Between-species patterns of covariation in plant size, seed size and fecundity in monocarpic herbs

Covariation in plant mass, seed mass and fecundity was investigated for 15 species of monocarpic herbs (annuals and biennials) harvested at Final developmental stage from recently disturbed habitats. Above-ground vegetative mass and fecundity varied by over three orders of magnitude and seed mass varied by over two orders of magnitude across species. Eighty-eight percent of the variation in fecundity across species was explained by covariation in the other two characters as predictor variables in a least squares regression models fecundity increased significantly and proportionately with increasing above-ground vegetative mass but decreased significantly and proportionately with increasing individual seed mass. Individual seed mass and above-ground vegetative mass were positively correlated across species when fecundity was held constant under partial correlation. The results indicated that none of the relationships between these characters are size-dependent, i.e., the traditional trade-off, predicted by life history theory, between seed size and fecundity is isometric across species, as is the increase in both fecundity and total seed mass with increasing vegetative mass. Allometric (size-dependent) relationships for these characters have been reported in previous studies, where developmental stage was not controlled across species; the detection of isometric relationships in the present study may therefore be a consequence of measuring whole-genet, lifetime fecundity and above-ground vegetative mass for all study species. A general hypothesis is presented for the interpretation of covariation in plant size, seed size and fecundity across species, in which the principal selection mechanism involves 'time limitation' The amount of time available for growth (before density-independent mortality, e.g., from disturbance) selects for the level of precocity necessary to reproduce before death. This affects the level of constraint on maximum attainable plant size, i.e., smaller when shorter-lived. This, in turn, affects reproductive output, which can be greater when longer-lived, expressed as greater fecundity and/or larger seed size. However, both cannot be maximized because of the inherent trade-off between seed size and fecundity for plants of a given size.