The seed bank dynamics of invasive Australian Acacia in the presence of biological control agents

Australian Acacia have become invasive in many parts of the world and are the most prominent invasive plants in South Africa. These nitrogen-fixing woody species change ecosystem structure and functioning, impacting on species richness and ecosystem services in their invaded ranges. These impacts cost economies billions of dollars annually. The environmental and economic effects have led to significant management efforts to control this invasion in South Africa. A large obstacle to the effective and sustainable management of invasive Australian Acacia, even in the presence of released biological control agents, is their ability to accumulate large seed banks. Despite the importance of seed banks for the successful management of these invasive plants and their relevance to monitoring management progress and success (e.g. biological control), there is a general lack of knowledge on their seed bank dynamics. Therefore, various aspects linked to the seed bank dynamics of four invasive Australian Acacia (A. longifolia, A. mearnsii, A. pycnantha and A. saligna) over space and time was studied. Through investigating this process it was aimed to assess the potential seeds that will have to be managed, whether the released biological control agents have an impact on the population dynamics of their invasive Australian Acacia hosts as well as when management should be conducted. Many seeds still resided in the seed banks of the studied species, regardless of the presence of the released biological control agents. These seed banks were not residual, i.e. the legacy of seed input from before biological control agents were released, but was shown to be the result of current seed accumulation over time. This seed bank accumulation suggested that seed production and survival remained high despite the impact of the biological control agents. Confirmation of this prediction was given through a study of seed production, which showed that many seeds are produced throughout the life-time of invasive Australian Acacia populations. Both the seed bank and seed production studies showed that irrespective of the seeds lost to biological control agents, these organisms have no impact on the population dynamics of invasive Australian Acacia, since more than enough seeds are produced to ensure population persistence. The development of the seed rain and seed bank over time indicated that the first few years of fruiting are important in determining seed bank size as well as the long-term persistence of seeds in the soil. Based on this finding a management strategy is proposed to control invasive Australian Acacia populations while they are still young, i.e. during the first few years post disturbance events. Focusing control efforts on the seedling stage will prevent the build-up of large seed banks, decrease the long-term persistence of seeds in the soil and will be more cost-effective, as removal of small plants requires fewer resources than large mature trees. Through investigating the seed bank dynamics of Australian Acacia, and establishing relationships between stand age (using tree size as a proxy), it was shown that seed banks can be used as an effective tool to monitor management effectiveness and progress.澳大利亚金合欢（Australian Acacia）已在全球诸多区域形成入侵，是南非最具代表性的入侵植物。这类固氮木本物种会改变生态系统的结构与功能，对入侵区域内的物种丰富度及生态系统服务造成负面影响，其带来的经济损失每年高达数十亿美元。这些环境与经济层面的影响，促使南非针对该入侵物种开展了大规模防治工作。尽管已释放生物防治剂（biological control agents），但有效且可持续防控澳大利亚金合欢入侵的一大阻碍，在于这类植物能够形成庞大的种子库。种子库对于这类入侵植物的成功防控至关重要，同时也与防治进度及成效的监测（例如生物防治效果监测）密切相关，但目前学界对其种子库动态普遍缺乏认知。因此，本研究针对4种入侵性澳大利亚金合欢——长叶金合欢（A. longifolia）、迈氏金合欢（A. mearnsii）、密花金合欢（A. pycnantha）以及柳叶金合欢（A. saligna）——的种子库动态及其相关的多维度时空特征展开了研究。本研究旨在明确需防控的潜在种子总量、已释放的生物防治剂是否会对寄主金合欢的种群动态产生影响，以及最佳防控时机。研究发现，无论是否已释放生物防治剂，受试金合欢物种的种子库中仍留存有大量种子。这些种子库并非残留的历史种子库，即并非源自生物防治剂释放前的种子输入遗留，而是长期持续的当前种子积累所形成的。该种子库积累现象表明，尽管受到生物防治剂的影响，金合欢的种子生产与存活仍维持在较高水平。针对种子生产的研究验证了这一推测：入侵性澳大利亚金合欢种群在整个生命周期中都会产生大量种子。无论是种子库还是种子生产相关研究均显示，即便有部分种子被生物防治剂取食，这类生物仍不会对寄主金合欢的种群动态造成显著影响，因为其产生的种子数量足以维持种群存续。种子雨与种子库随时间的变化规律表明，结果初期的种子输入对于决定种子库规模及土壤种子的长期留存至关重要。基于这一发现，本研究提出了针对入侵性澳大利亚金合欢种群的防控策略：应在种群仍处于幼龄阶段，即干扰事件发生后的最初几年内开展防控工作。将防控重点置于幼苗阶段，可阻止大型种子库的形成，降低土壤种子的长期留存率，同时也更具成本效益——因为相较于成熟大树，移除小型植株所需的资源更少。通过研究澳大利亚金合欢的种子库动态，并建立林分年龄（以树木体量作为替代指标）与种子库之间的关联，本研究证实，种子库可作为有效工具，用于监测入侵防控的成效与进展。