Studies on the Autecology and Population Biology of the Weedy Monocarpic Perennial, Pastinaca Sativa

Abstract

(1) Wild parsnip (Pastinaca sativa L.) is a monocarpic perennial herb which grows as a weed in North America in grassy habitats that are usually mown in summer. Flowering occurs in May and June, and seeds are mature by early July. Seeds remain attached to the dead erect shoots until autumn, unless the plants are disturbed, as by mowing. (2) At maturity small numbers of seeds can germinate at summer temperatures. As the seeds after-ripen during summer their ability to germinate at high temperatures increases, but there is little increase in ability to germinate at autumn temperatures. Stratification (moist chilling) over winter lowers the temperature requirement for germination, and therefore the majority of seeds in the field germinate in early spring. (3) Seedling mortality is high; of 3980 seedlings marked in a natural population in Kentucky, only thirty-three (0.83%4) flowered before dying. Cohorts of seedlings marked in August, November, December, January and April had concave survivorship curves; cohorts marked in February and March initially had concave survivorship curves, but the curves subsequently became linear. (4) Plants produce rosettes and grow until the second, third, fourth or fifth growing season before producing flowers. Vernalization is required for flowering, and plants reach the flowering stage quicker under long than under short days. (5) An old-field population showed a wide range of individual plant sizes in late May: flowering plants had root-crown diameters (RCDs) of 5-39 mm, and non-flowering plants had RCDs of < 1 (first-year seedlings) to 10 mm. It is suggested that plants must reach a critical size before they can respond to the environmental stimuli of vernalization and photoperiod.