Invasive native scrub and soil condition in semi-arid south-eastern Australia Matthew Tighe a, *, Nick Reid a , Brian Wilson a,b , Sue V. Briggs c a Ecosystem Management, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia b NSW Department of Environment and Climate Change, Armidale, NSW 2351, Australia c NSW Department of Environment and Climate Change, GPO Box 284, Canberra, ACT 2601, Australia 1. Introduction Increased scrub density and scrub encroachment occur across the globe. The causes are system-dependent, and include short- term weather variations, changes in climate, grazing pressure, fire regimes and various combinations of these (Westoby et al., 1989; Archer and Stokes, 2000). Invasive native scrub (INS) comprises plant species (usually woody plant species) that are encroaching or regenerating densely following disturbance. Management of INS that removes native vegetation is legally a form of clearing in New South Wales, Australia. Native shrub and tree species that have encroached or are growing densely following disturbance can be listed as INS, and assessed under the Native Vegetation Act 2003 (NSW Government, 2007). In semi-arid New South Wales, Australia, the dollar value of grazing domestic stock in INS has been calculated as half the value of uninvaded areas (Noble and Walker, 2006). This lower productivity has been attributed to low herbage production and to the degraded nature of the soil. The former is at least partly caused by competition for water (Johns, 1984; Anderson and Hodgkinson, 1997) (although competition for other resources and allelopathy may also occur). However, while deterioration in soil condition under INS has been found (Wilson et al., 1988), land use is a confounding issue, and evidence of a casual link between INS and soil degradation is largely anecdotal (Harrington et al., 1979; Gardiner et al., 1998). Removal of INS, followed by cropping and a return to pasture, has been stated by landholders as the best way to address the deleterious effects of INS on agricultural production (Cobar Vegetation Management Committee, 2006). In subtropical to semi-arid systems world-wide, scrub encroachment often leads to increased soil organic matter, nutrients, and soil microbial biomass, and changes to soil structure such as decreased compaction, when compared with uninvaded grasslands (Archer and Stokes, 2000; Hibbard et al., 2001; Zavaleta and Kettley, 2006). Open woodlands in semi-arid NSW also show these differences from native pasture where trees have been Agriculture, Ecosystems and Environment 132 (2009) 212–222 ARTICLE INFO Article history: Received 7 January 2009 Received in revised form 30 March 2009 Accepted 3 April 2009 Available online 2 May 2009 Keywords: Vegetation states Pasture Open woodland Ground cover Alkalinity Acid soil Scrub encroachment Soil nutrient status pH ABSTRACT 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. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +61 2 6773 2180; fax: +61 2 6773 2769. E-mail address: mtighe2@une.edu.au (M. Tighe). Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee 0167-8809/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2009.04.001