Colonization of grassland by sown species: dispersal versus microsite limitation in responses to management

1. Diversification of species-poor grassland often requires the introductio n of desirable species by sowing seed. Little is known about the factors co ntrolling the spread of introduced species, or how these interact with mana gement. We determined whether management affected spread rates of two grass land species by modifying seed dispersal or seedling establishment. 2. An experiment was set up in 1995 on a species-poor grassland. It compris ed five blocks, each with four treatments: (1) autumn grazed only; (2) cut July; (3) cut July and September; (4) cut July and aftermath grazed. Twenty -two plant species were separately slot-seeded into each treatment plot, pr oviding discrete linear colonization foci. 3. The mechanisms controlling spread were studied in two species: Rhinanthu s minor, an annual with large seeds adapted for wind dispersal; and Leucant hemum vulgare, a perennial with small seeds with no obvious dispersal adapt ations. 4. Perpendicular spread of each species by 1998 was described well by a sim ple inverse power model. Rhinanthus had spread further in the hay-cut treat ments (2-4) than in the grazed treatment (1). Leucanthemum spread poorly in all plots, with no treatment effects. 5. Seed dispersal from source slots was also described well by the inverse power model. Dispersal curves for Rhinanthus were much longer in the hay-cu t treatment (3) than in the grazed treatment (1), because more seed dispers ed during hay cutting than before, and cutting dispersed seed longer distan ces. There was no dispersal by grazing animals. Dispersal showed directiona l effects: seeds travelled further in the prevailing wind direction before the hay-cut and in the grazed treatment; dispersal by hay cutting was furth er in the cut direction than in the opposite direction. 6. Leucanthemum showed poor dispersal, with no treatment effects, except th at more seeds were dispersed in the grazed (1) than the hay-cut (3) treatme nt. 7. The establishment and survival of sown seeds showed no treatment effects for either species. 8. Management effects on the spread of Rhinanthus reflected effects on disp ersal, rather than establishment. Leucanthemum showed poor dispersal but go od establishment in all treatments, suggesting its spread may also have bee n dispersal-limited. Rhinanthus was positively affected by hay cutting beca use it set seed at the time of cutting, whereas Leucanthemum set seed later and cutting reduced its seed production. 9. The results indicate that management of grassland to enhance the coloniz ation of sown species might be best targeted at enhancing seed-dispersal di stances. Hay cutting can do this, but must coincide with seed set.