Abstract

Key Words

Introduction

In southwestern Australia, 10 % or 1.8 million ha of the farmed area is affected by dryland salinity and a further 6 million ha are threatened (1). One option for farmers is to plant salt-tolerant grasses, trees and shrubs in saline areas. These plants improve the visual amenity of saline land, provide feed for livestock, may reduce the spread of salinity and probably increase the biodiversity value of saline land. However, vegetation on saline land tends to remain patchy and there are few planting options available to farmers. The aim of this study was to assess botanical diversity within two paddocks that were once cropped and were subsequently sown with salt-tolerant species in the early 1980’s after becoming saline. It was predicted that botanical diversity would be low due to salinity, waterlogging and past farming practice.

Methods

The paddocks that were studied in 2001 are situated near Meckering (130 km east of Perth) and Tammin (180 km east of Perth). Soil samples were collected across paddocks in March 2001 to measure electrical conductivity and peizometers were used to monitor the water table. Botanical composition was assessed in autumn and spring using the dry-weight rank method of Mannetje and Haydock (2). Briefly, the paddocks were crossed by five transects in both autumn and spring, with botanical assessments made every 15 m along transects. For each assessment, the three herbaceous plant species that appeared to contribute most biomass were ranked in descending order from 1-3. Rankings were totalled and multiplied by a weighting factor to give a percentage of biomass/species.

Results

The Meckering paddock (19 ha) was the more saline and waterlogged site as 20% of the area had become a bare salt scald. On average the soil was highly saline (EC_e 48 dSm^-1) and the saline water table (EC_w 27 dSm^-1) was 66 cm below the soil surface in March. The soils in the Tammin paddock (12 ha) were on average highly saline (EC_e 25 dSm^-1) and the groundwater (164 cm below the soil surface in March) was highly saline (EC_w 56 dSm^-1). The topography of both paddocks was undulating.

Table 1 summarises the results. The Meckering paddock contained 31 herbaceous plant species, of which five were sown (early 1980’s) and 26 were volunteers. Only 23 % of the species were native to Australia. The most common species at the site in autumn were the grass Puccinellia ciliata and the halophytes Atriplex undulata, A. lentiformis, A. semibaccata and Halosarcia spp. In spring, the annual herbs Mesembryanthemum nodiflorum and Cotula coronopifolia formed 16.5 % of the botanical composition. The Tammin paddock contained 24 plant species of which five were sown and 19 had volunteered. One third of the recorded species were native to Australia. The most common species were Maireana brevifolia, Halosarcia spp. and Lolium rigidum.

Table 1. Botanical composition (%) in autumn and spring of two salt-affected paddocks.

Conclusion

Plant diversity within the paddocks was as high or higher than the diversity generally found in non-saline pastures in the same areas. The presence of so many species suggests that there is either little competitive exclusion, little agronomic exclusion or the paddocks are divided into botanical niches. It is likely that high levels of plant diversity within paddocks are maintained by niche differentiation; i.e. no one species can dominate (or perhaps even survive) in all of the functional niches within the paddock. The suggestion of niche differentiation was supported by successional changes in species composition across salinity and waterlogging gradients at both sites (data not presented). This diversity should be considered when evaluating options for revegetating saline land.

References

National Land and Water Resources Audit. 2001. Australian Dryland Salinity. Canberra.

Mannetje L.‘t and Haydock, K. P. 1963. J. Brit. Grassl. Soc., 18: 268-275.

• "Solutions for a better environment". Edited by Murray Unkovich and Garry O'Leary. Proceedings of the 11th Australian Agronomy Conference, 2-6 February 2003, Geelong, Victoria.