The interactive effects of salinity and water regime on two common wetland plants with contrasting growth forms: Melaleuca ericifolia and Vallisneria australis

It is generally accepted that predicting plant persistence in wetlands subject to a modified water regime requires an understanding of the response of different life history stages of key species to water regime, as longevity and tolerance of water regime may differ between seeds, vegetative propagules, seedlings and mature plants. The majority of studies examining the response of wetland plants to water regime, however, have focussed on only one life history stage, most notably germination from the seedbank. Few studies have examined how other environmental factors, such as salinity, might alter the likely persistence of plants under different water regimes. This study used the Wetland Sieve Model (van der Valk 1981) as a framework for examining the persistence of two perennial native plants of contrasting growth form, the wetland tree, Melaleuca ericifolia Sm. (Myrtaceae), and the submersed monocot, Vallisneria australis S.W.L. Jacobs & D.H. Les (Hydrocharitaceae), in response to water regime at a range of salinities likely to be experienced in the field. Both species were common at Dowd Morass State Game Reserve, a large (1500 ha) continually flooded (> 30 years, ~ 60 cm deep) mesohaline (up to ~ 20 dS m"1) wetland in the Gippsland Lakes region of southeastern Australia. As a woody emergent, M. ericifolia is likely to require drawn down conditions for establishment, whilst V. australis, being a submersed species, is likely to require flooding (van der Valk 1981). Mesocosm and laboratory-based experiments were used to determine the capacity of different life history stages (regeneration potential of seed and vegetative propagules, seedling establishment and mature plants) of both species to persist under a number of water regime and salinity scenarios. The response of both species to a series of large-scale, experimental drawdowns of water level at Dowd Morass was also examined. The findings of laboratory and field-based experiments supported the widely-held belief that prolonged drying or flooding can limit wetland plant diversity by favouring certain growth forms over others. Continual flooding prohibited seedling establishment of M. ericifolia and compromised the condition and vegetative expansion of mature trees. Conversely, drying of the sediment resulted in the loss of mature V australis and prohibited their vegetative regeneration. The results indicate that a dynamic water regime is likely to facilitate the presence of all life stages of both species at some stage, under freshwater conditions. Salinity, however, compromises the capacity of M. ericifolia and V. australis to recover from periods of flooding and drying, respectively. The Wetland Sieve Model was generally accurate in predicting the likely establishment of both species during drawn down or flooded periods under fresh conditions. Prediction of a species’ response to a particular wet-dry cycle could be improved by considering the effects of timing and duration of flooding and drying cycles on plant persistence. In addition, incorporation of salinity would improve its explanatory power and thus improve the utility of the model to wetland managers. This study shed new light on how salinisation can alter the response of wetland plants to water regime, and may restrict the ability of wetland plants to respond favourably to a reinstatement of dynamic wet-dry cycles.