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 abundan ce 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 b y Cuscuta as a host over Arthrocnemum subterminale, Limonium californi cum, and Frankenia salina. This result was confirmed with host-choice experiments in the field. Based on spatial correlations and host-choic e experiments, we hypothesized that Cuscuta indirectly facilitated Lim onium and Frankenia, increasing plant diversity and initiating vegetat ion cycles. This hypothesis was supported by sampling patches with dif ferent histories of Cuscuta infection. Patches with recent heavy Cuscu ta infection had reduced Salicornia biomass and increased Limonium and Frankenia biomass relative to controls. This effect was most pronounc ed at higher elevations and in larger, older patches. The hypothesis w as also supported by sampling permanent quadrats repeatedly over time. The probability that Cuscuta would invade a quadrat increased with in creasing cover of Salicornia. Quadrats containing Cuscuta increased in Limonium and Frankenia cover between censuses relative to quadrats la cking Cuscuta. Our results indicate that parasitic plants can have str ong effects on the structure and dynamics of natural vegetation assemb lages. However, these effects are mediated by physical and biological gradients across the landscape.