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

Abstract

More than 2 M ha of remnant vegetation in Australia is predicted to be at risk from shallow water tables by 2050. Currently, vegetation is considered to be at risk where the water table is predicted to be less than 2 m below the soil surface, yet casual observation of areas affected by secondary salinity in the Western Australian wheatbelt 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 Cl concentrations and water contents, and whether small changes in elevation were associated with major changes in tree health in two remnants of Eucalyptus wandoo Blakely woodland with secondary salinity. At one site there were strong dissimilarities between soil samples collected above or below relative elevations of 0.5 m in areas with a shallow (0.3 m deep in September 2001) and saline water table. This was reflected in almost complete tree mortality at relative elevations below 0.5 m. However, low rainfall in 2001 meant that it was unlikely that current soil conditions had caused tree death. When water table data for 1999 was overlaid over plots of tree health and transect topography, high levels of tree mortality corresponded with areas where the water table was at or above the ground surface. At the other site, there was no clear relationship between elevation, soil characteristics and tree health. 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 are likely to be related to extreme and episodic events, which we may have limited ability to predict.

Water table response to an experimental alley farming trial: dissecting the spatial and temporal structure of the data

“…Although within-treatment topography is minimal (typically 610-50 mm), designs of such systems need to take surface elevation into account. Utilization of microtopographic variation in agriculture and agroforestry site design is used to aid the management and application of resources to enable maximum output to be achieved (Cramer et al 2004). The high proportion of variation explained by the environmental variables for the SWL norm data sets highlights the importance of minor topographic variations on a small scale (between piezometers).…”

Altered Soil Properties Inhibit Fruit Set but Increase Progeny Performance for a Foundation Tree in a Highly Fragmented Landscape

“…Here, we investigated the impacts of habitat fragmentation and soil properties on Eucalyptus wandoo Blakely (Myrtaceae), a dominant tree species found across the landscape that has undergone significant recent habitat fragmentation and associated landscape changes through much of its range. Wandoo woodlands tend to occur low in the landscape on valley slopes and floors and therefore may be exposed to the effects of secondary salinity, which in some areas has led to mortality of wandoo trees (Cramer et al, 2004). Salinity-related mortality of wandoo appears to occur as a local ecosystem collapse in response to the breaching of critical salinity thresholds, often following extreme events, rather than as the result of a gradual decline in tree health with increasing salinity (Cramer et al, 2004).…”

“…Wandoo woodlands tend to occur low in the landscape on valley slopes and floors and therefore may be exposed to the effects of secondary salinity, which in some areas has led to mortality of wandoo trees (Cramer et al, 2004). Salinity-related mortality of wandoo appears to occur as a local ecosystem collapse in response to the breaching of critical salinity thresholds, often following extreme events, rather than as the result of a gradual decline in tree health with increasing salinity (Cramer et al, 2004). Two studies found no relationship between secondary soil salinity and the crown health of live E. wandoo trees (Cramer et al, 2004;Brouwers et al, 2013), but they did not investigate other soil properties or aspects of population ecology such as reproduction.…”

Composition and relative health of remnant vegetation fringing lakes along a salinity and waterlogging gradient

“…More than 93% of native vegetation in the wheatbelt has been cleared (Cramer et al 2004), so the fringing vegetation of playa lakes are important remnants of a biologically diverse region that are now threatened by altered hydrological regimes.…”

“…Subtle differences of less than 0.2 m in topography can buffer the seasonal effects of salinity and waterlogging, but vegetation can be severely impacted by extreme and episodic events (Cramer et al 2004). There has been a significant decline in annual rainfall in south-west Western Australia and more intense summer rainfall events from 1911 to 1990 (Yu and Neil 1993).…”