, Volume 11, Issue 2, pp 171-181
Date: 27 Feb 2008

Restoration of Themeda australis swards suppresses soil nitrate and enhances ecological resistance to invasion by exotic annuals

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Abstract

Understanding processes that underlie ecological resistance to weed invasion is critical for sustainable restoration of invaded plant communities. Experimental studies have demonstrated that invasive nitrophilic annuals can be controlled by addition of carbon to reduce soil nitrate concentrations, sometimes leading to enhanced establishment of native plants. However, effects of carbon supplements on soil nitrate are temporary, and the longer-term value of carbon supplementation as a restoration tool is dependent on the resistance of the re-established ecosystem to repeat invasion. We investigated whether re-established swards of the tussock grass Themeda australis (R.Br.) Stapf (a natural understorey dominant in mesic grassy woodlands of SE Australia) could suppress soil nitrate concentrations, and through this or other means, could impart ongoing resistance to exotic invasion in restored woodlands. In a remnant invaded by exotic annuals, we applied three plot treatments (carbon supplements, annual spring burns and untreated control) and two seed treatments (± Themeda seed) in a replicated, factorial design. Within 3 years, successful establishment of Themeda swards on burnt and carbon-supplemented plots was associated with a reduction in soil nitrate to levels comparable with non-invaded, Themeda-dominated reference sites in the region (<3 mg/kg), and significantly reduced exotic cover compared with unseeded plots. By contrast, on plots not seeded with Themeda, soil nitrate increased after cessation of carbon addition and exotic cover returned to levels comparable with untreated control plots, despite a high cover of other native perennial grasses. Few persistent effects of carbon supplements or spring burning on soil nutrients were evident 9–19 months after cessation of these treatments. Results suggest that Themeda is a keystone species that regulates nitrate cycling, thereby imparting ecological resistance to invasion by nitrophilic annuals.