Barriers to ecosystem restoration after clearing invasive Acacia species in the South African fynbos: soil legacy effects, secondary invaders and weedy native species

A significant proportion of the world's ecosystems are invaded by alien N2-fixing woody species such as Australian acacias. Invasive alien N2-fixing woody species often transform ecosystems through their negative impacts on soil chemistry, seed banks and microbial communities, and native plant diversity. Management interventions such as clearing are necessary to reduce these negative impacts. It is often assumed that clearing the invasive species will lead to a dissipation of their impacts and native plant diversity recovery. However, this is often not the case because the invasive species' negative impacts can become persistent soil legacy effects and present barriers to restoration of viable native plant communities. Understanding barriers to restoration can lead to improved restoration outcomes. Using Acacia saligna invasions in the South African fynbos as case study, this thesis explored soil legacy effects, secondary invasion and weedy native species dominance after clearing invasive acacias. In chapter one, I reviewed global literature to understand how soil legacy effects of invasive alien N2-fixing woody species present barriers to restoration, and identify management actions that could potentially be used to address them. In chapter two, I investigated how long soil legacy effects of invasive A. saligna persist after clearing using soil sample analyses. In chapter three, I explored the effect of invasive A. saligna's soil chemical and biotic legacies, and weedy native species on native species re-establishment using a greenhouse experiment. In chapter four, I identified species that are secondary invaders after clearing invasive A. saligna across several sites and investigated the effects of vegetation type and fire application on their establishment over three years after clearing using vegetation monitoring. In chapter five, I investigated interactions between secondary invaders and the extent to which soil nitrate levels, apparent after clearing invasive A. saligna, influence secondary invasion and weedy native species dominance using growth chamber and greenhouse experiments. I found that altered soil microbial communities, depleted native soil seed banks, elevated N status, secondary invasion and weedy native species dominance, and reinvasion can be barriers to restoration. Furthermore, management actions such as carbon addition, soil microbial treatments, herbicide or graminicide application and native species reintroduction can be used to address these barriers to restoration. Acacia saligna's soil chemical legacies persisted up to ten years after clearing. However, they did not have direct negative consequences on the re-establishment of native proteoid shrubs but soil biotic legacies and presence of weedy native species reduced their growth. Secondary invasion was not habitat specific, was promoted by fire application and persisted up to three years after clearing at levels similar to or higher than the first year. Lastly, growth of secondary invaders and weedy native species increased with an increase in soil nitrate levels. I conclude that practicing restoration ecologists should manage soil legacy effects, secondary invaders and weedy native species after clearing invasive A. saligna to improve restoration outcomes.