ROOT DYNAMICS OF MELALEUCA-HALMATURORUM IN RESPONSE TO FLUCTUATING SALINE GROUNDWATER

Melaleuca halmaturorum is a salt and waterlogging tolerant tree and th us often occurs in saline areas fringing permanent wetlands and in eph emeral swamps. The dominance of this tree in natural groundwater disch arge areas may result in M. halmaturorum transpiration making a major contribution to groundwater discharge. To quantify this the seasonal c hanges in tree water sources in response to fluctuating soil salinity and waterlogging were examined. This study was conducted in a natural system where seasonally fluctuating saline groundwater (64 dS m(-1); 0 .3 - 1.2 m deep) allowed the patterns of M. halmaturorum root water up take to be followed over a 15 month period. Tree water sources were ex amined using the naturally occurring stable isotopes of water, while n ew root growth was examined using a field root observation window and from soil scores. The presence of isotopic fractionation of H-2 under conditions of soil salinity and waterlogging was tested in a glasshous e experiment. Measurements of soil and leaf water potential were also made to examine the possible water sources and limits to water uptake. No isotopic fractionation was found by tree roots under conditions of salinity and waterlogging. M. halmaturorum trees were active in takin g up groundwater at most times and combined this with a shallower soil water source replenished by rainfall in winter. Water uptake was conc entrated in the deeper parts of the soil profile when the groundwater was at its deepest and salt had accumulated in the surface soils, at t he end of summer. When groundwater rose, at the end of winter, roots r esponded by extracting water from near the soil surface (0 - 0.1 m), a t the new watertable. This pattern of water uptake in response to grou ndwater fluctuations and salt accumulation in the surface soil was als o reflected in new root tip appearance at the root observation window. Fluctuations in leaf water potential fallowed fluctuations in surface soil (0.1 m depth) water potential at all times. In winter leaf water potential reflected the absolute values of the surface soil water pot ential but in summer it was between surface soil and groundwater water potentials. We conclude that M. halmaturorum used groundwater in summ er and a combination of rainfall and groundwater from the surface soil s in winter. The ability to take up water from saline substrates throu gh the maintenance of low leaf water potential, combined with this abi lity to rapidly alter root water uptake in response to changes in soil water availability contributed to the survival of M. halmaturorum in this saline swamp.