Individual study: Growth of the halophytic shrub Atriplex amnicola on salt-affected soils at Tammin, Western Australia
Davidson N.J., Galloway R. & Lazarescu G. (1996) Growth of Atriplex amnicola on salt-affected soils at Tammin, Western Australia. Journal of Applied Ecology, 33, 1257-1266
The Wheat Belt of Western Australia (a wheat cultivation and sheep grazing region) covers an area of around 15 million ha. Prior to European settlement it was predominantly an area of open Eucalyptus woodland. In the early 1900s it was known that clearing native vegetation could cause soils to become saline but despite this, most was cleared over the subsequent 70 years. Rising water tables resulted, bringing salts to the soil surface. By 1989 approximately 440,000 ha were adversely affected by saline soil conditions. If undisturbed, these salt-affected areas develop barley grass Hordeum marinum and samphire Halosarcia spp. communities, but these low productivity and fragile communities cannot withstand grazing and trampling, which exposes the soil and leads to further degradation due to erosion. In contrast, some halophytic shrubs e.g. Atriplex spp., can tolerate periodic grazing and can enhance soil stability. Little was known about environmental factors influencing growth of such halophytes, therefore this study investigated soil characteristics that determined the growth of the native Atriplex amnicola.
Study site: Atriplex amnicola seedlings were raised in a glasshouse and transplanted in the field at Tammin, 200 km east of the city of Perth, Western Australia, in 1989. The site was in a broad valley uncultivated for 2 years due to encroaching salinity.
Experimental plots: A total of 36 plots were established in a 200 x 200 m area. Twelve 30 x 30 m plots had low densities of seedlings (planted at 5-m spacing), six on areas originally dominated by barley grass and six by samphire. Adjacent to these plots, two high density plots (15 x 15 m) with plantings at 2.5 m spacing were established.
In each plot the central 16 plants were measured for growth characteristics at 3-month intervals for 2 years. Above-ground biomass was recorded from plant samples taken at 3-monthly intervals. Plants were also excavated to assess root characteristics.
Soil sampling: Soil samples were taken at various depths in February 1990 next to each plot and analysed for soil texture, bulk density, electrical conductivity (a measure of salinity), pH, sodium absorbtion ratio, nitrate and ammonium concentrations and available phosphorus. Inspection wells were used to record water table levels at monthly intervals.
Overall, growth of A.amnicola was poor and declined greatly in response to reduced depth to the water table during the summer. Growth was limited by a combination of adverse soil physical properties, salinity, drought and water-logging. Water-logging occurred after rains in low lying areas and also elevated regions where perched water tables developed above the dense, very poorly drained, clay subsoil.
Salinity varied greatly over short distances and in extreme cases ranged from 300 to 6,500 mS/m over a distance of only around 10 m, and growth was much reduced in high salinity patches.
In terms of root penetrability, the penetrability of the clay sub-soils was low, preventing the entry of A.amnicola roots except where old ‘fossil’ root channels existed.
Where roots were confined to shallow sandy surface soils plants suffered in summer droughts. Whilst the sandy A-horizons of the soil profile varied from 10 to 110 cm in depth, they were generally shallow (average depth c. 40 cm) and had low moisture reserves.
Conclusions The site at Tammin is typical of salt-affected soils in this region of Western Australia. It produced generally poor growth of A.armnicola, with an average growth rate in the Tammin plantation at 2.5 m spacings of only 0.7 tonnes/ha dry matter annually. There was no significant difference in growth of individual plants at spacings of 5 m compared with 2.5 m due to major limitations to growth posed by a harsh environmental conditions. The results of this study suggest that the cause is a combination of water-logging, adverse soil physical properties, salinity and drought which vary on a meso- and micro-environmental scale.
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