Browsing Journal of Range Management, Volume 41, Number 6 (November 1988) by Subjects
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Floristic changes induced by flooding on grazed and ungrazed lowland grasslands in ArgentinaChanges 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.
Grazing effects of the bulk density in a Natraquoll of the flooding pampa of ArgentinaThe influence of grazing by cattle on soil bulk density was studied in a typic Natraquoll of the Flooding Pampa of Argentina for a period of 33 months, by comparing a grazed situation to an enclosure deferred from grazing for 7 years. Floods took place in this period as usual. Bulk density (BD) at -33.3 kPa of water retention varied from 1.00 to 1.11 Mg m-3 in the ungrazed soil and in the grazed soil from 1.04 to 1.16 Mg m-3. Environmental factors were the primary agent controlling BD; only in some periods were there significant differences between treatments. Slight increases in BD occurred under grazing after the recession of the flood water, and significant decreases occurred in the ungrazed soil during the large and sudden falls in water content. In this case the effect of trampling, therefore, would consist mainly of impeding the decrease in BD. No compaction was observed in periods when no flood occurred or while soil remained submerged in water. The results indicated that the variations of bulk density caused by cattle trampling were superimposed on those produced by floods and showed an interaction between the effects of land-use and the particular environmental conditions of the region.