Plant Water Relations in a San Francisco Bay Salt Marsh

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Abstract
Diurnal and seasonal patterns of plant water potential (.psi.) were measured for 3 common salt marsh species, Spartina foliosa, Scirpus robustus and Salicornia virginica, that co-occur in the San Pablo Bay marshes of northern California. For all 3 spp., dawn maximum and midday minimum plant .psi. declined over the growing season in parallel with decreasing soil osmotic potentials. Maximum leaf conductances for Spartina and Scirpus also exhibited large seasonal decreases as salinities increased. Scirpus, which is more abundant in brackish marshes than in true salt marshes, occurs in a narrow zone between the other 2 spp. at the study site. Salinities in this zone were intermediate between those in the Spartina and Salicornia zones and Scirpus experienced intermediate plant .psi.. The persistence of the less halophytic Scirpus in a zone between 2 more halophytic species cannot be explained by lower salinities in this zone. The occurrence of Scirpus in this marsh may be due to its capacity for rapid growth during spring when salinities are low, plus a tolerance for high salinity later in the year after vegetative growth has ceased.