Grazing Regimes and Plant Reproduction in Semi-Natural Grasslands

Wissman, J. 2006. Grazing regimes and plant reproduction in semi–natural grasslands. The dominating grazing regime in semi-natural grasslands today is continuous grazing throughout the season, and a high grazing pressure is often advised in order to avoid litter accumulation. Traditional management, on the other hand, included late mowing and grazing, which favoured plant flowering and seed production. This thesis studies the effects of alternative grazing regimes that may ecologically mimic traditional management. The ingoing papers examines: 1) Plant reproduction under late onset of grazing compared to continuous grazing; 2) The tradeoff between disturbance and competition for plant seed production and establishment; 3) Effects of increased flower density in late grazed grasslands on pollinator abundance, visitation rate, and pollen limitation; 4) The impact of landscape, habitat, population, and management on pollination, reproduction, and population structure of Primula veris. The studies were performed by using grazing and pollination experiments combined with estimates of habitat and landscape properties. Late grazing enhanced the fruit set without increasing the litter depth. The management effect for individual plant species depended mainly on the species' phenology of reproduction. Late grazing and 1-yr grazing interruption increased seed production linearly, but reduced seedling establishment as a threshold function. The net effect was an increased seedling density and, after six years, species density. Increased flower density in late grazing attracted more pollinators, but resulted in competition for pollinators among flowers. No pollen limitation was, however, found in two experimental species. Direct grazing of reproductive organs outweighed indirect effects on plant reproduction through effects on pollination. Reproduction and population structure of P. veris was mainly controlled by seedling establishment, pollen limitation, and direct grazing of fruits. No constant landscape effects were found. Grazed populations had high proportions of seedlings and juveniles due to high recruitment and mortality of adult plants, whereas ungrazed populations had higher proportions of adult plants due to low recruitment. The net effect was similar densities of young plants in grazed and ungrazed populations. Management regimes such as less intense grazing, late summer grazing, or late summer mowing would increase both seed production and establishment, thereby favouring populations of P. veris.