Dynamics of seashore populations of Agrostis stolonifera, Festuca rubra and Poa irrigata

Sammanfattning: The dynamics of tillers of natural populations of three cohabiting perennial grass species Agrostis stolonifera, Festuca rubra and Poa irrigata were studied for five years in a Baltic seashore meadow. A demographic approach based on numerical changes over time was used. Analyses were made to determine the effects of temperature and precipitation and the effects of intraspecific and interspecific density of the species on rates of tiller birth and death were assessed. For A. stolonifera and F. rubra, temporal and spatial differences in vital rates were studied and consequences of that differences for long term population processes were studied by population matrix models. Competitive relationships between the three grasses were studied experimentally.Recruitment was mainly by vegetative tillers, produced continuously throughout the growing season. Rates of tiller birth and death were high, particularly in A. stolonifera consequently the life span of individual tiller was short. Considerable year -to-year was found in birth and death rates and the birth rates of tillers in all species were positively related to summer precipitation. Positive relationships were found between death and birth rates in F. rubra. In A. stolonifera the risk of tiller death was constant through the life but increased with age in the two other species. Most tillers of all species died for unknown reasons. Relationships between tiller death and density of tillers could not be confirmed nor could relationships between the rate of tiller death and weather functions. The proportion of flowering tillers was low in all species. Survival probabilities for tillers of A. stolonifera and F. rubra were not homogeneous in time or space and for that species the temporal and spatial variation in vital rates was large enough to have influences on long-term processes such as population growth rate. For A. stolonifera and F. rubra indications were fond that contribution of the youngest age class, through daughter tiller production, was of major importance to the population growth rate. All three species were affected by between species competition except F. rubra when grown as the more frequent component in mixture with P. irrigata. The pattern of interactions between the species was not influenced by initial density, but it was influenced by relative frequency of a species in mixture with another species.In this study a picture of everchangin tiller populations was found, turnover rate of tillers was high and considerable variation in vital rates was confirmed. For A. stolonifera and F. rubra mosaic of increasing an decreasing populations was found in time and space. Despite this the populations of each species remained relatively stable over the study period as a whole.