Effects of soil water potential on germination of co-dominant Brigalow species: Implications for rehabilitation of water-limited ecosystems in the Brigalow Belt bioregion

Abstract

Rehabilitation of disturbed and degraded land is a critical legal and ecological requirement to achieve stable and non-polluted ecosystems. In some semi-arid climates, such as the Brigalow Belt Bioregion in Eastern Australia, extensive areas have been affected by open-cut mining. With erratic rainfall patterns and clayey soils, the Brigalow Belt is a unique biome which is representative of other water-limited ecosystems worldwide. Direct seeding and native plant germination on post-mining land may be an effective and economically viable solution to re-establishing plant communities. Germination is governed by the amount of water the seed can imbibe, which is a function of the soil water potential and hydraulic soil properties rather than soil water content. The question remains how soil water potential triggers germination of Brigalow Belt plant species.We used six replicates of 50 seeds of three co-dominant native species from the Brigalow Belt Bioregion – Eucalyptus cambadgeana, Eucalyptus populnea, and Casuarina cristata – to investigate germination in relation to water potential as environmental stressor. Solutions of polyethylene glycol (PEG 6000) were applied to expose seeds to nine osmotic water potentials ranging from soil water saturation (0kPa) and field capacity (−10–30kPa) to the permanent wilting point (−1500kPa). We measured germinability (number of germinated seeds relative to total number of seeds per lot) and mean germination time (mean time required for maximum germination of a seed lot) to quantify germination. Further, we employed Hydrus-1D to simulate daily values of soil water potential and water content at 1mm depth of a Black Vertosol under the climatic conditions of Emerald in Central Queensland.While E. cambadgeana demonstrated the fastest and most prolific germination across the widest range of water conditions with germination being observed even at water potentials as low as −1500kPa, E. populnea and C. cristata required wetter conditions over longer periods to achieve maximum germination rates. For the 11 years of meteorological data examined in this study, soil water conditions were most favourable for germination of E. cambadgeana; this was primarily due to its high tolerance of water deficit in soils of lighter texture than that of the denser Black Vertosols. In contrast, for the seed-soil combination of C. cristata and Black Vertosol, modelled climatic conditions proved favourable to triggering germination only 25% of the time. The empirical data on seed germination can be used to parameterise and calibrate germination models.