Research Doctorate - Doctor of Philosophy (PhD) / The Hunter Estuary Wetlands (NSW, Australia) are important locally, nationally and internationally. They contain significant breeding and nursery grounds for commercial fisheries and are essential shorebird foraging and roost sites. Originally a mosaic of fresh- and salt-marsh, these wetlands have become degraded due to the erection of flood mitigation structures. Reintroduction of a more natural tidal regime is proposed, which is expected to decrease freshwater macrophytes and increase saltmarsh distribution. An a priori approach was undertaken to assess the relative salinity tolerance of three macrophytes, prior to restoration commencing. Study species included a glycophyte, Phragmites australis (Cav) Trin. ex Steudel, and two closely related estuarine saltmarsh species, the invasive exotic Juncus acutus L. and native Juncus kraussii Hochst.. Short- and long-term effects of salinity at key life stages were assessed for each species. For P. australis, the reliability of physiological and morphological responses to salinity stress was assessed under both laboratory and field conditions as potential indicators for future monitoring of initial restoration progress. Competitive/facilitative interactions between the two Juncus species under various salinity regimes were also examined. Results showed salinity affected viability of P. australis but not Juncus species seeds. Irrespective of species, cooler temperatures enhanced germination capabilities under saline conditions. Juncus species displayed superior germination capabilities ≤ 10 ppt salinity; however, unexpectedly, above 10 ppt germination of P. australis was higher. All three species are highly salt tolerant, although salt adaptation mechanisms were found to differ among species. P. australis excluded sodium (Na+) where possible, only accumulating Na+ to toxic levels beyond particular salinity concentrations (~ 20 ppt) and temporal duration (four months). Juncus spp. accumulated Na+ in both root and shoot tissue without noticeable damage. Overtime, J. acutus regulated Na+ uptake at exposure concentrations above 5 ppt salinity, while J. kraussii did not commence regulation until concentrations exceed 10 ppt. A 50% reduction in photosynthesis, biomass, height and density of P. australis was apparent at 20 ppt salinity and mortality at 30 ppt. In P. australis, although height and density were indicative of salinity stress under laboratory conditions, only density showed potential as an indicator of reduced vigour under field scenarios, providing a valuable potential tool to track initial expected restoration trajectories. Although affected, neither Juncus species experienced a 50% reduction in measured endpoints at 40 ppt salinity. However, biomass allocation was asymmetrical. Under stressful conditions, J. acutus maintained shoot increase at the expense of root development. Conversely, as salinity rose J. kraussii preserved root development rather than shoot growth. J. acutus was facilitated by the presence of J. kraussii under freshwater conditions, but suffered a competitive response at 10 ppt salinity. Juncus kraussii was detrimentally affected by being grown with J. acutus at 5 ppt, but unaffected under non-saline and 10 ppt salinity conditions. All three species possess overlapping salinity tolerances. Creating conditions that favour a particular species is perhaps not realistic, given the limited resources of many restoration initiatives. Flooding duration, depth and waterlogging may modify these results. However, the most plausible scenario is that P. australis will continue to dominate marshes after tidal reinstatement. With time, where soil salinity rises above 30 ppt, distribution of Juncus species will increase. The relative salinity tolerances of J. acutus and J. kraussii are analogous. Under mild salinity regimes J. acutus is likely to out-compete J. kraussii. Juncus kraussii is expected to be restricted to areas of high salinity stress.
| Identifer | oai:union.ndltd.org:ADTP/222116 |
| Date | January 2008 |
| Creators | Greenwood, Mary |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright 2008 Mary Greenwood |
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