Abstract:

We evaluated the effects of individual and joint effects of fire, mechanical removal of shrubs and grazing on natural vegetation of the eastern Monte Phytogeographical Province in Rio Negro, Argentina. Four enclosures were constructed in sites where large disturbances (fire, mechanical removal and both) had taken place. Measurements were made both inside and outside them in order to evaluate not only the grazing effects in each site, but also possible interactions between the latter and large disturbances, which have not usually been reported. Several structural characteristics (shrub density and cover, aerial biomass and basal cover of herbaceous layer, and of biological crust), plus a functional one (phenology of five grasses and two shrubs) were studied. Both the fire and the mechanical removal reduced the density and area covered by the shrubby layer, and also the biological crust cover, with the effects being larger at the site were both had taken place. Bare soil was higher in the disturbed sites respect to the control due to the disappearance of the biological crust. Fire produced a qualitative change in the floristic composition of the herbaceous layer through altering the dominance from Stipa tenuis Phil. to a more palatable species as it is Stipa papposa Nees. Grazing reduced homogeneously aboveground biomass of the herbaceous layer, but extended the vegetative period of grasses and increased the size of that budget though reducing the reproductive one. The last two effects will be beneficial for secondary production since they increase the energy flux to herbivory. Large disturbances, however, could have negative effects through the reduction of the biological crust, which plays an important role in seed germination, seedlings establishment and water balance, thus contributing to ecosystems sustainability.