Alicia H. Barchuk

Regeneration and structure of Aspidosperma quebracho-blanco Schl. in the Arid Chaco (Córdoba, Argentina)

The Aspidosperma quebracho-blanco is an important sclerophyllous evergreen species of the Argentine Chaco. There is scarce information about demographic patterns of A. quebracho blanco in the Arid Chaco. This paper focused on describing consequences of the process of degradation of the vegetation on the regeneration and size structure of A. quebracho-blanco population, compared to the contrasting scenarios of traditional management (cutting, grazing and fire) and a long undisturbed situation. Seedling and sapling establishment of A. quebracho-blanco has seldom been observed in Cordobas Arid Chaco. A. quebracho blanco did not have significant seedling and sapling recruitment in any of the stands submitted to the effects of traditional management (extensive grazing and uncontrolled cutting). It had substantial seedling and sapling recruitment in the Chancani Forest Reserve, which was different to the other sites. In protected areas, the adult plant densities were higher than in areas of low grazing and cutting. Disturbed habitats will be often considered to be suboptimal, whereas more stable undisturbed habitats have higher success of regeneration. Though, the general patterns of size distributions are similar, the disturbing effect promotes changes in their distribution parameters.

A woody plant community and tree-cacti associations change with distance to a water source in a dry Chaco forest of Argentina

In semiarid regions, livestock is concentrated around water sources generating a piosphere pattern (gradients of woody vegetation degradation with increasing proximity to water). Close to the water source, livestock may affect the composition, structure and regeneration strategies of woody vegetation. We used the proximity from a water source as a proxy of grazing pressure. Our objectives were (1) to compare woody vegetation attributes (richness, diversity, species composition, density and basal area) and ground cover between sites at two distances to a water source: near (higher grazing pressure) and far from the water source (lower grazing pressure), and (2) to quantify and compare cases of spatial association among the columnar cacti Stetsonia coryne (Salm-Dyck) Britton and Rose (Cactaceae), and the dominant tree Bulnesia sarmientoi Lorentz ex Griseb. (Zygophyllaceae). We used a paired design with eight pairs of rectangular plots distributed along a large and representative natural water source. We found lower total species richness, plant density and soil cover near than far from water source, and more cases of spatial associations between the two species studied. Our results show evidence of increased livestock impacts around water sources. However, we found no difference in terms of species composition or basal area at near versus far sites. We conclude that grazing pressure might be changing some attributes of the woody plant community, and that the association of young trees with thorny plants (grazing refuge) could be a regeneration mechanism in this semiarid forest with high grazing pressure.