NUCLEATION AND FACILITATION IN SALT-MARSH SUCCESSION - INTERACTIONS BETWEEN SPARTINA-MARITIMA AND ARTHROCNEMUM PERENNE

1 The construction of a raised dike across the coastal saltmarshes of Odiel (Huelva, south-west Spain) in 1977 divided a uniform area of low -lying sediment into two lagoons with very different drainage properti es. To the east of the dike, there is rapid drainage into the estuarin e channel of the Odiel and Tinto rivers, whereas to the west, the deve lopment of sand spits has impeded drainage such that standing water pe rsists long after high tides, despite the slightly higher elevation of this area. 2 Both lagoons have been colonized by isolated clones of S partina maritima, which have locally enhanced accretion to form domed tussocks ('nucleation'). The diameter of the tussocks was highly corre lated with the elevation of their sediment surface, the larger tussock s having been the earlier colonists in a generally accreting system. S partina in the interior of the tussocks showed reduced tiller density and vigour. 3 Only in the better-drained lagoon have the central, high er areas of the Spartina tussocks been invaded by Arthrocnemum per per enne, although seed is freely available in both lagoons. Arthrocnemum formed a sprawling, dense canopy and a superficial, relatively impenet rable root system; it rapidly suppressed the remaining tillers of Spar tina, tina, eventually leaving only a fringe around the edge of the tu ssock. Areas invaded by Arthrocnemum were characterized by a superfici al layer (10 cm) of oxidizing sediment (E(h)>+80 mV). Spartina-dominat ed areas in both lagoons remained highly reducing, even in the surface layers (E(<)-150 mV). 4 In terms of successional mechanisms, primary colonization by Spartina maritima facilitates the invasion by Arthrocn emum, which only becomes established from seed on relatively well-drai ned, oxidising sediments. The interaction is, however, more complicate d than this: declining tiller density within the tussocks prior to inv asion by Arthrocnemum is consistent with an inhibition mechanism, and the superior competitive ability of Arthrocnemum, once established, su ggests that a tolerance mechanism may also operate.