Relationships between water availability and Eucalyptus camaldulensis growth in a riparian forest

The effects of short-term flooding on soil water content and subsequent tree response were examined in a riparian Eucalyptus camaldulensis forest which was dissected by a series of shallow ephemeral channels, locally known as runners. Twelve isolated plots, each approximately 0.8 ha, were established in three blocks of four treatments. One of the blocks was underlain by a moist, sandy aquifer 2–4 m below the surface. The four treatments were (1) flooding each spring; (2) flooding each summer; (3) flooding each spring plus each summer; (4) control (zero flooding). Depth of water percolation after a summer flooding varied from 1.3 to over 6 m below the surface. Horizontal movement away from the edge of the floodwater ranged from almost zero on some plots to at least 38 m. The extensive horizontal movement was confined within narrow aquifers which occurred under some plots. Trees in plots underlain by a shallow aquifer always had higher xylem pressure potential (XPP, MPa) than other trees, and flooding these plots increased XPP by a non-significant quantity (−0.14 MPa to −0.12 MPa). However, on the other plots, flooding resulted in a statistically significant increase in XPP from −0.45 to −0.10 MPa. The effect of flooding on XPP was evident for between 22.5 and 37.5 m from the floodwater. This was ascribed to root interception and some horizontal movement of water. Increased flood frequency from zero to one to two per year resulted in mean leaf areas of 11.0 cm², 12.2 cm² and 13.2 cm², respectively. Trees in the runner, at 8 or at 38 m from the channels, had mean leaf areas of 12.9 cm², 13.6 cm² and 9.9 cm², respectively. The presence of shallow aquifers increased mean leaf area from 11.5 to 13.3 cm². Increased flood frequency significantly increased relative growth rate of trees up to 22.5 m from the edge of the floodwater. We conclude that short-term flooding of channels that occupied 15–20% of the forest floor temporarily improved tree moisture status and this increased tree growth rate in up to 70% of the forest.