GROWTH AND WATER-USE OF THE MANGROVES RHIZOPHORA-APICULATA AND RHIZOPHORA-STYLOSA IN RESPONSE TO SALINITY AND HUMIDITY UNDER AMBIENT AND ELEVATED CONCENTRATIONS OF ATMOSPHERIC CO2

Two mangrove species, Rhizophora apiculata and R. stylosa, mere grown for 14 weeks in a multifactorial combination of salinity (125 and 350 mol m(-3) NaCl), humidity (43 and 86% relative humidity at 30 degrees C) and atmospheric CO2 concentration (340 and 700 cm(3) m(-3)). Under ambient [CO2], growth responses to different combinations of salinity and humidity were consistent with interspecific differences in distrib ution along natural gradients of salinity and aridity in northern Aust ralia. Elevated [CO2] had little effect on relative growth rate when i t was limited by salinity but stimulated growth when limited by humidi ty. Both species benefited most from elevated [CO2] under relatively l ow salinity conditions in which growth was vigorous, but relative grow th rate was enhanced more in the less salt-tolerant and more rapidly g rowing species, R. apiculata. Changes in both net assimilation rate an d leaf area ratio contributed to changes in relative growth rates unde r elevated [CO2], with leaf area ratio increasing with decrease in hum idity. Increase in water use efficiency under elevated [CO2] occurred with increase, decrease or no change in evaporation rates; water use c haracteristics which depended an both the species and the growth condi tions. In summary, elevated [CO2] is unlikely to increase salt toleran ce, but could alter competitive rankings of species along salinity x a ridity gradients.