Increased death of new leaves of coastal banksia (Banksia integrifolia L.f) around ocean sewage outfall sites

Salt-spray damage to coastal vegetation is a worldwide phenomenon. Worsening of such damage by surfactants present in sewage disposed of at sea has been observed in Europe and on trees planted around metropolitan beaches in Australia. Whether adverse impacts of sewage occur on the more widespread native vegetation around non-metropolitan outfalls in Australia is unknown. The present study investigated the survivorship of existing leaves, and of new shoots and leaves, on Banksia integrifolia around three outfall and three non-outfall sites in spring/summer of 1997-1998. Survivorship of existing leaves was almost identical at outfall and non-outfall sites. When plants initiated new shoots over the spring/summer, plants at outfall sites initiated more than plants at non-outfall sites; but the proportion of these new shoots lost subsequently differed little between outfall and non-outfall sites. Survivorship of newly initiated leaves on the new shoots differed between outfall and non-outfall sites, with 65% of the new leaves on average being lost at outfall sites, and 47% being lost at non-outfall sites by the end of summer. The probability of observing a difference of this magnitude ranged between P= 0.057 and 0.11 (February-April 1998). This is the first report of a possible adverse impact on an Australian native plant from sewage discharged to the ocean.

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