The effects of propagule size on germination and seedling growth in Mirabilis hirsuta

Abstract

Mirabilis hirsuta, a fugitive prairie plant species, occurs only on disturbances (mounds) resulting from the foraging activity of badgers on the Cayler Prairie Preserve in northwestern Iowa, U.S.A. Seedling survivorship and later competitive success are predicated upon the root's rapid extension to and occupation of a moisture-retentive layer of buried vegetation which forms the base of these disturbances. Seed size, which varies from <2 to >6 mg, is a predictor of the likelihood of establishment success through effects upon germination and seedling vigor. Regressions indicated that germination increased by 10.2% per milligram increase in seed weight, leaf area of 30-day-old seedlings increased by 47.8 mm2/mg seed weight, and dry weight biomass of these seedlings increased 40.2 mg/mg seed weight. Germination also occurred more rapidly with heavier seeds. Differences in seedling growth with increases in seed size are largely due to the capital provided by initial food reserves. The instantaneous rate of increase in seedling biomass is nearly identical for seeds of all different weights. Absolute growth rates of seedlings are thus proportional to seed size, at least through the establishment phase. These results are of demographic importance because droughts frequently begin in early summer on this prairie.