Effect of fire on vegetation dynamics and plant types in subtropical grassland in southern Brazil

Extensive parts of southern Brazil are covered by grasslands ( campos ), even though climatic conditions allow for forest development. It is being supposed that grasslands are relicts from cooler and drier periods, stabilized until today by the action of grazing and fire. Based on data from a grassland area subjected to frequent (anthropogenic) burns close to Porto Alegre, RS, this thesis analyses the impact of fire on vegetation dynamics and composition of plant functional types. The studied grassland, dominated by caespitose grasses with the C 4 photosynthetic passway, possesses about 450 to 500 vascular plant species in an area of about 220 ha and thus – like South Brazilian grassland in general – can be considered very species-rich. There seems to be no strong relation between variation in species composition and soil conditions. Fire has a high importance for fine-scale, especially temporal, diversity. Species number and dynamics of the vegetation are highest about one year after a burn, as fires reduce competitive interactions while recruitment possibilities and resource availability are high. With time since fire, caespitose grasses increase in dominance and cause the loss of forb species from above-ground vegetation, even though total species composition apparently will not be affected greatly by fire. The species seem to be well-adapted to the current fire regime: grasslands may have evolved under the impact of fire. In general, grasslands under climatic conditions allowing for forest development show high productivity and would give place to forests or at least shrublands in the absence of fire. The dominating caespitose grasses are responsible for persistence of grasslands, as their biomass leads to high flammability and thus to the cyclic vegetation development with frequent burns. As a consequence, forb and shrub species that contribute little to flammability need to possess attributes allowing for post-fire regeneration. (Re-)colonization of burned areas by seed germination only plays a very small role when compared to regeneration from belowground organs. The perception of fire as a disturbance in Brazilian nature conservancy has lead to the policy of fire exclusion in protected areas and thus to shrub and tree encroachment in grasslands and to shifts at the forest-grassland-border. Together with large-scale transformation into agri- or silivicultural lands, this policy endangers grasslands in the region and thus their high and up to now only poorly studied high diversity.