The effectiveness of weed control in a threatened plant community: A grassland case study

Abstract

Habitat loss due to altered land use has resulted in the fragmentation of remnant grasslands, resulting in an increase in the edge to area ratio and a subsequent rise in the risk of weed invasion. Restoration in threatened remnant grasslands focus on spot-spraying high threat herbaceous weeds with herbicides (i.e. weed control) as a dominant management action, but there is limited knowledge about the ecological outcomes of weed control effectiveness in these systems.In this study, we conducted a Before-After-Control-Impact analysis over three years using 32 linear grassland remnants to test whether weed control can improve grassland condition. Our predictions were that weed control would: (i) reduce the cover and health of pervasive exotic perennial grasses (i.e. target weeds), (ii) increase the cover of native plants by removing competition from target weeds, and (iii) not affect native plant richness as any recolonization will likely be from species already occurring in the area of sampling. We also predicted that weed control would shift the cover of ground layer attributes towards levels more appropriate for a grassland in good condition.Our results show that short-term weed control can be effective in reducing the cover and health of target weeds with minimal effects on native vascular plants. Changes in bare ground and litter cover following weed control were dynamic and complementary. A reduction in soil crust cover after two consecutive weed control treatments could lead to altered survival and recruitment dynamics of native grassland plants.Effective weed control requires actions that limit subsequent re-invasion by target weeds and promote a microenvironment suitable for native species to re-colonise. Our findings suggest a need to assess (i) if changes in ground layer cover post herbicide application affects native vegetation recruitment, and (ii) if other management activities, such as burning (in combination with, or independent of, weed control), are effective in controlling weeds while promoting better outcomes for ground layer cover (i.e. biological soil crust does not decrease) and native species (i.e. cover increases). If a strategy that eliminates target weeds while maintaining biological soil crust cover and increasing native cover can be identified, it will greatly improve the long-term restoration of these highly threatened grasslands.