• Floristic changes induced by flooding on grazed and ungrazed lowland grasslands in Argentina

      Chaneton, E. J.; Facelli, J. M.; Leon, R. J. C. (Society for Range Management, 1988-11-01)
      Changes in community composition of 2 grassland sites exposed to a flood of unusual intensity and duration were investigated in the Flooding Pampa. These grasslands are subject almost annually to floodings of lesser magnitude. The study sites were adjacent to each other, and differed in vegetation structure and composition. One had been grazed continuously by cattle and was showing signs of intense deterioration. The other had remained ungrazed during 15 years. Basal cover by species was measured in summer, before and after the flooding event. Compositional difference between sites decreased with flooding from 68.9 to 39.1%. In the grazed site the cover of alien forbs was reduced by 48%. After the flooding native graminoids represented 99.7 and 86.7% of the cover, inside and outside the exclosure respectively. Total basal cover was not affected but was redistributed among species already present before the flood. Floristic changes would have led to an improvement of the forage source. We conclude that plant community response to the event was influenced by the previous grazing history of the site. The large flood acted as an overriding environmental factor which partially reverted the effects of grazing upon grassland composition.
    • Stability of grazed patches on rough fescue grasslands

      Willms, W. D.; Dormaar, J. F.; Schaalje, G. B. (Society for Range Management, 1988-11-01)
      Continuous stocking usually leads to the formation of grazed patches. However, the effect of patches on the grassland community is related to their stability. Therefore, we studied the spatial stability of grazed patches on Rough Fescue Grasslands by mapping forage removal classes on 10 sites over a 4-year period, testing stability using the Kappa index (K), and characterizing the soils and vegetation of overgrazed and undergrazed patches. Spatial stability of grazed patches between consecutive years was good (K is greater than or equal to 0.26) on sites experiencing low grazing pressure. However, on sites having high grazing pressure, spatial stability was less consistent between consecutive years (0>K is lesser than or equal to 0.45) and low over a 4-year period (K is lesser than or equal to 0.10). Overgrazed patches were dominated by grazing-resistant seral species, but undergrazed patches were dominated by climax species. Rough fescue (Festuca scabrella) and Parry oat grass (Danthonia parryi) plants were 50% shorter, and forage production was about 35% less, on overgrazed than on undergrazed patches. Soil organic matter, carbohydrates, and depth of Ah horizon were significantly greater on undergrazed patches but urease activity, NO3-N, NH4, and available phosphorus were greater on overgrazed patches. Overgrazed and undergrazed patches were stable in the long term, although patch boundaries fluctuated.