Interplay of Morphology and Development on Size Inequality: A Polygonum Greenhouse Study

Increases in size inequalities with time in cohorts of plants have been attributed to two causes: (1) variation in the exponential relative growth rates (RGR) of plants and (2) asymmetry of competitive interactions, whereby large plants can more readily suppress the growth of smaller neighbors than vice versa. It follows, then, that species with a longer exponential growth phase should develop greater size inequality over time. And since asymmetry is thought to be greater when competition is for light than when it is for below—ground resources, competitive interactions should be more asymmetric and size inequality should increase more in response to competition in species whose morphology results in greater between—plant shading. I tested these two hypothesis in greenhouse studies of two annual species, Polygonum arenastrum and P. douglassii, which differ in the duration of exponential growth and the likelihood of between—plant shading as a result of differences in development and morphology. Polygonum arenastrum has a shorter exponential phase of growth because it switches from vegetative growth to reproduction earlier than P. douglassii. It also has a prostrate growth form, more diffuse branching, and smaller leaves than P. douglassii. There should therefore be less shading of one plant by another in P. arenastrum than in P. douglassii. Seedlings from wild—collected seeds were planted in monoculture arrays at four different densities. The plants were harvested at five dates up to 10 wks; growth and branching pattern were measured at each harvest. As expected, in populations of P. douglassii, the species with the longer exponential growth phase, greater size inequality developed over time. In the absence of intraspecific competition, RGR was independent of plant size and size hierarchies were quite permanent. In contrast, P. arenastrum exhibited a period of decelerating growth which resulted in a negative correlation between RGR and plant size, frequent reversals in the size rankings of individuals over time, and declining size inequality through time under noncompetitive conditions. At high densities the morphology and RGRs of plants were size dependent in both species. Large individuals were more branched, had longer branches, and had higher RGRs than small individuals, and these shape and growth—rate differences became more pronounced as competition intensified. Furthermore, the size—dependence of shape and RGR was stronger in P. douglassii at all densities, and became relatively more pronounced as density increased, compared with P. arenastrum. These results are consistent with the expectation of greater competitive asymmetry in the species whose morphology should have led to greater between—plant shading. Size inequality increased with competition in both species. However, contrary to expectation, it did not increase more as density increased in P. douglassii. Reasons for the discrepancy between the morphological, RGR, and size inequality responses of the two species to density are discussed. The link between size inequality and plant development and morphology is examined in light of the effects the latter may have on the size—dependence of RGRs and the asymmetries of competitive interactions.