The population dynamics of field pansy (Viola arvensis) and red deadnettle (Lamium purpureum) in winter cereal and oilseed rape fields

The population dynamics of Viola arvensis Murr. and Lamiuni purpureum L. were studied in winter cereal and oilseed rape fields in North Staffordshire and Cheshire. The seedling emergence, seedbank density, population survival and potential seed input were recorded. Transition probabilities for each stage were calculated. For both species, annual emergence was positively related to the seedbank. For Lamiun purpureun, the adult population was also positively related to the seedbank. The seedbank of V.arvensis was positively related to the sandiness of the soil and to the number of years sown to winter barley. In a garden experiment, emergence was greater in more sandy soil (though containing less inorganic nitrate). Emergence patterns in garden experiments reflected those in the fields despite monthly cultivation and are explicable by seasonal changes in germination requirements for temperature and light, which were studied on a thennogradient bar. Peak emergence coincided with minimum dormancy and would occur in the second autumn of burial for V.arvensis and in each autumn for L. purpureum. Collection date and burial time did not affect dormancy of L.purpureum. However, for V.arvensis, earlier dispersed and earlier buried seeds were less dormant. Doimancy changes of seeds in a Stevenson Screen resembled those of buried seeds for V.arvensis. though changes were more abrupt. There was no dormancy cycle in seeds stored in a refrigerator for either species. A greenhouse experiment revealed phenotypic and genotypic variation in dormancy in both species. Genetic variation was apparent both between plants and fields for V.arvensis but only between plants for L. purpureun. Life cycle data from the fields were incorporated into a Leslie ffatrix population model and the effects on the population of varying the seed were simulated. The implications for control measures are discussed