Discussion Paper - Part A - Victorian Environmental Assessment ...

obtain water from groundwater, rain and flooding. Thelowland floodplains have low rainfall and highevaporation, and growing conditions alternate betweenvery dry and flooded (Roberts & Marston 2000). Inextended dry conditions, some trees rely on groundwater. The roots of river red gums at Barmah forestextend 10 m into the sub-soil to extract water in thesandy aquifers (Bacon et al. 1993b). They can alsoreduce water use by shedding leaves.Nonetheless, particularly in more arid regions, most riverred gums require periodic flooding and both thefrequency and duration of flooding are importantdeterminants of growth and condition (see Table 5.6).For example, Bacon et al. (1993a) found that short-termflooding of channels that occupied 15-20 percent of theforest floor temporarily improved tree moisture statusand this increased tree growth rate in up to 70 percentFigure 5.11 Black box at Leaghur State Park.of the forest. However, short duration floods may onlyresult in short-term benefits. Soil water storage returnedto pre-flood levels within 40 days of flood recession inBarmah forest (Bacon et al. 1993b).Although river red gums require periodic flooding, theycannot survive permanent inundation. Roots requireoxygen and become increasingly anoxic as the oxygen isdepleted leading to stress in the tree. Prolongedinundation reduces growth and can kill trees (Jacobs1955). How long it takes to stress trees depends partiallyon the soil, the tree species and growth stage and theflooding regime, but may be several years or more. InBarmah forest, river red gums have survived permanentflooding for two years (Bren 1987) but others died afterfour years (Chesterfield 1986).Some river red gum forests are in poor condition due toeither insufficient flooding, or near-permanent flooding(Figure 5.10). They are in especially poor conditiondownstream of Swan Hill where there has been nonatural flooding for ten years and the climate is arid(Brett Lane & Associates Pty Ltd 2005). These forestshave been given emergency watering (see chapter 15) tohelp them recovery. Earlier studies on river red gumcondition also found that over 30 percent of river redgums and 35 percent of black box trees at sitesdownstream of Echuca and into South Australia wereunhealthy (>40 percent of crown dead) or dead(Margules & Partners Pty Ltd et al. 1990). Thesemeasures of condition broadly correlate with thevegetation condition measures discussed earlier thischapter (Map 5.1). It should be noted that factors otherthan the water regimes, for example insect attack,disease and salinity, also affect the condition of the riverred gums (see below).Black box (Figure 5.11) is an inland species distributed inVictoria from Strathmerton in the east, along the MurrayRiver to the South Australian border and south toaround Horsham (Figure 5.1). Black box grows toapproximately 20 m depending on flood frequency(Roberts & Marston 2000). In open habitat it has a shorttrunk and drooping form with some lower branchestouching the ground. The leaves are narrow and a dullgreen colour and the bark is dark, rough and persistentto the small branches.Black box trees were widely cleared for agriculture andtheir heavy, durable timber harvested for fencing andfirewood. The largest remaining black box forests inVictoria are within the western part of the study area,e.g. Leaghur State Park and Hattah-Kulkyne NationalPark.Black box trees are less tolerant of flooding but moretolerant of prolonged dry periods than river red gumsand thus tend to occur higher on the floodplain thanriver red gum (Roberts & Marston 2000). This is evidentin Barmah forest where increases of only 10–20 cm inelevation cause changes in eucalypt species. Black boxare adapted to dry conditions through very lowtranspiration rates, small canopy leaf area and leaveshanging vertically to reduce water loss (Roberts &Marston 2000). Mature black box die after inundationfor approximately 18 months (Roberts & Marston 2000).Black box often regenerate after flooding but seedlingsgrow slower if they are flooded for longer than a month.68 River Red Gum Forests Investigation 2006