Lower limits of Spartina densiflora and S-maritima in a Mediterranean saltmarsh determined by different ecophysiological tolerances

Castillo, JM Fernandez-Baco, L Castellanos, EM Luque, CJ Figueroa, ME Davy, AJ

Jm. Castillo et al., Lower limits of Spartina densiflora and S-maritima in a Mediterranean saltmarsh determined by different ecophysiological tolerances, J ECOLOGY, 88(5), 2000, pp. 801-812

45

INGLESE

Article

Environment/Ecology

JOURNAL OF ECOLOGY

0022-0477 → ACNP

88

5

2000

801 - 812

ISI

0022-0477(200010)88:5<801:LLOSDA>2.0.ZU;2-0

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.