Effects of plant morphology and emergence time on size hierarchy formation in experimental populations of two varieties of cultivated peas (Pisum sativum)

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