S. alterniflora has strong adaptability and tolerance on stress environment with broad distribution (30°-50° N). Furthermore, the species can grow well in different substrate (Xu & Zhuo, 1985) and have a large degree of plasticity and create some growth forms in different habitats. These growth forms have a genetic basis, either in the basic coding information or regulatory mechanisms that becomes permanently set at an early development stage (Gallagher et al., 1988). However, 55% phenotypic variability of S. alterniflora is due to environmental variation (Richard et al., 2005). Salt marsh is one of the most stressful habitats in the world. There are direct relationship between successful expansion and outbreak of S. alterniflora and its powerful tolerance to the environmental factors of this stressful habitat, and the powerful tolerance consequently resulted to its competitive superiority (Pennings et al., 2005).
S. alterniflora is a facultative halophyte. It can tolerate a broad range of salinities from near fresh water to ocean concentrations. Salt crystals can often be seen on the leaves during the growing season. The highest growth rate occurred at salinities of 20%o or less, with the upper limit for salt tolerance being 60%. S. alterniflora could adopt multi-approach to tolerate salinity stress. In salinity habitats, the species tolerate the most of salinity stress by exclusion, then excretion and accumulation (Bradley and Morris, 1991). In addition, S. alterniflora has also been shown to combat high intracellular salinity with osmotic adjustment (Cavalier, 1983). Furthermore, the cell structure of the species presents high stability to salinity stress. Only part of mitochondria structure of S. alterniflora seedling had been damaged under 200 m mol/L NaCL treatment, and the treatment did not effect the other cell organelles and cell wall ultramicrostructure.
The salt marshs are flooding regularly by tide. Flooding results in plant hypoxia and leads to the reduction of photosynthesis, and consequently decreases plant growth. The extensive aerenchyma system of S. alterniflora may supply submerged portions of the plant with atmospheric oxygen as well as lower metabolic demands of the plant (Maricle & Lee, 2002). Furthermore, the efficiency of oxygen transport exhibits Matthew Effect. Dense growth and expansion facilitate the tolerance of hypoxia and enhance the invasiveness of the species (Bertness, 1991). The species can stand 12 h inundation each tide (Xu & Zhuo, 1985). The root growth was restrained when oxidation-reduction potential (ORP) of soil was lower than +350 mv, and it was seriously impacted when ORP under +200 mv (Pezeshki, 1997).
As marsh elevation increases to landward, tide related abiotic factors, such as the frequency and duration of inundation, are reduced and competition for resources becomes increasingly important in structuring plant communities (Gray, 1992; Huckle et al., 2002; Daehler, 2003). In inundated and high salinity habitats, S. alterniflora showed significantly competitive superiority (Wijte & Gallagher, 1996a, 1996b). The level of nitrogen nutrition affects the result of competition of S. alterniflora with co-existed species. Lower nitrogen level was disadvantageous for S. alterniflora growth, whereas, the environmental pollution and eutrophication increased the nitrogen nutrition and enhance the species invasion and expansion (Levine et al., 1998; Tyler et al., 2007; Zhao et al., 2007).
High sulfidic concentration in salt marsh limited the growth of some plant species. Increased sulfide in the rhizosphere reduces the ability of Phragmites to take up nutrients. However, S. alterniflora are better-adapted to sulfidic soil conditions (Stribling, 1997; Chambers et al., 1998). The presence and distribution of sulphate Bacteria on the roots and rhizomes of S. alterniflora were more than that of other salt marsh species, and high sulphate reduction rates (SRR) could mitigate negative action of sulfide (Nielsen et al., 2001). Furthermore, S. alterniflora could exclude the competitor through releasing sulfide to soil. The sulfide concentration in sediment of S. alterniflora communities was more than ten times of that of Phragmites australis communities (Chambers et al., 1998; Seliskar et al., 2004).
The relationships between plants always vary with habitats change, i.e. the relative importance of competition and mutualism varies with environmental condition (Brook & Callaghan, 1998; Callaway & Aschehoug, 2000). The relationship between plants presents competition rather than mutualism in better habitats, and the reversed relationship is more common in stress conditions (Bertness & Callaway, 1994; Bruno, 2000; Callaway & Penning, 2000). S. alterniflora could benefit through the mutualism relationship when it coexists with Ascophyllum nodosum. The latter provides more nutritions for S. alterniflora to increase the biomass accumulation, reversion, and the shade of the canopy of S. alterniflora decrease the leaf-surface evaporation of Ascophyllum nodosum (Gerard, 1999).
The symbiosis of S. alterniflora and fixed nitrogen bacteria could mitigate the limitation of poor nitrogen soil condition (Dai & Wiegert, 1997; Boyer & Zedler, 1998; Piehler et al., 1998; Tyler et al., 2003). The symbiosis between S. alterniflora and sulfidic bacteria could improve the rhizosphere microhabitat (Nielsen et al., 2001).
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