Background/Question/Methods Intraspecific and interspecific facilitation increasing seedling survivorship at increasing plant densities occurred in a greenhouse experiment using both native and exotic annual plants from California. To test for mechanisms of facilitation, seedlings of Vulpia microstachys, Vulpia bromoides, Erodium brachycarpum, Geranium dissectum, and Plantago erecta were grown at varying intraspecific densities with three treatments. Pots with toothpicks added, with plants cut at surface level, and with holes poked in the soil were compared against controls. These treatments tested the effects of increased drainage, presence of plant roots, and increased soil aeration, respectively. The presence of mycorrhizal fungi as a mediating influence on the facilitative interaction was tested by treating half the pots with fungicide.

Results/Conclusions Pots with toothpicks added had greater survivorship than pots with cut plants. It appears that the facilitation we observed was due to increased soil drainage due to the root structures of neighboring plants. Pots poked to increase soil aeration had the most mortality, and cut plant pots were similar to controls. Fungicide-treated pots had higher survivorship than untreated pots, indicating that the effects of pathenogenic fungi outweigh any positive effects that mycelial networks might have on increasing plant survivorship in the high phosphorous soil type used in this experiment. Species responded positively to the fugicide treatments with different magnitudes: P. erecta was the most affected while E. brachycarpum was the least affected. While competitive interactions among annual plants reduce seed set per individual plant, facilitation in survivorship acts to maintain genetic diversity in these populations by reducing the variance in reproductive outputs.