SEASONAL WATER-UPTAKE AND MOVEMENT IN ROOT SYSTEMS OF AUSTRALIAN PHRAEATOPHYTIC PLANTS OF DIMORPHIC ROOT MORPHOLOGY - A STABLE-ISOTOPE INVESTIGATION

A natural abundance hydrogen stable isotope technique was used to stud y seasonal changes in source water utilization and water movement in t he xylem of dimorphic root systems and stem bases of several woody shr ubs or trees in mediterranean-type ecosystems of south Western Austral ia. Samples collected from the native tree Banksia prionotes over 18 m onths indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral ro ots acquired water mostly by uptake of recent precipitation (rain wate r) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of thes e two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high su mmer, shallow lateral roots remained well-hydrated and shoots well sup plied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete th eir seasonal extension growth during the long (4-6 month) dry season. Parallel studies of other native species and two plantation-grown spec ies of Eucalyptus all demonstrated behavior similar to that of B. prio notes. For B. prionotes, there was a strong negative correlation betwe en the percentage of water in the stem base of a plant which was deriv ed from the tap root (ground water) and the amount of precipitation wh ich fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources w hile in the wet season most of the water they use is derived from shal lower sources supplied by lateral roots in the upper soil layers. The data collected in this study support the notion that the dimorphic roo ting habit can be advantageous for large woody species of floristicall y-rich, open, woodlands and heath lands where the acquisition of seaso nally Limited water is at a premium.