Technical Abstract: When plant size is strongly correlated with plant reproduction, variance in growth rates results in a lognormal distribution of seed production within a population. Fecundity variance affects effective population size (Ne), which reflects the ability of a population to maintain beneficial mutations and to respond to natural selection. When growth rates vary as little at 0.3%, the resulting variance in seed production can strongly affect genetic drift processes, reducing the Ne/N ratio of populations to a low fraction where drift is largely determined by this variance in seed outputs. Increasing the length of the growing season also reduces Ne/N. While intrinsic plant growth and development (e.g., growth rate variance, degree of correlation between plant size and growth rate) can affect Ne/N ratios, ecological factors such as competition, herbivory and abiotic stress also play a role in determining growth rate variance and thus affect the Ne/N ratio. Even if an ecological factor has a small influence on growth rate variance, if variance is increased by at least 0.3% by this ecological factor it could be the primary determinant of drift processes within a population.