The influence of local elevation on soil properties and tree health in remnant eucalypt woodlands affected by secondary salinity

More than 2 M ha of remnant vegetation in Australia is predicted to be at risk from shallow water tables by2050. Currently, vegetation is considered to be at risk where the water table is predicted to be less than 2 mbelow the soil surface, yet casual observation of areas affected by secondary salinity in the Western Australianwheatbelt has suggested that small differences in elevation (<0.5 m) are important in determining plant health.In this study, we investigated how small changes in elevation (and hence depth to the water table) affected soil Clconcentrations and water contents, and whether small changes in elevation were associated with major changesin tree health in two remnants of Eucalyptus wandoo Blakely woodland with secondary salinity. At one site therewere strong dissimilarities between soil samples collected above or below relative elevations of 0.5 m in areaswith a shallow (0.3 m deep in September 2001) and saline water table. This was reflected in almost complete treemortality at relative elevations below 0.5 m. However, low rainfall in 2001 meant that it was unlikely that currentsoil conditions had caused tree death. When water table data for 1999 was overlaid over plots of tree health andtransect topography, high levels of tree mortality corresponded with areas where the water table was at or abovethe ground surface. At the other site, there was no clear relationship between elevation, soil characteristics and treehealth. Localised variation in abiotic conditions and ecosystem processes at a fine-scale may buffer, to some extent,the spatial impact of soil salinity and waterlogging in remnant vegetation. Collapses in tree health at some sites arelikely to be related to extreme and episodic events, which we may have limited ability to predict.