Is sheep grazing a disturbance agent in arid-semiarid Patagonia of Argentina?.

BERTILLER, M.B.

Forest to Combat desertification. Afforestation and sustainable Forests as a mean to Combat Desertification

Conferencia; Forest to Combat desertification. Afforestation and sustainable Forests as a mean to Combat Desertification; 2007

IUFRO-KKL-JNF

Vegetation dynamics under sheep grazing in ecosystems of Patagonia-Argentina may be described in the conceptual framework of the state-and-transition model. Sheep grazing in these ecosystems leads to an overall reduction of plant cover, species replacement within functional groups (grasses or shrubs), and the replacement of functional groups (grasses for shrubs). Vegetation states induced by grazing shift along different physiognomic types, and more diverse changes occur in the most humid ecosystems.  These changes are associated with specific steps in soil degradation and in the status of the soil seed bank. Losses in soil organic C and N by erosion of the bare soil and limiting perennial grass seeds reduce the establishment of perennial grasses promoting the subsequent increase of shrub cover and shifting in the dominant shrub species. Shrub shifting or encroachment lead to changes in the quality of litterfall and soil organic matter and to deceleration in N cycling, representing the crossing of a resilience threshold. Release of grazing and animal redistribution with fencing of degraded spots to avoid recurrence of grazing in the most degraded patches, could drive transitions from the first states of degradation to more conserved ones. Strategies like addition of organic matter, nitrogen, and seed to soil or planting of target species in favorable seasons or years under grazing exclusion may be included among management tools to restore the most degraded vegetation states. In conclusion, sheep grazing is an important disturbance agent in Patagonian rangelands of Argentina and in most of them its effects cannot be reversed by reducing stocking rates.