Establishment of trees on saline, waterlogged soils

The East Barmoya and adjoining catchments in coastal central Queensland are suffering classic symptoms of dryland salinity, brought on by clearing of dry-rainforests in their upper catchments. The problem became manifest in the 1920-30s, expanded rapidly in the 1950s and has intermittently increased until the present, lagging some 20 to 30 years behind the period of large scale clearing in the catchment. Plant succession onto the new saline environment is considered, and a production system involving marine couch (Sporobolus virginicus) and swamp-oak (Casuarina glauca) is advocated. The role of soil mounding and mulching for the establishment of Casuarina glauca onto waterlogged, salinized land (water-table within 1 m of soil surface, surface soil EC1:5 0.0-0.1 m c. 10 dS.m -1) in Central Queensland was investigated in a factorial experiment involving two levels of soil mounding (0.05 and 0.15 m) and four mulch conditions (no mulch, hay, black and white plastic). Mounding was of little effect on plant survival, growth, soil pH or conductivity, however mulching greatly influenced these parameters, with the exception of soil pH. Plastic mulch is recommended over hay mulch. Installation of the plastic mulch was mechanized, involving cultivation of the soil to mix surface salts, mounding of soil into an M cross-section to harvest rain water to the seedling, and use of a commercial mulchlayer for plastic film installation. An established individual swamp-oak was found to use 8-11 L.d-1 of groundwater. Using a steady state model for the water-table depression due to water extraction from a well and assuming that the tree used only groundwater, the depression was calculated to be c. 1 m (steady state) for the low hydraulic conductivity soils (clays) that typify dryland salinity discharge sites. During a forty day period, the tree caused a water-table depression of 10 cm, relative to a reference point 10 m from the tree. Further, a diurnal oscillation of 1 cm occurred in the water-table under the tree. With water use by the single tree considered to be sufficient to cause localized inhibition of the unsaturated flow of groundwater to the soil surface, an estimation was made of the density of trees required to lower the water-table beyond the capillary fringe and thereby reverse the salinization process.