Declining Diversity in Natural and Restored Salt Marshes: A 30-Year Study of Tijuana Estuary

Between 1974 and 2004, Tijuana Estuary's natural salt marsh underwent pulse disturbance (an 8-month nontidal period in 1984), which caused the sudden loss of two short-lived halophytes ( Salicornia bigelovii [Sb] and Suaeda esteroa [Se]) and rapid dominance of a productive native succulent ( Sa. virginica [Sv]), plus ramp disturbance that led to gradual codominance by Jaumea carnosa (Jc) (another productive succulent) by 1994. Species richness was high in 1974 (4.2 species/0.25-m² plot), low in 1984 (1.4 species), and not fully recovered by 1994 (3.7 species) or 2004 (3.9 species). Restoration efforts (reseeding former habitat and excavating and planting new sites) did not recover the populations of Sb or Se. In a 1997 project, plantings of these and six other native halophytes survived initially, but by 2005, short-lived species were lost and Sv and Jc dominated, as in the natural marsh. In a 2000 restoration site, planting mortality was high for five species, but Sv recruited voluntarily and dominated by 2005. We attribute recent vegetation changes to frequent catastrophic storms, flooding, and sedimentation, which contrasted strongly with the benign conditions of decades prior to 1974. Sediment blocked tidal channels in 1984 and gradually elevated the marsh plain, degrading the diverse salt marsh and hindering efforts to restore it. Future restoration efforts will require even greater control over sediment inflows plus contouring sites to include natural topographic features that appear critical to sustaining high species richness and evenness.