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