EDAPHIC FACTORS AND COMPETITION AFFECT PATTERN FORMATION AND INVASION IN A CALIFORNIA GRASSLAND

We used soil analyses, a greenhouse experiment, and a field experiment to examine the roles of competition and abiotic soil factors in plant community pattern formation in a California grassland. We collected soils from five microhabitats: one dominated by the native perennial grass Sporobolus airoides Torrey (alkali sacaton), two dominated by the nonnative annual grass species Hordeum marinum (Hudson) ssp. gussoneanum (Parl.) Thell (foxtail) and Bromus diandrus Roth (ripgut grass), and bare soil created either by gopher mounds or by high salt concentrations (sodic soils). We used principal components analysis (PCA) to determine whether soils from these five microhabitats differed in any discernible pattern. In the greenhouse we examined the emergence, growth, and survival of the native perennial in soil from the five habitat types. In the field we then examined a combined survival and reproduction measure to assess how competition with the two nonnative annual grasses affected the fitness of S. airoides. PCA revealed that B. diandrus soils were distinct from soils occupied by S. airoides, although the perennial emerged, grew, and survived well on these soils in the greenhouse. Despite some differences in soil quality in the greenhouse experiment, sacaton emergence, growth, and survival were possible in all five soils. Those S. airoides plants that survived in sodic soils grew far larger than plants grown in other soils; survival, however, was very low in sodic soils. Sacaton actually performed worst in soils from areas dominated by S. airoides. In the field experiment, total available soil nitrogen affected reproduction and survival. In addition, soil ammonium levels interacted with competition from B. diandrus to produce lower survival than in control plots. There was no survival in the presence of high densities of H. marinum gussoneanum. The two nonnative species seem to represent threats from two distinct types of invaders: (1) “selective invaders,” which only invade a narrow range of habitats but can restrict natives from those habitat types, and (2) “twin invaders,” which have similar niches to the native and can exist over almost the same range of habitats as the native. Twin invaders can potentially reduce fitness or threaten the persistence of the native over much of its range.