- ARE INVADERS DISTURBANCE-LIMITED? CONSERVATION OF MOUNTAIN GRASSLANDS - 195 Applied Vegetation Science 5: 195-202, 2002 © IAVS; Opulus Press Uppsala. Abstract. Extensive areas in the mountain grasslands of cen- tral Argentina are heavily invaded by alien species from Europe. A decrease in biodiversity and a loss of palatable species is also observed. The invasibility of the tall-grass mountain grassland community was investigated in an experi- ment of factorial design. Six alien species which are widely distributed in the region were sown in plots where soil distur- bance, above-ground biomass removal by cutting and burning were used as treatments. Alien species did not establish in undisturbed plots. All three types of disturbances increased the number and cover of alien species; the effects of soil disturbance and biomass removal was cumulative. Cirsium vulgare and Oenothera erythrosepala were the most efficient alien colonizers. In conditions where disturbances did not continue the cover of aliens started to decrease in the second year, by the end of the third season, only a few adults were established. Consequently, disturbances are needed to main- tain alien populations in tall-grass mountain grasslands. Burn- ing also increased the species richness of native species. We conclude that an efficient way to control the distribution of alien species is to decrease grazing pressure while burning as a traditional management tool may be continued. Keywords: Alien species; Burning; Grazing; Invasibility; Management; Species richness. Nomenclature: Cantero & Bianco (1986). Are invaders disturbance-limited? Conservation of mountain grasslands in Central Argentina Petryna, L. 1 ; Moora, M. 2 ; Nuñes C.O. 1 ; Cantero, J.J. 1 & Zobel, M. 2* 1 Universidad Nacional de Rio Cuarto, Facultad de Agronomia y Veterinaria, Estafeta Postal No. 9, 5800 Rio Cuarto, Cordoba, Argentina; 2 Institute of Botany and Ecology, Tartu University, Lai 40, Tartu 51005, Estonia; * Corresponding author; Fax +3727376222; E-mail mzobel@ut.ee Introduction Invasions in grasslands are common and, in many cases, have been associated with changes in grazing regime (Sala et al. 1986; D’Antonio & Vitousek 1992; Mack 1996). Argentinian grasslands that have evolved under low grazing pressure have even shown an increase in species diversity as a result of the introduction of livestock (Sala et al. 1986), while cessation of grazing may result in a decrease in diversity (Pucheta et al. 1998). Rusch & Oesterheld (1997) found that in grasslands of central-eastern Argentina, grazing increased species rich- ness through the addition of exotic forbs, without reduc- ing the richness and cover of the native flora. However, one may currently observe the rapid extension of the distribution area of several exotic species in the mountain grasslands of central and southern Argentina, especially where land use is, or has been, intensive (Mack 1996). There is evidently a need to control the distribution and abundance of aliens. For these reasons, one has to know the mechanisms enhancing establishment and persistence of alien species populations. Following arrival in a certain locality, successful establishment and subsequent expansion of the alien species depends on local abiotic conditions and on bi- otic interactions such as competition from established native plants, predation, parasitism and on the presence of mutualistic organisms (Crawley 1987; Vitousek 1990; Williamson 1999). Rejmánek (1996) showed that com- munities in mesic environments are more invasible as xeric environments are not favourable for seedling es- tablishment and wet habitats are characterized by fast growth and high competitiveness of resident species. Several pieces of evidence show that the number of alien species is greater in more disturbed habitats (Kitayama & Mueller-Dombois 1995; Burke & Grime 1996; Crawley et al. 1996; Rejmánek 1995, 1999; Rejmánek & Richardson 1996; Morgan 1998), while traditional management of semi-natural grasslands may