Growth, water use efficiency, and adaptive features of the tree legume tagasaste (Chamaecytisus proliferus Link.) on deep sands in south-western Australia

Four-year-old tagasaste trees in dense plantation and wide-spaced alley cro pping layouts at Moora, Western Australia, were cut back to 0.6 m high and their patterns of coppice regrowth and water use monitored over 3 years. Tr ees reached a permanent fresh watertable at 5 m depth by means of deeply pe netrating sinker roots. Dry matter (DM) accumulation and transpiration loss were closely similar at the 2 planting densities despite higher soil water contents in alley plots. Yearly transpiration at plantation density amount ed to 0.55 and 0.63 of Penman-Montieth potential evapotranspiration (E-0) i n the second and third years, respectively. Mean water use efficiency over the 3 years was 247 L/kg DM, compared with values in the range 186-320 L/kg for younger pot- and column-grown trees. Using a combination of neutron mo isture metre (NMM) assays of soil moisture and deuterium: hydrogen ratios o f groundwater and xylem water of tagasaste and annual weeds, it was shown t hat trees became increasingly dependent on groundwater over time and had th e capacity to switch rapidly between soil and groundwater sources. Seasonal changes in carbon isotope composition of new shoot tip dry matter indicate d that plantation trees were less stressed than alley trees by the third su mmer as they adapted to heavy dependence on groundwater. In the third seaso n, when plantation trees were transpiring at rates equivalent to 2.3 times annual rainfall, NMM profiles and time domain reflectometry (TDR) assays in dicated that no free drainage occurred and that trees were capable of hydra ulically lifting groundwater to near surface soil in the dry season. Additi onal adaptive features of importance to this environment included heat stre ss induced leaf shedding, development of perennial root nodules on lower pa rts of tap roots, and an ability to respond in summer to artificial irrigat ion or a seasonal rainfall by rapidly increasing transpiration 2-3-fold to values equalling E-0.