The colonization of bare areas in two-phase mosaics of an arid ecosystem

Two-phase mosaics (densely vegetated patches regularly alternating with almost bare areas) occur on smooth slopes of many arid lands of the world as a result of rainwater redistribution through sheet-flow run off. This paper reports the vegetation dynamics (through synchronic and diachronic approaches) of both the upslope and downslope fringes of vegetated patches (vegetation arcs) located in the Chihuahuan Desert, Mexico. Ten vegetation arcs were used in a synchronic study and one of them in an 8-year diachronic study (1982-90). Data gathered in 2-m-wide, 8-15-m-long grids of 25-cm X 25-cm contiguous quadrats were used to study the variation of species richness, life-form dominance, dominance-diversity relationships and species composition across the fringes. A colonization process developed in the upslope fringes; (i) species richness increased gradually to a peak and then decreased, as would be expected in an ecotone: (ii) this peak coincided spatially with a change in life form and floristic dominance; and (iii) communities with geometric-like distributions of abundance were replaced by communities with log-normal-like distributions of abundance, which are common in successional processes. The diachronic study showed that the peak of species richness had moved upslope with a concomitant colonization of new space in that direction. The displacement of that peak, coupled with changes over time in the dominance of life forms and in the dominance-diversity relationships, provided additional evidence that a successional process of colonization develops. Likewise, long-term variations in floristic composition of the upslope quadrats were explained by time, position along the grid and rainfall, in order of decreasing importance. There was a determinism both in the life form and in the floristic replacement processes observed upslope, suggesting a general scheme of obligatory succession. The bare area was colonized by short-lived perennials and/or facultative therophytes that almost never appear in the main body of the arcs. These species are gradually replaced by tussock grasses and seedlings of woody species which are the dominants in the main body of the arcs. A different process can be inferred from the structural variability in space observed in the downslope fringes. A spatially non-contagious variation in species richness, a spatially non-patterned variation in life-form dominance, and a constancy in geometric-like dominance-diversity relationships suggested that colonization processes were not developing. A long-term decrease in species richness as well as in life-form diversity and a reduction in the abundance of tussock grasses suggested that a process of senescence without new recruitments was occurring. Analysis of the variation of floristic data through time showed the floristic impoverishment of a grassland community dominated by, Hilaria mutica coupled with a reduction in the abundance of that species. The dynamics of colonization of bare areas in two-phase mosaics offers an example of succession in arid lands that is probably present in all the world areas where these two-phase mosaics have been reported (the Middle East, Africa, Australia and North America).