Tiller dynamics of Spartina maritima in successional and non-successional Mediterranean salt marsh

Tiller demography was compared in two populations of Spartina maritima present at similar elevations in the coastal saltmarshes of Odiel (Huelva, S.W. Spain). The successional population consisted of colonizing tussocks in a littoral lagoon, and the non-successional population comprised a stable award that had fringed a major channel for 40 years. At both sites S, maritima was replaced by Arthrocnemum perenne at higher elevation, where sediments were less reducing. Rapid, consistent sediment accretion confirmed the successional nature of the lagoon site but there was little net accretion in the stable sward. Census of permanent quadrats at the successional site chronicled moving concentric 'waves' of high tiller density as tussocks expanded. Initially high densities declined after one year to low values at the end of the second year but they had almost recovered after 3 years. The decline represented a combination of reduced numbers of births and increased numbers of deaths. Tiller densities were substantially higher in the stable sward and showed relatively small fluctuations with time. The underlying risk of tiller mortality was similar in the two populations for much of the time but after two years there was increased mortality, mainly associated with flowering, at the successional site; very few tillers flowered in the sward. This mortality contributed to a shift to a younger age structure in the successional population. Data aggregated over consecutive 3-monthly periods were examined for density dependence. None was found in the successional population. In the sward population there was evidence of density-dependent adult and juvenile mortality of tillers, particularly over the first Is months of the study, when there were compensatory responses to subtle variations in density. The lack of density dependence and relatively low peak density of about 2000 m(-2) near to the leading edges of the expanding tussocks at the successional site suggest that tiller placement there was regulated mainly by physiological mechanisms affecting rhizome growth and bud development in well integrated clones.