Most salt marsh environments are unpredictable for plants, because of the occurrence of both salinity and flooding stress (Cantero et al. 1998). When conditions for seed germination are not favorable, ungerminated seeds of halophytes often remain under enforced dormancy in the soil and serve as a transient or persistent seed bank (Ungar 1995). Thus, seed banks are particularly important in maintaining populations of halophytes in saline marshes (Coteff and Van Auken 2006). Salt marshes are among the most heavily invaded systems in the world (Grosholz 2002). The ability of introduced species to bank seeds can contribute to invasion success, since seeds can persist while waiting for favorable conditions. This ability is especially useful in environments where opportunities for seed germination are infrequent or unpredictable such as salt marshes (Parker et al. 1989). Management of invasive species in salt marshes can be challenging because monitoring and control must continue for at least as long as their seeds persist in the seed bank (Panetta and Timmins 2004).

Rafts of dead plant material, or wrack, are common and abundant in tidal marshes. Due to high primary productivity rates and low consumption rates by herbivorous, halophytes, such as eelgrasses and cordgrasses, produce high quantities of wrack, especially in dye-back areas. This wrack is added up to that transported by rivers to salt marshes in estuaries. Then, tidal creeks provide corridors for wrack transport into salt marshes where it can disturb plant communities (Reidenbaugh and Banta 1980; Bertness and Ellison 1987; Valiela and Rietsma 1995; Tolley and Christian 1999; Brewer et al. 1998; Minchinton 2002). As part of the natural disturbance regime, deposited wrack mats often create bare spots by killing not only the surface vegetation, but also the below-ground biomass (Hartman 1988). Seeds of different salt marsh plants can germinate in the spaces opened by wrack accumulation, playing a key role in plant distribution (Hartman et al. 1983; Pennings and Richards 1998), but germination and emergence may decrease significantly by seed burial in plant debris. Buried seeds are recruited only when they are brought back to the surface by disturbances (Facelli and Pickett 1991). However, these effects are poorly understood since there is a general lack of research that examines the effects of wrack burial in salt marshes.

The austral cordgrass, Spartina densiflora Brongn. (Poaceae) is invading salt marshes in southern Europe, Northwest Africa and the West Coast of North America (Bortolus 2006). In invaded marshes, Spartina densiflora develops very dense populations where large amounts of dead matter are deposited (Nieva et al. 2001; Castillo et al. 2008). Moreover, Spartina densiflora seeds float so they are dispersed by currents and tides together with plant debris (Howard and Sytsma 2013). One of the keys to the success of Spartina densiflora invasion is its ability to produce large quantities of viable seeds (Nieva et al. 2001a, b). Spartina densiflora produces many seedlings in intertidal mudflats but its recruitment is very low in marsh zones where wrack is accumulated.

While cordgrasses (Spartina genus) are one of the most abundant and frequent halophytes in salt marshes, there have not been any studies that examined how their seeds and seedlings respond to wrack burial. We examined under controlled greenhouse conditions the impact of five wrack burial depths (0, 1, 2, 4 and 8 cm) on germination, seed viability and seedling establishment of Spartina densiflora. Main abiotic factors conditioning seed germination and seedling establishment in salt marshes (sediment pH, electrical conductivity, redox potential and temperature) were also recorded for every wrack treatment. We hypothesized that Spartina densiflora germination and establishment would be reduced by wrack burial due to anoxia and/or low temperature fluctuations.

This study shows that germination and establishment of Spartina densiflora, an invasive cordgrass in Europe, North America and North Africa, is greatly limited by wrack burial.

Sediment pH increased with wrack burial load, which may be related to the presence of shells in the debris that would add carbonate to the sediment. However, Spartina densiflora germination would not be altered within the narrow recorded pH range (6.2–7.7) (Curado et al. 2010). Similarly, electrical conductivity varied within a range (0.11–0.23 mS cm^-1) that would not influence Spartina densiflora germination significantly (Castillo et al. 2005). In contrast, sediment redox potential under the debris may have affected germination under 8 cm of wrack since it was as low as -83 ± 7 mV, and germination decreased and quiescence increased at low redox potentials as it has been described previously in anaerobic environments under organic material in lakes (Rich and Wetzel 1978) and in accordance with our hypothesis. Negative values of redox potential can decrease Spartina densiflora germination in the field (Mateos-Naranjo et al. 2008). Sediment anoxia affects seeds by consuming oxygen resulting from degradation of organic matter (Wu et al. 2009). Thus, poor soil aeration may induce quiescence (Vleeshouwers et al. 1995).

The highest germination was recorded without debris (96%), decreasing markedly (to values between 21–43%) between 1 and 4 cm deep with positive redox potentials (> +150 mV). Therefore, other environmental factors in addition to anoxia seemed to be limiting Spartina densiflora germination under the debris. The effects of burial on germination can be mediated by changes in the light regime as it has been described for some halophytes (Pons 1992; Khan and Gul 2002), which is not the case for Spartina densiflora germination that is similar in light and darkness under fresh water conditions (Nieva et al. 2001b). However, synergistic effects between light and other abiotic factors cannot be excluded. Germination of Spartina densiflora increased at lower sediment daily minimum temperatures, conditions that were recorded without or under low loads of wrack, indicating that burial probably caused an unsuitable temperature environment for germination (Benvenuti et al. 2000). Thus, quiescence and dormant seed percentages increased at higher sediment minimum temperatures. In temperate regions, many grass species require exposure to low winter temperatures to come out of dormancy (Baskin and Baskin 1998). Sensitivity to temperature fluctuation functions as a depth- or gap-detecting mechanism; in this way, germination is activated when temperature fluctuation increases at unvegetated areas exposed directly to solar radiation (Thompson and Grime 1983). Furthermore, allelopathic effects from the plant debris inhibiting the germination of Spartina densiflora seeds cannot be excluded (Li et al. 2010; Sieg and Kubanek 2013).

Spartina densiflora did not show primary dormancy since all its seeds germinated or died (only 4%) without wrack in optimal conditions. Instead, Spartina densiflora seeds entered a non-deep physiological dormancy (Baskin and Baskin 1988) under the plant debris. Dormancy percentage increased with higher daily minimum temperature determining a lower daily temperature variation under the wrack. Secondary dormancy may be induced by environmental factors such as high CO₂ levels produced by debris decomposition (Harper and Obeid 1967), poor aeration (Simpson et al. 1989) and low temperature fluctuations (Baskin and Baskin 1998). Longer seed dormancy at greater depths within the debris would be ecologically advantageous because seeds would survive in the dormant state in the seed bank until the upper layer of wrack would be removed. Nevertheless, wrack burial increased seed mortality.

The establishment of Spartina densiflora seedlings was also greatly influenced by wrack. Only six seedlings emerged above the plant debris from a burial depth of 1 cm, while no seedling emerged from deeper than 1 cm (wrack load > 3 kg DW m^-2). The seedling of Spartina densiflora has just one thin and sharp cotyledon that grows easily along an axis, being able to emerge straight away from 4 cm depth in water (Abbas et al. 2012) and in sand (A.M. Abbas, personal observation). In contrast, when a seedling grew within wrack it had to find the few hollows left open in the wrack as is reflected in its cotyledon growing in curves. Every Spartina densiflora seedling dying within the debris was shorter than 2 cm, which seemed to be the longest length they were able to grow under these conditions. The dense structure of the debris would prevent Spartina from emerging from deeper than 1 cm due to the exhaustion of food reserves before getting to the surface (Martínez et al. 1992; Bond et al. 1999). It has been previously described how deeply sown seedlings in sediment died before emerging in wetlands (Hartleb et al. 1993; Jurik et al. 1994; Wang et al. 1994; Dittamar and Nelly 1999; Spencer and Ksander 2002; Ke and Li 2006). Live and dead standing biomass of Spartina patens (Aiton) Muhl prevented seedling emergence of subordinate annuals and perennials in coastal marshes (Brewer and Grace 1990; Baldwin et al. 1996). Spartina densiflora seed germination and quiescence presented a gradual response to wrack burial but seed dormancy, seed and seedling mortality and seedling emergence showed a threshold dynamic in response to wrack burial, increasing markedly even under just 1 cm depth. These results characterized Spartina densiflora as a very sensitive species to wrack burial during the establishment period, which may limit its invasion in those marshes accumulating high debris loads.

Spartina densiflora seedling development was also significantly reduced by wrack burial. Seedlings growing without wrack were ~5.8 cm tall at the end of the experiment while seedlings emerging from 1 cm deep under the wrack were ~2 cm tall (including their buried part), coinciding with lower growth rates. In adittion, seedlings in the control treatment had longer and more roots. In this sense, it has been reported that phytotoxins generated in anaerobic decomposition can inhibit the growth of freshwater plants (Barko and Smart 1986; Maun and Lapierre 1986).

Our experimental results are in accordance with our field observations. In tidal salt marshes, wrack is accumulated mainly coinciding with the mean higher high water. We have observed in the field (Odiel Marshes) that the wrack depth in these areas ranges from 2 to 14 cm. In these marshes, we have seen no Spartina densiflora seedling growing from within the wrack. Just very few Spartina adult clumps were observed within the debris areas, which seemed to have established before wrack accumulation or within open patches in the wrack. Spartina densiflora tussocks accumulate high densities of dead tillers in middle and high marshes (Nieva et al. 2001a) and when this necromasa is detached from the tussocks is accumulated as wrack. In view of our results, as Spartina densiflora invades a location it would decelerate its own invasion rate through the accumulation of wrack that may limit its establishment.

Data gathered during this study confirmed an inverse relationship between germination and emergence with wrack burial for the invasive cordgrass Spartina densiflora. Germination decreased from 96% without wrack to 14% at 8 cm deep in debris (ca. 6 kg DW m^-2). No seedling emerged above the wrack surface for seeds germinated at wrack burial depths greater than 1 cm (a wrack load of ca. 1 kg DW m^-2). The results from this study improved our understanding of Spartina densiflora invasion and they are useful to predict invasion dynamics and to plan the management of invaded marshes. Thus, wrack may be used to limit Spartina densiflora colonization and should not be removed from those areas sensible to the invasion of this cordgrass.