1 Salt marshes in south-west Spain are being invaded by Spartina densiflora, 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 flats.
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 fluorescence. 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. densiflora 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. densiflora 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 fluorescence parameters. In contrast, in S. densiflora the rate of CO2 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. densiflora. The progressively reduced growth and survival of S. densiflora at lower levels appear to result primarily from impairment of photosynthesis when root conditions are hypoxic. This effect is mediated through effects on the photochemical apparatus rather than on stomatal resistance to CO2 uptake.
6 These studies define 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 fluorescence and long-term survival of S. densiflora suggests that this short-term measure may be a valuable tool.