1 Salt marshes in south-west Spain are being invaded by Spartina densiflora
, a South American introduction, although the native S. maritima still domi
nates many lower marshes. A transplant experiment was used to investigate t
he means by which physical and chemical factors may determine lower vegetat
ion limits in the tidal frame. Both species were transplanted from a mid-le
vel marsh to lower, unvegetated tidal flats.
2 The survival and growth of transplanted clumps and their constituent till
ers were monitored on an elevational gradient. The photosynthetic competenc
e of transplants was assessed by measurements of leaf gas exchange and the
fast kinetics of chlorophyll fluorescence. Submergence period, salinity, re
dox 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 (+ 1.0
4 m relative to Spanish Hydrographic Zero). At + 1.41 m elevation, S. marit
ima survived well but all clumps of S. densiflora died. At higher elevation
s (+ 1.46-1.67 m), clumps of both species had high survival rates. Tiller g
rowth rates in surviving clumps of both species increased with elevation, b
ut that of S. densiflora was more sensitive to low elevation.
4 S. maritima showed no impairment of photosynthetic performance, even at t
he lowest elevation; its rates of gas exchange were independent of elevatio
n, as were its chlorophyll fluorescence parameters. In contrast, in S. dens
iflora the rate of CO2 uptake declined and stress to photosystem II (Fv/Fp)
increased at lower elevations; both of these photosynthetic measurements w
ere 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 re
duced growth and survival of S. densiflora at lower levels appear to result
primarily from impairment of photosynthesis when root conditions are hypox
ic. 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 sp
ecies in a Mediterranean-type salt marsh and thus provide a basis for futur
e investigations of interactions between them. The correlation between chlo
rophyll fluorescence and long-term survival of S. densiflora suggests that
this short-term measure may be a valuable tool.