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

1 The tiller dynamics of the two dominant species in a grassland, Agro stis stolonifera and Lolium perenne, were followed for two years in a field grazing experiment. Sheep grazing was applied in three seasons a t two grazing intensities; winter, spring (in both seasons grazed or u ngrazed) and summer (hard or light grazed). 2 Tiller densities of both species showed peaks in late summer and troughs in spring due to vari ation in tiller birth rates. Tiller death rates were not seasonal. Yea r-to-year weather variation also affected the dynamics. 3 Harder summe r grazing increased tiller birth and death rates of both species, incr easing tiller turnover, but did not affect tiller densities. The tille r death rates were higher and the summer peak in birth rates was earli er in the winter-grazed paddocks. Therefore, winter grazing decreased tiller densities over the two years by reducing densities in the non w inter-grazed paddocks during the late summer months. There were no mai n effects of spring grazing treatment or interactions among grazing tr eatments. Tiller birth and death rates were density-dependent, indicat ing that the populations were regulated. 4 Agrostis generally had high er tiller birth and death rates and therefore had a more rapid tiller turnover than Lolium. Fewer tillers flowered and there was less seedli ng establishment in the Agrostis populations. However, both species sh owed 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 t o winter grazing. 5 The tiller populations of both species were regula ted by density-dependent birth and death rates but showed seasonal dyn amics upon which were superimposed effects of grazing treatment and ye arly variation in the weather. However, their tiller population number s were little affected by grazing level in spring or summer. Only graz ing in winter decreased their densities and this is the treatment most likely to bring about changes in species composition in the grassland .