Impact of a parasitic plant on the structure and dynamics of salt marsh vegetation

We investigated the effect of a native parasitic plant, Cuscuta salina, on the structure and dynamics of the plant community in a California salt marsh. Cuscuta was common in the middle marsh zones. The abundance of Cuscuta was positively correlated with the abundance of Limonium californicum at two sampling scales (0.25- and approximate to 40-m(2) quadrats). Sampling at the scale of individual plants indicated that the dominant plant in the marsh, Salicornia virginica, was preferred by Cuscuta as a host over Arthrocnemum subterminale, Limonium californicum, and Frankenia salina. This result was confirmed with host-choice experiments in the field. Based on spatial correlations and host-choice experiments, we hypothesized that Cuscuta indirectly facilitated Limonium and Frankenia, increasing plant diversity and initiating vegetation cycles. This hypothesis was supported by sampling patches with different histories of Cuscuta infection. Patches with recent heavy Cuscuta infection had reduced Salicornia biomass and increased Limonium and Frankenia biomass relative to controls. This effect was most pronounced at higher elevations and in larger, older patches. The hypothesis was also supported by sampling permanent quadrats repeatedly over time. The probability that Cuscuta would invade a quadrat increased with increasing cover of Salicornia. Quadrats containing Cuscuta increased in Limonium and Frankenia cover between censuses relative to quadrats lacking Cuscuta. Our results indicate that parasitic plants can have strong effects on the structure and dynamics of natural vegetation assemblages. However, these effects are mediated by physical and biological gradients across the landscape.