GROWTH-RESPONSES TO SALINITY IN RELATION TO DISTRIBUTION OF 2 MANGROVE SPECIES, SONNERATIA-ALBA AND S-LANCEOLATA, IN NORTHERN AUSTRALIA

1. There were interspecific differences in salt-tolerance. Sonneratia alba grew in salinities ranging from fresh water to sea water, with growth being maximal in 5 to 50% sea water. In contrast, Sonneratia lanceolata grew in salinities ranging from 0 to 50% sea water, with maximal growth occurring in 0 to 5% sea water. 2. Under optimal conditions for growth of both species (i.e. 5% sea water), the less salt-tolerant species, S. lanceolata, achieved twice the height, leaf area and biomass of the more salt-tolerant species. It appears that increasing salt-tolerance is at the expense of growth and competitive ability under low salinity conditions. 3. Growth analysis showed that change in net assimilation rate accounted for most of the differences in growth between species and for changes in growth by a species with increase in salinity from 0 to 100% sea water. 4. Interspecific differences in salt tolerance were consistent with (but do not fully explain) differential distribution of S. alba and S. lanceolata along natural salinity gradients in northern Australia, with seasonal variation in salinity apparently being an important factor to their survival in many environments.