Plant community organization in New South Wales saltmarshes:species mosaics and potential causes

Abstract

Saltmarsh communities recognised by Outhred and Buckney (1983) bad Adam et al. (1988) were analysed for similarity, and we suggest that the vegetation of the majority of the mid-intertidal marsh in NSW be considered a single community complex, i.e. a mosaic dominated by Sporoblous virginicus, Sarcocornia quinqueflora and Triglochin striatum, wither alone or in combination. Triglochin appeared to be abundant more often in Outhred and Buckney's (1983) study of Kooragang Island that in the broader survey of NSW saltmarshes conducted by Adam et al. (1988). Our field sampling at Kooragang Island suggested that widespread disturbance by cattle could explain such a difference. We characterised spatial variability within the mid-intertidal saltmarsh mosaic at Kooragang and explored evidence suggesting species interactions. All three dominants were positively associated at the square-meter scale in a heavily grazed saltmarsh at Kooragang Island, but not at reference sites having less disturbance from cattle. Negative correlations of cover estimates between pairs of co-occurring species supported the hypothesis that the three dominants are competitors. We hypothesised that Sporoblous is capable of excluding Triglochin in the absence of canopy and/or rhizosphere disruption. Cattle graze the thick canopy of Sporoblous, and their hooves disrupt its dense rhizosphere allowing pooling of tidal water. These wet gaps appear highly suitable for establishment of the waterlogging-tolerant Triglochin. Observations of Sarcocornia in areas of high salinity, regardless of intertidal position, suggest that hypersalinity shifts the competitive advantage. These hypotheses about what controls dominance need to be tested experimentally, paying careful attention to the size of treatment plots and number of replicates in order to detect small changes in species occurrence and cover.