COMPETITION, FACILITATION, SEED DISTRIBUTION AND THE ORIGIN OF PATCHES IN A PATAGONIAN STEPPE

The Patagonian steppe is composed mainly of shrubs and tussock grasses organized in two types of patches: (1) scattered grass tussocks in a matrix of bare soil and (2) shrubs, each surrounded by a dense ring of grass tussocks. We analyzed the variation of competition, facilitation, and seed distribution through space and time as major driving forces in the development of this patch structure. Emergence and survival of grass seedlings increased with distance from shrubs when the ring of grasses was left intact. On the contrary, when the ring of grasses was experimentally removed, seedling survival decreased with distance from the shrub. Differences in root density, soil water potential, and evaporation accounted for these patterns. Density of naturally dispersed seeds of grasses decreased with distance from the shrub. Simulated recruitment near shrubs without the ring of grasses was twice as high as near shrubs with the ring. High root competition near the shrubs surrounded by a ring of grasses decreased the survivability of seedlings and overshadowed the aerial protection provided by the shrubs. We suggest that when shrubs are young and small, facilitation is more important than competition which results in the formation of the dense ring of grasses. When the shrub becomes large and the ring complete, competition overshadows facilitation. After shrub death, the ring may disintegrate and remnant tussocks may form the other patch type, scattered grass tussocks.