Abstract
Background and aims It is generally accepted that in terrestrial ecosystems the occurrence and abundance of plant species in late successional stages can be predicted accurately from prevailing soil conditions, whereas in early succession their presence is much more influenced by chance events (e.g. propagule availability). Late successional vegetation stages would therefore be deterministically structured, while early succession would be dominated by more stochastic features. To test this hypothesis in salt marsh conditions, we compared the effect of abiotic environmental factors on vegetation composition and probability of occurrence of individual species in two adjacent salt marshes, differing in age (i.e. successional stage). Material and methods In 2002, a new salt marsh was created on substrate devoid of plant diaspores in the nature reserve The Uzermonding (Nieuwpoort, Belgium). From 2002 onwards, primary colonization started on that sterile substrate by hydrochoric seed dispersal, induced by tidal water currents from an adjacent 5 ha relic of the old salt marsh. In 2005, three years after the start of the colonization process of the new salt marsh, vegetation and three abiotic environmental factors (soil texture, salinity and elevation) were recorded in a set of 155 releves on the new and old salt marsh. Key results In contrast to the general observation in other terrestrial ecosystems, the vegetation composition of the early successional stage of the new salt marsh appeared to be at least as much determined by the combined effect of the measured abiotic factors as that of the old salt marsh. As revealed by logistic regression the presence/absence of perennial species as well as annual species oldie young salt marsh could be well predicted by the measured abiotic variables. For the old salt marsh this also held for the perennial species, but not for the annual species. The stochastic appearance of gaps in the perennial vegetation cover appeared to be important for the establishment of annuals in the older salt marsh. Conclusion In the case of salt marsh succession, the generally accepted hypothesis of early successional stochasticity dominance versus late successional environmental determinism must be rejected.
Highlights
One of the main aims of plant ecology is to try to understand the causes for patterns in plant distribution
To test whether a unimodal or a linear response curve should be expected, we first carried out a Detrended Correspondence Analysis (DCA) with Hill’s scaling, for both datasets separately
From our comparative study of two adjacent salt marsh areas with different age, we conclude that the generally accepted hypothesis that plant distribution patterns in early successional stages is primarily stochastic in nature while these patterns are much more deterministic in late successional stages, is false in a salt marsh environment
Summary
One of the main aims of plant ecology is to try to understand the causes for patterns in plant distribution. In salt-marsh habitat, the spatial distribution of individual species is usually linked to the concept of succession, i.e. the replacement of plant species in an orderly sequence by colonization and population development (Clements 1916, Odum 1969, Chapman 1976, Glenn-Lewin et al 1992). On emerging salt marshes, after an initial colonization phase, the substrate becomes more stable and sediments are trapped by the vegetation. This will lead to a change in topography, decreasing the frequency of inundation with salt water, and allowing other species to colonize the marsh which would eventually result in a mature and stable climax ecosystem (Odum 1971, Denaeyer et al 1968 for salt marsh conditions). The dynamics of salt-marsh vegetation does not always seem to follow a deterministic succession scheme and many other
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