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Abstract. The three dominant tree species, Casuarina obesa, Eucalyptus rudis and Melaleuca rhaphiophylla, of the riverine vegetation of the Harvey River delta showed a general decline in canopy condition over the four years following the opening of the Dawesville Channel. M. rhaphiophylla also showed significant depression in diameter increment for the population closest to the river mouth. The most likely explanation of the tree decline on the Harvey delta is in changes to the salinity regime of the lower Harvey estuary following the opening of the Channel. Further decline of the vegetation is expected.
Key words: vegetation change, salinity, growth rates, canopy decline
Introduction
The Dawesville Channel south of Mandurah was constructed to increase tidal flow within the Peel-Harvey estuary and decrease the occurrence of toxic algal blooms resulting from excess inputs of nutrients from the catchment. Following the opening of the Channel to the sea in April 1994, the tidal amplitude of the Harvey estuary has increased 55% with a significant influx of marine waters and decreased residence time within the estuary (Anon 1998). A study was instigated in late 1994 to monitor the tree health of the lower 4 km of the Harvey River to ascertain if the changes in the salinity regime of the lower Harvey estuary and the greater penetration of salt water up the Harvey River (JAK Lane, CALM, personal communication) were adversely impacting on the riverine vegetation (Fig 1).
Methods
Tree species appear to segregate along the lower Harvey River in response to salinity levels, with the most tolerant species, Casuarina obesa Miq (Casuarinaceae, Saltwater Paperbark) restricted to the lower 1.5 km of the river, and the least tolerant species Eucalyptus rudis Endl (Myrtaceae, Flooded Gum) appearing as dense stands 2 km above the river mouth and continuing far upstream. Melaleuca rhaphiophylla Schauer (Myrtaceae, Swamp Paperbark) is widely distributed along the 4 km length of the lower Harvey.
Thirty individuals of each species were marked at the lower and upper ends of their range in the lower Harvey River. Each individual was marked with a steel tag and girth at breast height over bark (GBHOB) was measured and condition of the tree was scored on a one to five scale (1 < 20% canopy alive, to 5...