The Comparative Ecology of Two Sand Dune Biennials: Lactuca Virosa L. And Cynoglossum Officinale L.

S ummary This paper compares two contrasting biennial plant species of the sand dunes of Holkham, North Norfolk. A survey of distribution, in relation to various habitat factors, showed that Lactuca virosa L. is characteristic of the tall, Ammophila‐dominated grassland on the younger dunes. It is favoured by gaps in the vegetation cover such as those produced by local wind erosion, but it is very sensitive to rabbit grazing. Cynoglossum officinale L. is a plant of older dune grassland and is not normally grazed by rabbits. It can exploit the bare ground around rabbit warrens, benefiting from the higher nitrogen levels resulting from rabbit excreta. Studies on seed production and germination showed that Lactuca can produce large numbers (> 60000 per plant) of very small ( c. 0–8 mg) wind‐dispersed seeds that germinate readily in the autumn. Cynoglossum , on the other hand, produces fewer (< 2500) seeds per plant that are larger ( c. 30 mg) and animal‐dispersed. Cynoglossum seeds are inherently dormant; most germination is in the late winter and early spring, after a period of cold. Lactuca compensates for small seed size by having a higher growth rate than Cynoglossum as well as by its earlier germination. Despite all these differences, the two species survive at comparable densities.