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.
Abstract. Photosynthetic stress in response to a natural episode of frost and seasonal drought was assessed in a ‘dehesa’ grassland of SW Spain with a portable fluorimeter. Chlorophyll fluorescence characteristics of dark‐adapted leaves of 11 abundant species of Mediterranean grassland were measured over the course of a growing season from November 1992 to July 1993. Concomitant changes in population size were estimated from censuses of permanent quadrats. There was a general decline in the photochemical efficiency (Fv/Fm) during the growing season and this was particularly evident late in the growing season (spring and early summer) when ambient temperatures were increasing rapidly and rainfall was low; it coincided with the period of most intense mortality for most species. A frost in early March (‐ 5 °C), when photosynthetically active radiation was relatively high, resulted in a small decrease in Fv/Fm that was consistent across many species. The mechanisms of protection in species of Mediterranean grassland appear to be sufficiently effective to avoid damage to PSII for most of the year. For most species there was little evidence of photosystem II damage, as initial fluorescence (F0) usually did not increase. Many of the effects observed were due to a reduction in Fm and thus were consistent with non‐photochemical quenching. This could be adaptive in protecting PSII from damage in species that show little evidence of stress. The sharp increase in stress toward the end of the life cycle coincided with the fall in net population size.
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