Muriel Davies

Date of Award


Document Type

Thesis - ECU Access Only


Edith Cowan University

Degree Name

Master of Science (Environmental Management)


Faculty of Computing, Health and Science

First Supervisor

Dr Eddie van Etten


This thesis examines the death of mulga (Acacia aneura) adjacent to a mine haul road in the north-eastern Goldfields of Western Australia. The death of trees along roadsides in the arid and semi-arid regions of Australia is often referred to as a ‘road shadow effect’ and is thought to occur because of sheetflow blockage. As sheetflow redistributes water across the land surface (rather than into channels) any impediment to flow is likely to have consequences for vegetation downslope of the impediment. The study was important as many mining companies in semi-arid Western Australia have reported tree deaths along their mine haul roadsides and the problem has also been identified across other areas of Western Australia including grazing land and road verges.

The haul roads at many mines are watered to suppress dust and the water is often hypersaline. This study compared vegetation and soil parameters at sites up and downslope of both the haul (treated) and control roads to ascertain whether factors additional to sheetflow contributed to roadside tree deaths. At each site, a series of adjoining quadrats was established perpendicular to the road. The data collected at each site was analysed on a quadrat by quadrat basis to establish trends with increasing distance from the road edge. Where possible analysis techniques which could test for differences between control and treated sites or upslope and downslope sites and the interaction between sites were used as this would address the aims of the study. If data were normally distributed either two-way Analysis of Variance or t-tests were carried out and where data were not normally distributed the non-parametric Mann Whitney Utest was performed.

The significant differences noted in several vegetation parameters between up and downslope sides of both the control and treated roads provided a good argument for road presence leading to road shadow effect. These differences included a greater proportion of dead and recently dead trees and a greater reduction in canopy fullness for living trees on the downslope sides of the roads. The presence of roads and the associated interruption in sheetflow water movement have contributed to the decline and eventually death of some mulga trees. However, the downslope side of the treated road often showed much greater differences for the parameters measured when compared with the control road. The results show that there was an interactive effect between salt water treatment and roadside and a strong to modest positive relationship between soil salt levels and tree health in close proximity to roadsides. At the haul road, the addition of hypersaline water as a dust suppressant exacerbated the decline and death of mulga trees downslope as well as having an influence on the trees upslope.

Road location in the landscape strongly influences the type of interactions between roads and pathways of water movement. The roads monitored for this study mostly acted as a barrier to sheet water flow which resulted in a drought situation for mulga located on the downslope roadside and lead to their eventual decline or death. Road designs incorporating culverts or under road drainage systems are widely suggested for areas of mulga dependent on sheetflow however, their effectiveness requires further study. On occasion the haul road also acted as a source of water which had been contaminated with excess salts that accumulated during road watering for dust suppression. This resulted in an alteration of the surface soil salinity along the haul roadside and further exacerbated the effect of the altered hydrological regime. While dust suppression on mine haul roads is essential for a variety of reasons a product other than hypersaline water is highly recommended.

LCSH Subject Headings

Edith Cowan University. Faculty of Computing, Health and Science -- Dissertations

Acacia -- Western Australia -- Goldfields Region

Roads -- Environmental aspects -- Western Australia -- Goldfields Region



Paper Location