The Population Ecology of Senecio Jacobaea in a Sand Dune System: I. Reproductive Strategy and the Biennial Habit

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Abstract
(1) Field observations of three sand-dune populations of ragwort (Senecio jacobaea L.) in the Netherlands showed that flowering and seed production were related to plant size. Small-sized plants might not flower in their second year, but live on for 3 or more years. This perennial behaviour was not accompanied by a repeated seed production, however. (2) Delay of flowering may result in an increase in seed production by individual plants (because they gain in size). Small-sized plants, however, have a high death-risk. It is therefore doubtful whether the extra seed production of the larger plants compensates for the loss in potential reproduction of the plants that die. (3) Damage by herbivores may also lead to a delay in flowering. As an alternative to generative reproduction, these damaged plants show clonal growth. (4) Laboratory experiments on seed germination showed that the achenes do not possess innate dormancy. Light is required for germination. Coverage by sand to a depth of more than 4 mm leads to reduced light transmission and enforced dormancy in the achenes; thin coverage of sand (1-2 mm), however, strongly stimulates germination as compared to uncovered achenes, due to more favourable moisture conditions. As a result the achenes are well-adapted to areas of local disturbance. (5) Temperature, soil moisture and air humidity have a very important influence on the timing and level of germination: short periods of frost or drought, following wetting of the achenes, may induce dormancy; high temperature, low soil moisture and low air humidity reduce the capacity for germination even after transfer to more favourable regimes. (6) Germination behaviour was studied in field plots in three different sand-dune communities. Germination and survival were consistently higher in cleared plots than in undisturbed plots, thus indicating the importance of local disturbance and suppression of seedlings of ragwort by a dense vegetation cover. (7) Achenes of ragwort in the soil may remain viable for some years, so that extinct populations may be re-started again from germination of buried dormant achenes.

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