Invasive native scrub and soil condition in semi-arid south-eastern Australia

Invasive native scrub (INS) comprises shrub and tree species that have encroached or regenerated densely following disturbance. We used a paired-site approach to assess soil condition in vegetation states of INS (>700 stems/ha) and adjacent, recently established pastures (<40 years of age) where the INS had been cleared, as well as in reference areas of long-established pasture (50–100 years of age) and open woodland in semi-arid south-eastern Australia. Surface soil condition and ground cover in INS differed from open woodland, with INS generally having lower nutrient status, lower biological activity and harder-setting soils that were more compacted than open woodlands. Soil condition was related to the types and proportions of ground cover in the different vegetation states. Differences in soil condition beneath individual shrubs and large trees were marked, following the same trends as differences in soil condition between INS and open woodlands. Surface soil condition varied little between INS, recent pastures and long-established pastures, but INS exhibited an acid soil profile (reaching pH'ca' < 5.0 at 10 cm depth). We propose a mechanism in which the woody plants in INS act as ion pumps, redistributing alkalinity from the shallow soil depths both into the plant and to the soil surface as litterfall, which is subsequently exported via water and wind erosion from INS areas. This is in contrast with the ion pumping action of large trees in open woodlands, which redistribute alkalinity from deeper subsoil through the tree to the surface as litterfall. This litter is retained and incorporated into the surface soil, resulting in higher pH in the shallow soil layers of open woodlands compared with INS. This is consistent with processes that operate in these semi-arid systems, and illustrates the differences in ecological function between grassy open woodland and INS. This work emphasises the need to link soil condition with ecological processes to understand productivity and resource distribution, and to apply state-and-transition concepts to these systems.