Interactions between native and exotic plants in the context of grassland restoration and the importance of below-ground processes

Abstract

The importance of native ecosystems is being ever more realised as human-induced environmental change leads to ecosystem degradation. This is spurring increased efforts to restore ecosystems. In previously cultivated landscapes (old-fields) the legacy of farming practices can persist for decades and present many challenges for restoration. This thesis is focussed on identifying and overcoming some of these challenges that limit restoration efforts. The overall aims were to develop a mechanistic understanding of the processes hindering native grass establishment and to improve the effectiveness of techniques used in the restoration of native grasslands. Two glasshouse experiments (chapters 2 and 3) were designed to investigate whether soil microbial communities present in old-fields hinder native plant establishment and allow exotic plants to dominate. The results indicate that native grasses performed better in the presence of soil microbes from remnant grassland. However, these microbial effects were heavily influenced by nutrient availability in the soil. Characterisation of the microbial communities, using molecular barcoding, revealed that they differed between old-field sites and remnant grassland. Differences in soil physiochemical properties between soil types, as well as the presence of different plant species, appear to explain the observed differences in microbial community composition. In turn, these changes in microbial communities affected plant performance, particularly when soil nutrient availability was low. High nutrient availability in old-fields from past farming practices usually results in dominance of fast-growing annual exotic plants. Reducing soil fertility is therefore seen as an effective approach to restoration. I trialled four methods (carbon supplements, slashing, burning, and scalping; chapter 4) to 1) reduce biomass of exotic species, 2) reduce soil nutrients, and 3) increase biomass of native grasses. Overall, scalping was the only method to achieve all three aims whereas carbon supplements and slashing reduced exotic biomass with no apparent benefit to native species. Both carbon supplements and scalping resulted in changes to the soil microbial community. Given the importance of plant-soil interactions, the implications of these result for future restoration works are discussed. One strategy to promote resistance to invasion in a revegetated community is to plant species that use resources in a complementary way, i.e. planting a diversity of functional groups. In a field trial (chapter 5), grass species from complementary functional groups (chosen based on phenology) were grown in different combinations and densities to test whether native communities are more resistant to invasion if resources are utilised all year round (niche saturation). Overall, high density planting was most effective at lowering exotic biomass. Planting C3 (winter-growing) and C4 (summer-growing) grasses together did not reduce invasibility, in contrast to my predictions. Instead, planting C3 plants alone was effective at reducing exotic biomass, providing evidence that planting functional groups that match the functional group of potential invaders could be an effective strategy for restoration. Findings presented in this thesis demonstrate the importance of soil amendments, both abiotic and biotic, and planting arrangements in ecological restoration. Greater consideration of these should lead to more successful and sustainable restoration outcomes in grassland habitats.