PARTITIONING OF SPACE AND WATER BETWEEN TWO CALIFORNIA ANNUAL GRASSLAND SPECIES

We examined the potential for resource partitioning between two sympatric species with similar phenologies but different rooting morphologies. The annual grass, Bromus diandrus (Roth.), and annual forb, Erodium botrys (Cav.), were grown in monoculture and 50:50 mixed stands at each of three densities (10, 30, 100 seeds/dm²) in a randomized complete block design. Plants were grown outdoors, in 1-m-tall 15-cm-diameter containers. Comparison of seed number produced per plant in mixture and in monoculture indicated greater effects of intraspecific than interspecific competition for Erodium. Such differences were not detected for Bromus seed number, and the converse relationship was suggested from Bromus shoot biomass. Final size inequality of Bromus populations tended to be higher in monoculture than in mixture; no patterns in Erodium size distribution over time or stand composition were evident. Bromus roots were primarily in the upper 10 cm of soil, while Erodium roots were bimodally distributed in the surface and deep soil. Roots of the two species in mixture showed a distribution pattern intermediate between those of the two monocultures. The rate of soil water depletion was higher in the high density than in the low density stands, but was not dependent on stand composition at a given density. Partitioning of belowground space and water resources by groups of species with different root morphologies may partially explain the high species diversity in the grassland.