TILLER DYNAMICS OF 2 GRASSES - RESPONSES TO GRAZING, DENSITY AND WEATHER

1 The tiller dynamics of the two dominant species in a grassland, Agrostis stolonifera and Lolium perenne, were followed for two years in a field grazing experiment. Sheep grazing was applied in three seasons at two grazing intensities; winter, spring (in both seasons grazed or ungrazed) and summer (hard or light grazed). 2 Tiller densities of both species showed peaks in late summer and troughs in spring due to variation in tiller birth rates. Tiller death rates were not seasonal. Year-to-year weather variation also affected the dynamics. 3 Harder summer grazing increased tiller birth and death rates of both species, increasing tiller turnover, but did not affect tiller densities. The tiller death rates were higher and the summer peak in birth rates was earlier in the winter-grazed paddocks. Therefore, winter grazing decreased tiller densities over the two years by reducing densities in the non winter-grazed paddocks during the late summer months. There were no main effects of spring grazing treatment or interactions among grazing treatments. Tiller birth and death rates were density-dependent, indicating that the populations were regulated. 4 Agrostis generally had higher tiller birth and death rates and therefore had a more rapid tiller turnover than Lolium. Fewer tillers flowered and there was less seedling establishment in the Agrostis populations. However, both species showed similar seasonal dynamics, density-dependent regulation, effects of between year climatic variation and responses to spring and summer grazing treatments. Agrostis showed a significantly greater response to winter grazing. 5 The tiller populations of both species were regulated by density-dependent birth and death rates but showed seasonal dynamics upon which were superimposed effects of grazing treatment and yearly variation in the weather. However, their tiller population numbers were little affected by grazing level in spring or summer. Only grazing in winter decreased their densities and this is the treatment most likely to bring about changes in species composition in the grassland.