Colonization of grassland by sown species: dispersal versus microsite limitation in responses to management.
Diversification of species-poor grassland often requires the introduction of desirable species by sowing seed. We determined whether management affect spread rates of two grassland species by modifying seed dispersal or seedling establishment. An experiment was set up in 1995 on a species-poor grassland in Oxfordshire, UK. It comprised 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, providing discrete linear colonization foci. The mechanisms controlling spread were studied in two species: Rhinanthus minor, an annual with large seeds adapted for wind dispersal; and Leucanthemum vulgare, a perennial with small seeds with no obvious dispersal adaptations. Perpendicular spread of each species by 1998 was described well by a simple inverse power model. Rhinanthus had spread further in the hay-cut treatments (2-4) than in treatment 1. Leucanthemum spread poorly in all plots, with no treatment effects. Seed dispersal from source slots was also described well by the inverse power model. Dispersal curves for Rhinanthus were much longer in treatment 3 than in treatment 1, because more seed dispersed during hay cutting than before, and cutting dispersed seed longer distances. There was no dispersal by grazing animals. Dispersal showed directional effects: seeds travelled further in the prevailing wind direction before the hay-cut and in the grazed treatment; dispersal by hay cutting was further in the cut direction than in the opposite direction. Leucanthemum showed poor dispersal, with no treatment effects, except that more seeds were dispersed in treatment 1 than in treatment 3. The establishment and survival of sown seeds showed no treatment effects for either species. Management effects on the spread of Rhinanthus reflected effects on dispersal, rather than establishment. Leucanthemum showed poor dispersal but good establishment in all treatments, suggesting its spread could also have been dispersal-limited. Rhinanthus was positively affected by hay cutting because it set seed at the time of cutting, whereas Leucanthemum set seed later and cutting reduced its seed production. Results indicate that management of grassland to enhance the colonization of sown species might be best targeted at enhancing seed-dispersal distances. Hay cutting can do this, but must coincide with seed set.