Grazing effect on the seed bank of a halophytic steppe of the Flooding Pampa, Argentina

Abstract

Rotational grazing, or even grazing cessation, improves soil and plant community structure on the halophytic steppe of the Flooding Pampa. However, no information exists about the effect of those grazing regimes on the soil seed bank of this fragile plant community. The objective of this work was to evaluate the effect of different grazing regimes on four attributes of the soil seed bank: 1) total size, 2) size and relative contribution of the functional groups of winter grasses (annual and perennial), summer grasses (annual and perennial), non-grass monocotyledonous, legumes and non-legume dicotyledonous, 3) diversity and 4) floristic composition. We studied five plots subjected to different grazing regimes for 14 years (two plots under continuous grazing; two plots under rotating grazing and one excluded from grazing), sampling in June and February to characterize the summer and winter soil seed bank. We found that continuous grazing reduced total seed bank size, and especially that of winter annual grasses and monocotyledonous, and also reduced floristic diversity, richness and evenness, especially in the summer seed bank. In addition, continuous grazing triggered important species replacements within each functional group, especially in the winter annual grasses, summer perennial grasses and dicotyledonous, the three groups explaining most proportion of the soil seed bank. Therefore, plots under continuous grazing were floristically different from those under rotational grazing or ungrazed. This work suggests that continuous grazing is a severe disturbance which can reduce the regeneration potential of the halophytic steppe.