Stress and Disturbance: Vegetation Dynamics in the Dry Chaco Region of Argentina

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