal ability, measured in the laboratory as the lateral movement of propagules in the upper 5% (right tail) of the distribution of distances, is inversely correlated (r, =-0.94) with biomass abundance. Under constant conditions, seeds of sparse species travel farther than seeds of common species. Dispersal ability is more closely related to a populational attribute (abundance) than to a more proximate characteristic (diaspore weight). Diaspore weight is closely related (r, = 0.97) to culm biomass of the parent, in apparently simple allometry, and more weakly related (r, = 0.71) to biomass abundance. Large parents make large seeds. The sparsest species, which is small in stature, has light (0.08 to 0.19 mg), flattened diaspores with low terminal velocities (82 to 87 cm sec-1) and achieves lateral movement (up to 460 mm) by tumbling and gliding. Species of intermediate abundance have intermediate stature and diaspore weight (1.0 to 1.8 mg), unornamented propagules, high terminal velocities (191 to 209 cm sec-1) and intermediate dispersal ability (up to 388 mm). The three most common species have heavy propagules (2.2 to 2.8 mg), ornamented with long hairs. Although the ornamentation slows fall (96 to 194 cm sec-1), it acts as a "guide parachute," orienting the propagule so that it falls nearly straight downward. The common species, which morphologically appear adapted to longer dispersal, achieve little lateral movement (up to 235 mm) under laboratory conditions. Terminal velocity is a misleading descriptor of dispersal because diaspores with similar terminal velocities have differing movement patterns. When diaspores of two species (one tall common species and one short sparse species) are released in still air from their natural height, differences in maximum fall distance observed under constant height of release are lost. When dispersal is observed in the field by means of sticky seed traps, the slopes of the dispersal curves (regression coefficient of ln-transformed number of seeds caught on distance from the clump) do not differ from common and sparse species, despite the fact that seeds of the common species are released from greater heights. The orderly differences in dispersal behavior among species due to subtleties of morphology observed under laboratory conditions are masked in

The effects of plant form and emergence time on size hierarchy formation in populations of two morphologically and genetically distinct varieties of peas (leafless and leafed) were studied. There were no significant differences in germinability between the two varieties, although leafless peas imbibed more rapidly than the leafed ones did. Monocultures of leafed and leafless peas were established at two densities: plants grown alone in small pots and plants grown at 576 m-2. Time emergence was noted, and plant shape, biomass and seed production were measured at two-week intervals for ten weeks. Seedlings emerged continually over an eight-day period, and two cohorts of seedlings were distinguished (seedlings emerging 6-7 days after planting, and seedlings emerging > 7 days after planting). Dominance and suppression were observed in the high-density populations, and early-emerging plants had less hierarchical biomass distributions than did late-emerging ones. Although leafless peas were larger and suffered less mortality than leafed ones did at identical densities, there were no differences in the degree of size inequality between the two genotypes (emergence cohorts pooled), or within emergence cohorts between genotypes. The degree of size inequality increased with time among dominant individuals and decreased with time among suppressed individuals. These results broadly support Weiner and Thomas's (1986) hypothesis that plant form may affect the extent but not the existence of competitive asymmetry in plant populations. THE PURSUIT of causal mechanisms that generate unequal size distributions ("size hierarchies") in plant monocultures has been a recurrent theme in plant population biology over the last thirty years (e.g., Koyama and Kira, 1956; Ford, 1975; Harper, 1977; Rabinowitz, 1979; Gates, 1982; Turner and Rabinowitz, 1983; Hara, 1984, 1986a, b; Weiner, 1985; Benjamin and Hardwick, 1986; Weiner and Thomas, 1986; Ellison, 1987a). Two hypothI Received for publication 25 April 1988; revision accepted 15 September 1988. We thank Dr. E. T. Gritton (Univ. of Wisconsin) for supplying us with the peas, Dr. G. Marx (Cornell Univ. Agric. Exp. Station) for advice on growing peas, Dr. A. R. Watkinson (Univ. of East Anglia) for providing us a copy of Dr. Butcher's dissertation, and Dr. P. Dixon (Savannah River Ecology Laboratory) for providing the Gini coefficient program. Beth Farnsworth and Cliff Cunningham assisted with planting. Debra Vam Vikites and Steve DiCamillo provided invaluable assistance in harvesting plants and/or data analysis. Jeannie Ngai maintained the germination experiment. Phil Dixon, Beth Farnsworth, Sana Gardescu, Laura Jackson, and Jake Weiner provided constructive criticisms of early drafts of this manuscript that resulted in a much-improved final product. Supported by McIntyre-Hatch-Stennis funds through contract NY(C)183419. 2 Current address: Organizacion para Estudios Tropicales, Estacion Biologica La Selva, Apartado Postal 676, 2050 San Pedro de Montes de Oca, Costa Rica. 3Deceased 18 August 1987. eses that explain the genesis of size hierarchies in dense monocultures have been proposed: the dominance and suppression hypothesis, and the growth rate hypothesis (reviewed in Turner and Rabinowitz, 1983; Weiner and Thomas, 1986). Briefly, if dominance and suppression are occurring, then size distributions of individuals in dense, competing stands should be more hierarchical (sensu Weiner and Solbrig, 1984) than size distributions of noncompeting individuals. If, on the other hand, intrinsic differences in plant growth rate alone generate observed size hierarchies, there should be no difference between the degree of inequality among individuals of equal size in competing and noncompeting stands. While these two hypotheses are often considered to be mutually exclusive because each has a different predicted outcome (e.g., Turner and Rabinowitz, 1983), it is likely that growth rate and competitive effects interact to produce observed variation in plant sizes (Benjamin and Hardwick, 1986; Ellison, 1987a). To date, of the dozens of studies that explicitly have examined hierarchy development in plant populations (reviewed in Benjamin and Hardwick, 1986; Ellison, 1986; Weiner and Thomas, 1986), only two have supported the growth rate hypothesis. Turner and Rabinowitz (1983) found no differences in the

Biomass abundance and distance from a random point to the nearest flowering culm were measured during two seasons for eight species of native perennial grasses on a continuum from locally rare to common in a Missouri tall prairie. Mean abundance from the most common to the most rare ranges over three orders of magnitude (nine octaves) from 73 to .25 g/m2. The means of the distance to a culm range over two orders of magnitude (four octaves) from 0.3 m to 5.0 m. The logarithm of distance to a culm is linearly correlated with the logarithm of abundance. Distance to an inflorescence is greater for sparse species than for common species.