How waterlogged microsites help an annual plant persist among salt marsh perennials

Annual plants that coexist among perennial dominants might persist in microsites that are stressful to their competitors. In Californian salt marshes, where cover of annual and perennial Salicornia species are negatively correlated, we hypothesized that waterlogged depressions support the annual (Salicornia bigelovii) but not the region's dominant perennial (Salicornia virginica). In a large restoration site, S. virginica cover was low in naturally formed pools, and our 10-cm depressions decreased its cover by approximately 30% compared to the controls. S. bigelovii grew taller and produced more flowers in waterlogged sites with low soil redox potential, and it completed its life cycle in the 5-cm-deep depressions that we created. Experimentally reducing S. virginica canopy cover in shallow depressions also increased the survival of the annual. In the greenhouse, rhizosphere oxidation was indicated as a mechanism for tolerating waterlogging, as S. bigelovii elevated the soil redox potential by 50 mV more than S. virginica did. Also, in the greenhouse, S. bigelovii seedlings actually suppressed the growth of S. virginica seedlings under increased flooding. We conclude that waterlogged microsites help sustain S. bigelovii in Californian salt marshes and that this increasingly rare plant could be managed by adding shallow depressions to restoration sites.