Stress and disturbance: vegetation dynamics in the dry Chaco region of Argentina.

This paper analyses two processes acting on The second process is the dynamics of gallery forest vegetation dynamics in the dry Chaco region of Argentina. resulting from intensive river-bed migrations which The major one is the response of herbaceous/woody species characterize the region. A main process model relates the to overgrazing. Extensive cattle breeding reached a peak largest and most frequent floods to the complex structure, shortly before the 1940s, when animal production in the high diversity and Amazonian lineage of the floristic region became saturated. Thus over-exploitation reached a composition of the forests. It also attributes the null water crisis during this decade and there followed a sharp supply of abandoned river-beds to Chaco lineage forests, decrease in the number of cattle and the efficiency of the adjusted to rainfall seasonality. However, owing to the high production system. The number of puestos (stations), on the morphological instability of the region these courses may contrary, remained fairly constant. Thus overgrazing has become active or inactive throughout time by means of led to degradation of the natural systems. Today the most digression or filling in, which leads to well-developed intensely degraded areas form a 25-50 ha fringe around forests growing on ancient river-beds and, conversely, dry each puesto headquarters and represent less than 1 % of the forests growing along the margin of permanent rivers. A total area studied. Here, marked changes in soil model is presented which, by analysing river bank physiochemical properties can be seen together with the community dynamics in terms of the geomorphological elimination of trees and shrubs. Away from this fringe, instability (constant river-bed migration, river-bed filling, degradation basically involves the herbaceous cover, bank formation and disintegration), shows how the changing the original landscape of forests and savannas structure and composition of these forests would be into one of forests and shrub patches. There are not only determined by past floods rather than by the present one. structural alterations but also changes in the system's dynamics due to modifications in the relationships between Key words. Overgrazing, riverine, savanna vegetation its components. The sequence of changes seems to exhibit dynamics, disturbance, gallery forests, stability, Chaco, resilient behaviour showing hysteresis. Argentina.