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.全球有相当比例的生态系统遭受外来固氮木本物种（alien N2-fixing woody species）入侵，其中以澳大利亚金合欢属植物（Australian acacias）为典型代表。此类入侵性外来固氮木本物种往往会通过破坏土壤化学性质、耗竭种子库与微生物群落，以及降低本土植物多样性等负面影响，彻底重塑入侵区域的生态系统。为缓解这些负面影响，清除入侵物种等管理干预措施必不可少。过往研究通常假设，清除入侵物种即可消除其带来的生态影响，并推动本土植物多样性恢复。然而实际情况往往并非如此——入侵物种造成的负面影响可能会转化为持久性土壤遗留效应，进而对本土可存活植物群落的修复形成阻碍。理解修复过程中的各类障碍，有助于提升生态修复的整体成效。 本研究以南非菲因波斯植被区（South African fynbos）的银叶金合欢（Acacia saligna）入侵事件为案例，系统探究了清除入侵金合欢后出现的土壤遗留效应、次生入侵以及本土杂草化物种占优的现象。第一章，笔者通过综述全球相关学术文献，明确了外来固氮木本入侵物种的土壤遗留效应如何对生态修复构成阻碍，并梳理了可用于应对此类障碍的潜在管理手段。第二章，通过土壤样本理化分析，笔者探究了清除入侵银叶金合欢后，其土壤遗留效应的持续时长。第三章，借助温室控制实验，笔者分析了入侵银叶金合欢的土壤化学与生物遗留效应，以及本土杂草化物种对本土物种重新定植的影响。第四章，笔者在多个样地中识别出清除入侵银叶金合欢后出现的次生入侵物种，并通过为期三年的植被监测，探究了植被类型与火烧处理对这些次生入侵物种定植的影响。第五章，通过生长箱与温室控制实验，笔者分析了次生入侵物种间的相互作用，以及清除入侵银叶金合欢后出现的表观土壤硝态氮水平对次生入侵与本土杂草化物种占优现象的影响程度。 研究结果表明：土壤微生物群落结构改变、本土土壤种子库耗竭、土壤氮素水平升高、次生入侵、本土杂草化物种占优以及再次入侵，均会对生态修复构成阻碍。此外，碳添加、土壤微生物调控、除草剂（herbicide）施用、禾本科杂草除草剂（graminicide）施用以及本土物种重新引种等管理措施，可用于应对此类修复障碍。 银叶金合欢的土壤化学遗留效应可在清除后持续长达十年之久，但此类效应并未对本土类排根灌木（proteoid shrubs）的重新定植造成直接负面影响；不过土壤生物遗留效应与本土杂草化物种的存在，会显著抑制其生长。次生入侵并无特定生境偏好，火烧处理会促进其发生，且在清除后的三年内，其发生水平可维持在第一年及以上的程度。最后，次生入侵物种与本土杂草化物种的生长速率随土壤硝态氮水平升高而显著提升。 综上，笔者认为从事修复生态学的从业者应当在清除入侵银叶金合欢后，针对性管理土壤遗留效应、次生入侵物种与本土杂草化物种，以切实提升生态修复的整体成效。