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Population ecology of the biennials Cirsium vulgare and Cynoglossum officinale in a coastal sand-dune area

(1) Demographic data were collected on the biennials Cirsium vulgare and Cynoglossum officinale in a coastal sand-dune area in the Netherlands. Over ninety local populations were observed in 2 m x 2 m plots for a three-year period (1981-84). (2) Within two years Cirsium vulgare disappeared from 51% of the plots, while Cynoglossum officinale disappeared from only 7% of the plots. Reduction of the total dune population during the study period was evident for both species; the number of seedlings that established was insufficient to compensate for the losses from the population because of death of rosettes and flowering plants. (3) Large temporal and spatial variation existed in reproductive success, measured as the number of surviving seedlings per reproductive individual. Seedling survival in the two species was lowest in the dry summer of 1983 and in exposed vegetation. As a result of the low establishment in open habitats the main distribution shifted from exposed habitats to plots in the direct vicinity of shrubs and trees. This shift was most obvious in Cynoglossum officinale. (4) Using a modified key factor analysis, plots with high early mortality also showed high total mortality during the life of the 1981 cohort: events early in the life of individuals determine the spatial differences in abundance of the adults. These events include failure to produce viable seeds, seed predation and early seedling mortality in Cirsium vulgare, and failure to produce viable seeds, failure to germinate and early seedling mortality in Cynoglossum officinale. (5) The reduction of the population appeared not to be due to systematic processes such as the accumulation of litter or the increase of perennial vegetation. Stochastic factors such as rainfall and seed predation had most influence on population recruitment. (6) Dispersal, delayed germination and delay of flowering permit a monocarpic organism to bridge years or sites with failing reproduction. In these two species the importance of a persistent seedbank in the soil is limited. The dispersal rate between populations in different vegetation types was probably low. Delayed flowering was most pronounced in Cynoglossum officinale. The proportion of the rosettes of one year and older delaying reproduction varied between 75% and 95%. In Cirsium vulgare this figure varied between 53% and 80%. The large proportion of rosettes that delayed reproduction buffered the population against low seedling recruitment in some years.