Patch structure dynamics and mechanisms of cyclical succession in a Patagonian steppe (Argentina)

The plant community in the Patagonian steppe is arranged in two structural patch types: scattered grasses and shrubs, which are often surrounded by a ring of grasses. Previous studies suggested that this pattern results from a cyclical succession driven by plant interactions. Although there is a good understanding of the factors affecting the formation of the grass rings encircling the shrubs, there is little evidence testing the causes of its fragmentation when the shrub dies, and the relation between the two structural patches. We evaluated the physiological status of two Stipa species in different degenerative stages of the ring surrounding Adesmia volckmanni shrubs, measuring microclimate and soil nutrients. Results support the cyclical succession model. As long as Adesmia shrubs are alive, both Stipa species coexist under its canopy, supported by facilitation via improved microclimate and nutrients under the shrub. After shrub death, degeneration of the ring of grasses is caused by competition. The species spatial distribution suggests that Stipa speciosa withstands poorer soil nitrogen levels than Stipa humilis. We suggest that when facilitation by the shrub ceases, S. speciosa may have a competitive advantage over S. humilis that may result in the dominance of S. speciosa in gaps between shrubs. (C) 2008 Elsevier Ltd. All rights reserved.