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

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.

Use of Soil Properties to Determine the Historical Extent of Two Western Washington Prairies

“…Recent research indicates that C addition to the soil surface is the most reliable method for altering soil C:N ratios (Perry et al 2010) and that, regardless of the C source used, alterations to the soil lose effectiveness rapidly after cessation of treatment (Prober and Lunt 2009;Kirkpatrick and Lubetkin 2011). Rapid loss of C effectiveness was supported in our study, as few significant effects remained detectable in the second growing season after application.…”

Suppression ofBromus tectorumL. by Established Perennial Grasses: Potential Mechanisms—Part One

“…Prober and Lunt [35] reported that the native perennial grass Themeda australis reduced soil NO 3 − -N and thereby decreased invasion by nitrophilic annuals. Evidence for a similar mechanism operating in this study included lower tissue nitrogen concentrations for the 1st and 2nd growth cycles for B. tectorum in competition, relative to B. tectorum grown without competition (Table 5) and the increase in absolute growth and tissue nitrogen concentrations of B. tectorum in competition after fertilizer addition.…”

“…Due to the limitations of activated carbon in allelopathic research [51,52], we cannot state with confidence that the perennial grasses exuded allelopathies, which suppressed B. tectorum. Published literature, however, reports that allelopathies, exuded from native flora, assist in the resistance to invasive plants [35].…”

“…Some invasive species exude allelopathies to gain a competitive advantage in their new host environments [33,34]. Conversely, some native plants resist invasion by producing allelopathies [35,36]. We ask another question: do healthy perennial grasses resist B. tectorum invasion by coopting biological soil space or by possibly exuding allelopathies?…”

Restoring Competitors and Natural Enemies for Long-Term Control of Plant Invaders

“…In some cases, well-timed burning or mowing can also reduce competition from invasive species and help natives to get established. 1 In other suppressing invasive species. 3 On the other hand, like the invasive species they replace, they can also reduce the diversity of native species and, depending on the objectives of the restoration, can even be considered invasive themselves.…”