Drought tolerance and rainforest tree growth on a North Queensland rainfall gradient

Three instrumented sites were established in 1985 along a 160-km transect from coastal evergreen rainforest on lowlands near Cairns, through rainforest on the Atherton Tableland, to semideciduous vine forest southwest of Mount Garnet. Mean annual rainfall graded from 2800 mm at coastal site (1), through 1400 mm on Atherton Tableland (2), to 760 mm at inland site (3). Each site was equipped with scaffold towers to provide regular access to upper and middle canopy.

Two shade-intolerant rainforest tree species which occurred at all three sites were used to compare tree growth and water relations; these were Acacia aulacocarpa (brown salwood) which was evergreen, and Melia azederach var. australasica (white cedar) which was leafless during onset of drought. Drought responses from coastal to inland sites were qualitatively different between Acacia and Melia. While Acacia foliage endured persistent low-moisture status, the deciduous habit of Melia provided a means of drought-avoidance.

Mean minimum leaf water-potential for deciduous Melia varied from −1.7 MPa to −2.3 MPa across all sites and seasons. By contrast, evergreen Acacia experienced increasing seasonal drought stress from coast to inland, reducing dry-season water-potentials from −2.1 Mpa to −6.4 Mpa, respectively.

Annual patterns of growth in stem cross-sectional area also varied according to species and site. For Acacia, current annual increment ( ) was 54.1 and 56.9 cm² year⁻¹ for coastal and tableland sites respectively. Acacia at the inland site was only 3.1 cm² year⁻¹.

Present results are relevant to species selection criteria in the tropics, where establishment of rainforest species can be limited by seasonal drought.