In this new author post Anthony Davy provides an insight into a 35-year longitudinal study of salt marsh development from intertidal flat to a mid-marsh platform at Odiel Marshes in south-western Iberia. The unexpected rapidity of this primary succession highlights the central role of facilitation.
Salt marsh is celebrated as a classic example of primary succession. In fact, succession on the coast is probably the earliest example. Theoretical elements that we have come to apply to salt marsh succession were described by G.M. Lancisi as far back as 1714 (Pignatti & Savoia, 1989). He recognised the importance of the accumulation of alluvial sediments from the River Tiber at the coast, referring to colonization on small hummocks (‘tumuleto’) by pioneer plants (‘plantae primigeniae’), with an explicit time scale for development (‘successio’) of sea shore vegetation through four successive stages.
More recently, our view has perhaps been coloured by the fact that most salt marshes were initiated when coastlines stabilised in their current positions about 6000 years ago, following the rapid rise in sea level after the last glaciation. With average apparent accretion rates of the order of mm per year, succession must have been going on for centuries or millennia you might think? Well, not necessarily and our paper shows that a few decades are quite sufficient under the right circumstances.
In 1977 a 20-km raised dike (Dique Juan Carlos I) was constructed across the Odiel saltmarshes on the Atlantic coast of Spain. Its primary purpose was to protect the navigable channel to the port of Huelva in the joint estuary of the rivers Odiel and Tinto but an unintended consequence was a remarkable ecological experiment. In 1984 this opportunity was seized by a group of ecologists led by Enrique Figueroa at the University of Sevilla (I joined the project on a sabbatical stay there in 1988).
To cut a long story short, protection from the open sea allowed the deposition of fine sediments on unvegetated intertidal flats within tidal lagoons associated with the dike – and thus precipitated succession to salt marsh. Our paper chronicles that developmental process with a 35-year study of one such lagoon, the Laguna de Don Claudio. The striking early colonisation was reported previously (Castellanos, Figueroa & Davy, 1994; Figueroa et al., 2003) but with the passage of time we have a much more complete picture. Furthermore, it now evident that similar changes to those in Don Claudio marsh have been happening in many salt marshes in the Gulf of Cadiz in recent decades.
What we found has unmistakeable echoes of Lancisi but, more than 300 years later, it can provide scientific insights into the mechanisms of primary succession beyond his imagining – in particular the emerging central roles played by facilitation and the stress-gradient hypothesis. Ironically, when our study began, the ecological and evolutionary role of facilitation was far from accepted as a general rule. Neither was it clear in those days that environmental stress would be associated with a shift in species interactions from competition to facilitation.
The first colonists at Laguna de Don Claudio were sporadic clumps of Spartina maritima, a perennial halophytic grass highly tolerant of waterlogged settings low in the tidal frame. It proved efficient at trapping sediment to form raised tussocks, which increased in elevation and expanded in area. The greater elevation of the tussocks reduced tidal inundation and improved local drainage, which in turn alleviated the low redox potentials of their sediments. Higher redox potentials allowed colonization by succulent halophytes that are less tolerant of reducing conditions: Sarcocornia perennis, and then its hybrid with the high-marsh S. fruticosa invaded the tussock centres, and expanded radially, displacing Spartina. Finally, unlike its centrifugally colonising predecessors, the shrub Atriplex portulacoides invaded progressively from raised tussock edges, where drainage was best, to become largely dominant.
Apart from this sequence of species dominance, there was also an interesting geomorphological development. After having increased in elevation by a whole metre (c. 29 mm/year), the tussocks coalesced to form a continuous mid-marsh platform, dissected only by sinuous creeks and levees that occupied some of the former tussock boundaries. Moreover, this phenomenon occurred at an elevation corresponding with the mean level of the high tides. This supports a theoretical punctuated transition from ‘submergence marsh’ to ‘emergence marsh’, previously postulated for such a tidal elevation.
What of the future? This story has probably not reached its conclusion. There is no obvious reason why vertical accretion could not continue until marsh elevation approaches that of the highest astronomical tides, and there are several halophytes typical of high marshes waiting in the wings. With less frequent inundation under dry Mediterranean summers, hypersalinity is increasingly likely. Although salinity has not been a decisive factor so far, in an ‘emergence marsh’ scenario redox potential may give way to salinity as the main environmental stress.
Anthony Davy University of East Anglia, UK
Puede leer una copia de esta publicación de blog en español aquí
Read the full research article online: Primary succession in an Atlantic salt marsh: From intertidal flats to mid-marsh platform in 35 years