1 Salt marshes in south-west Spain are being invaded by Spartina densi¯ora, a South American introduction, although the native S. maritima still dominates many lower marshes. A transplant experiment was used to investigate the means by which physical and chemical factors may determine lower vegetation limits in the tidal frame. Both species were transplanted from a mid-level marsh to lower, unvegetated tidal¯ats. 2 The survival and growth of transplanted clumps and their constituent tillers were monitored on an elevational gradient. The photosynthetic competence of transplants was assessed by measurements of leaf gas exchange and the fast kinetics of chlorophyll¯uorescence. Submergence period, salinity, redox potential and sulphide concentration in the sediment were also examined at the transplant sites. 3 Neither species survived for a year at the lowest transplant point ( 1.04 m relative to Spanish Hydrographic Zero). At 1.41 m elevation, S. maritima survived well but all clumps of S. densi¯ora died. At higher elevations ( 1.46±1.67 m), clumps of both species had high survival rates. Tiller growth rates in surviving clumps of both species increased with elevation, but that of S. densi¯ora was more sensitive to low elevation. 4 S. maritima showed no impairment of photosynthetic performance, even at the lowest elevation; its rates of gas exchange were independent of elevation, as were its chlorophyll¯uorescence parameters. In contrast, in S. densi¯ora the rate of CO 2 uptake declined and stress to photosystem II (Fv/Fp) increased at lower elevations; both of these photosynthetic measurements were linearly related to sediment redox potential. Stomatal conductance did not vary with elevation. 5 S. maritima has a potentially wide elevational tolerance and an absolute lower limit substantially below that of S. densi¯ora. The progressively reduced growth and survival of S. densi¯ora at lower levels appear to result primarily from impairment of photosynthesis when root conditions are hypoxic. This eect is mediated through eects on the photochemical apparatus rather than on stomatal resistance to CO 2 uptake. 6 These studies de®ne lower limits to the fundamental niches of the two species in a Mediterranean-type salt marsh and thus provide a basis for future investigations of interactions between them. The correlation between chlorophyll¯uorescence and long-term survival of S. densi¯ora suggests that this short-term measure may be a valuable tool.

The growth responses of the hygro-halophyte A. portulacoides to salinity appear largely to depend on changes in its rate of photosynthetic gas exchange. Photosynthesis appears to be limited mainly through stomatal conductance and hence intercellular CO(2) concentration, rather than by effects on PSII; moderate salinity might stimulate carboxylation capacity. This is in contrast to more extreme halophytes, for which an ability to maintain leaf area can partially offset declining rates of carbon assimilation at high salinity.

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Summary 1We examined salt marsh development over 17 years (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) in a rapidly accreting, well-drained lagoon of a macrotidal marsh in south-west Spain. Scattered, elevated tussocks of colonizing Spartina maritima expanded radially until they either coalesced or were separated only by narrow drainage channels. We recorded changes in elevation of the tussocks and investigated the cover and biomass of successive species invading them. 2 Sediment accretion produced a mean annual increase in tussock elevation of 3.5 cm. Sarcocornia perennis had begun to invade the raised centres of the tussocks by 1984, subsequently displacing Spartina maritima radially to become dominant by 1990. A hybrid form of Sarcocornia , which appeared on some of the tussocks occupied by S. perennis in 1997, also expanded radially and had achieved dominance on many of the invaded tussocks by 2001, by virtue of its more erect growth form and rapid accumulation of high above-and below-ground biomass. 3Tussocks not yet invaded by the hybrid remain dominated by S. perennis and have maintained sediment accretion rates and redox potentials similar to those that have been invaded. 4 Genetic analysis, using random amplified DNA (RAPD) markers, indicated that the hybrid was a cross between the diploid Sarcocornia perennis (2 n = 18) and the octaploid S. fruticosa (2 n = 72), a high-marsh species, and suggested that each hybrid individual may have resulted from a separate pollination of an indigenous S. perennis plant. 5 Invasion by the new hybrid thus probably occurred as a result of pollen flow from high-marsh S. fruticosa , some 1 km distant, to the stigmas of the established dominant S. perennis . Succession might therefore be facilitated genetically rather than simply by the enhanced sediment accretion, which ameliorated the effects of submersion and low sediment redox potentials that presumably exclude S. fruticosa from lower parts of the marsh.

SUMMARY1. Alien plant species are rapidly spreading in aquatic ecosystems around the world, causing major ecological effects. They are typically introduced by humans, after which natural vectors facilitate their further spread. Migratory waterbirds have long been recognised as important dispersal vectors for native and aquatic plants, yet little is known about their role in the spread of alien species. 2. We determined experimentally the potential for long-distance dispersal of native and alien wetland plants in Europe by two abundant waterfowl: mallards Anas platyrhynchos and greylag geese Anser anser. We fed seeds from two plants alien to Europe and two native plants to 10 individuals of each bird species, testing for the effects of bird and plant species on the potential for dispersal. 3. Intact seeds were retrieved from faeces for up to 4 days after ingestion. The proportion of seeds retrieved intact varied significantly between plant, but not bird, species. Retrieval was highest for the invasive water primrose Ludwigia grandiflora (>35% of ingested seeds), lowest for the invasive cordgrass Spartina densiflora (<3%) and intermediate for the native glasswort Arthrocnemum macrostachyum and seablite Suaeda vera (5-10%). 4. Seed retrieval patterns over time varied between both plant and bird species. Contrary to expectations, seeds were retained in the gut for longer in the smaller mallards. No Spartina seeds germinated after retention for over 8 h, whereas some seeds of the other species germinated even after retention for 72 h. Germinability was reduced by gut passage for Ludwigia and Arthrocnemum seeds. Ludwigia seeds recovered from geese were more likely to germinate than those recovered from mallards. Time to germination was reduced by gut passage for Spartina and Ludwigia, but increased with retention time. 5. Ducks and geese evidently have the potential for long-distance transport of alien and native plant seeds, with maximal dispersal distances of well over 1000 km. The much greater potential of Ludwigia than Spartina for dispersal by waterfowl is consistent with its faster expansion across Europe. Maximum retention times of wetland seeds have been underestimated in previous experimental studies that lasted only 1-2 days. Contrary to previous studies, wetland plants with large seeds, such as Ludwigia, can still show high potential for long-distance dispersal. More attention should be paid to the role of waterbirds as vectors of alien plants and to the role of migratory geese as vectors of plants in general.