Environmental determinism versus biotic stochasticity in the appearance of plant species in salt-marsh succession

摘要

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.