Contrasting water relations of three coastal tree species with different exposure to salinity

This field study examined the ecophysiological responses of three tree species to salinity in the Austin Bay Nature Reserve, adjacent to the Peel‐Harvey Estuary in Western Australia (115°46′ E 32°37′ S). The area is at increased risk of flooding with saline water during storm surges due to the construction of a channel between the estuary and Indian Ocean in 1994.Banksia attenuataR.Br. occurs on small sandy ridges adjacent to a seasonal wetland, whileMelaleuca cuticularisLabill. andCasuarina obesaMiq. occur in a seasonally flooded wetland. Landscape position determined exposure to salinity, withM. cuticularisandC. obesaexperiencing high soil and groundwater salinity during summer (electrical conductivity, EC, up to 70 dS m−1) whileB. attenuatawas not exposed to soil or groundwater with EC greater than 20 dS m−1.B. attenuatahad relatively stable leaf water status throughout the year and did not osmotically adjust as root‐zone salinity increased. By contrast,M. cuticularisandC. obesahad large variation in stem water potential and exhibited osmotic adjustment during summer. Whereas the sap flow rates ofM. cuticularisandC. obesaremained high throughout the year, sap flow ofB. attenuatadecreased during summer which may have limited uptake of salt. The three species also exhibited differences in traits associated with tissue‐level salt tolerance, asM. cuticularisandC. obesaproduced compatible organic solutes (methyl proline inM. cuticularisand proline inC. obesa), whereasB. attenuatadid not. The distributions of these species within the Austin Bay Nature Reserve are determined in part by their tolerance to salinity, which will influence their responses to hydrological disturbance.