Does diversity influence soil nitrate, light availability and productivity in the establishment phase of Australian temperate grassland reconstruction?

Abstract

This research arose from the need to progress the use of direct seeding in the restoration of species-rich temperate Australian grassland.Summary The successful conservation and restoration of the temperate native grasslands of south-eastern Australia is critical to reversing the decline in range and diversity of these threatened plant communities. Yet the goals of high native species diversity and weed management are difficult to achieve in grassland restoration projects. To increase our understanding of whether synergies exist between these goals (i.e. whether early introduction of a larger number of species might improve both outcomes in the reassembly of native grassland), we examined the relationships between plant species number, functional group number and resource use during the establishment phase of direct-sown grassland. We did this by sowing a representative suite of species (at varying levels of species number and functional group number) into experimental plots and then measuring and analysing the extent to which the newly established assemblages captured available resources, i.e. used soil nitrate, absorbed light and produced biomass (vegetative cover). Statistically significant correlations were common between the predictor variables (species number, functional group number, percentage vegetative cover, plant number, presence of idiosyncratic (dominating) species) and responses (soil nitrate concentration, light reduction or 'extinction'). Higher diversity was associated with lower soil nitrate, while percentage vegetative cover and the presence of idiosyncratic species best predicted light extinction. The relationship between diversity, and plant biomass (measured as vegetative cover) and plant number was positive in the first year of the study. The diversity/biomass relationship became negative in the second year due to the higher numbers and cover of 'idiosyncratic' species. The diversity/plant number relationship also became negative in the autumn of the second year and was reduced to a trend by the winter. We found that lower nitrate and increasing plant numbers and vegetative cover were most strongly linked to increasing species number in the early stages of this study. This suggests that introducing and maintaining high diversity early in a native grassland reassembly or enhancement project will improve the resistance (e.g. to weed) of these communities. At later stages of grassland development, this function may be provided by the more dominating idiosyncratic species. The maintenance of diversity, an important goal in its own right, will therefore necessitate managed disturbances to periodically reduce the vegetative dominance of idiosyncratic species, releasing resources for the diverse range of other species whose early introduction will have allowed them to persist in the soil seed bank or as suppressed rootstocks.

Expanding horizons for herbaceous ecosystem restoration: the Grassy Groundcover Restoration Project

“…plains grassland, plains grassy woodland, grassy dry forest) and to identify regional seed collection sites. Gibson‐Roy et al. (2009) found that high levels of plant diversity (species and functional) could be established early in the grassland restoration cycle.…”

Resource pre‐emption, rather than extending the growing season of native grass assemblages, reduces invasion by exotic species

“…The establishment time for the C4 species in our study may not have been sufficient to make them successful competitors. Another southern Australian study (but with higher average rainfall, particularly in summer), found that T. triandra, the dominant C4 species in our study, took longer than other species to become established (Gibson-Roy et al, 2009). We attempted to overcome this delay by planting mature tubestock plants rather than using direct seeding.…”

Establishment of native grasses and their impact on exotic annuals in degraded box gum woodlands

“…For example in box gum woodlands, increased species diversity during the early phase of restoration has been associated with reduced soil nitrate concentrations, suggesting greater resistance to exotic annual invasion during this critical phase (Gibson‐Roy et al . ).…”

“…Indeed, processes associated with both resistance and resilience have been attributed to various elements of species composition within an ecosystem. For example in box gum woodlands, increased species diversity during the early phase of restoration has been associated with reduced soil nitrate concentrations, suggesting greater resistance to exotic annual invasion during this critical phase (Gibson-Roy et al 2009). Fargione & Tilman 2005;Prober et al 2007).…”

“…Fargione & Tilman 2005;Prober et al 2007). For example in box gum woodlands, increased species diversity during the early phase of restoration has been associated with reduced soil nitrate concentrations, suggesting greater resistance to exotic annual invasion during this critical phase (Gibson-Roy et al 2009). This paper reports an experiment to investigate the benefits of nutrient reduction (by the addition of carbon) and seed depletion (by spring burning or spring grazing) for re-establishing a range of C3 and C4 native grasses (representing combinations of species and functional identity), and the extent to which these combinations influence soil resources and control exotic annuals.…”