Casuarina invasions: a multi-scale assessment of an important tree genus

Understanding the processes that drive the invasion of non-native species is often essential for effective management. This thesis focuses on Casuarina spp. – an economically and ecologically important tree genus with taxa that have been widely disseminated by humans. I explore the effects certain taxa can have on community dynamics in recipient environments, investigate the factors that mediate invasion of Casuarina species, and from this aim to develop recommendations for managing the group. First, I assessed the global introduction history, invasion ecology and the evolution of management approaches of Casuarina. Ten of the 14 species in the genus have been introduced outside their native ranges to over 150 countries, but only three species are recorded as naturalized or invasive. As with other groups there is a correlation between native range size and invasiveness – the three invasive species also have the largest introduced ranges. Propagule pressure explains much more of the variance in observed invasiveness between Casuarina taxa than any known combination of life-history traits. Large-scale plantings of casuarinas in some climatically suitable areas have not yet resulted in large-scale invasions; there is a substantial global Casuarina invasion debt. Experiences in Florida and the Mascarene Islands highlight that casuarinas have the potential to transform ecosystems with significant control costs. Despite modest progress with managing invasions in some areas, substantial problems remain. As with most other invasive tree taxa, complex conflicts of interest are particularly challenging. Second, I looked at mechanisms underlying naturalization and assessed invasion risk at a regional scale. Here, I examined Casuarina cunninghamiana invasions in the south-western Cape of South Africa – the part of the country with the largest contiguous area climatically similar to the native range of the species. Propagule pressure is a key driver of naturalization of C. cunninghamiana populations in climatically suitable areas. The species also naturalizes in regions with suboptimal bioclimatic conditions, but then only very near (&lt;10 m) planted trees. Risk models indicate that C. cunninghamiana is likely to spread across a greater region of the Western Cape than it currently occupies. Naturalized populations of C. cunninghamiana are young and expanding. The capacity of the species to resprout and attain reproductive maturity at an early age suggests that this species could become a widespread and damaging invader in South Africa. We conclude with some recommendations for management, and argue that if particular steps are taken (e.g. the immediate removal of all female plants from proximity to dams and water-courses; all future sales and plantings to be restricted to male plants) then it might be possible to safely utilise the species in future. Third, I examined the invasion dynamics of a single Casuarina species at the landscape scale. I explored how interactions between disturbance and invasion govern successional trajectories, using the remarkable invasion of C. equisetifolia on the volcanic island of Réunion. Invasive populations of C. equisetifolia have increased substantially in extent over 40 years. Lava flows have facilitated the spread of C. equisetifolia and invasion of this species has radically changed successional trajectories, increasing the rate of succession sevenfold. This case mirrors work done on Morella faya and Falcataria moluccana on Hawa'ii, which shows the extent to which invasive species can alter ecosystem function and benefit from natural disturbances generated by volcanic lava flows. The continued spread of C. equisetifolia poses a major threat to the small area of remaining native lowland rainforests on Réunion which cover &lt; 2 % of their original extent. The studies in this thesis have uncovered patterns, processes and invasion risks for invasive trees that are not well represented in the literature. Some insights derived from well-studied tree genera, such as Acacia and Pinus, seem to apply fairly well to tree invasions in general. However, special ecological features of Casuarina species and the ways they are used by humans call for unique considerations when piecing together changing global distributions and creating effective strategies for management.阐明驱动外来物种入侵的相关过程，是开展高效入侵管理的核心前提。本论文聚焦木麻黄属（Casuarina spp.）——这一兼具经济与生态价值的乔木类群，其多个分类单元已被人类广泛传播。本研究旨在探究特定木麻黄分类单元对入侵群落动态的影响，剖析调控木麻黄属物种入侵的关键因子，并据此提出针对该类群的管理建议。首先，本研究梳理了木麻黄属的全球引种历史、入侵生态学特征以及管理策略的演变历程。该属14个物种中有10个已被引种至原生分布区以外的150余个国家，但仅3个物种被记录为已归化或具有入侵性。与其他入侵类群一致，木麻黄的原生分布范围大小与其入侵性存在显著相关性：这3个具有入侵性的物种同时拥有最广阔的引种分布范围。相较于已知的任何生活史性状组合，繁殖体压力对木麻黄不同分类单元间入侵性差异的解释度更高。部分气候适宜区域的大规模木麻黄种植尚未引发大规模入侵，全球范围内存在显著的木麻黄入侵债务。佛罗里达州与马斯克林群岛的治理经验表明，木麻黄具备彻底改变生态系统的潜力，其防控成本极高。尽管部分地区的木麻黄入侵治理已取得一定进展，但仍存在大量亟待解决的问题。与多数其他入侵乔木类群类似，复杂的利益冲突是治理过程中尤为棘手的难题。其次，本研究聚焦于归化入侵的内在机制，并在区域尺度上评估了入侵风险。本次研究以南非西开普省的细枝木麻黄（Casuarina cunninghamiana）入侵事件为对象——该区域是南非境内与该物种原生分布区气候相似度最高、连片面积最大的区域。在气候适宜区域内，繁殖体压力是细枝木麻黄种群实现归化的关键驱动因子。该物种亦可在生物气候条件欠佳的区域实现归化，但仅能存活于距种植木<10米的范围内。风险模型预测，细枝木麻黄在西开普省的扩散范围将远超当前的分布区域。南非境内的细枝木麻黄归化种群尚处于年轻阶段且正持续扩张。该物种具备萌芽更新能力且可早期达到生殖成熟，这意味着其极有可能在南非发展为分布广泛、危害严重的入侵物种。本研究最终提出了一系列治理建议，并指出若采取针对性措施（例如立即移除水库与河道周边的所有雌株；未来所有木麻黄的销售与种植均仅限雄株），未来仍可安全利用该物种。最后，本研究在景观尺度上探究了单一木麻黄物种的入侵动态。本次研究以火山岛留尼旺岛的滨海木麻黄（Casuarina equisetifolia）大规模入侵事件为案例，解析了干扰与入侵的交互作用如何调控群落演替轨迹。40余年来，滨海木麻黄的入侵种群分布范围已大幅扩张。熔岩流活动促进了滨海木麻黄的扩散，而该物种的入侵彻底改变了群落演替轨迹，使演替速率提升了7倍。这一案例与夏威夷岛的香杨梅（Morella faya）与南洋楹（Falcataria moluccana）相关研究结果一致，均表明入侵物种可极大程度改变生态系统功能，并从火山熔岩流引发的自然干扰中获益。滨海木麻黄的持续扩散，对留尼旺岛仅剩的原生低地雨林构成严重威胁——该类雨林的现存面积仅为原始分布范围的2%以下。本论文的研究揭示了现有文献中鲜有记载的入侵乔木的分布模式、过程与入侵风险。部分基于金合欢属（Acacia）与松属（Pinus）等研究较为充分的乔木类群得出的结论，似乎可较好地推广至一般乔木入侵研究。但木麻黄物种独特的生态特征，以及人类对其的利用方式，要求在梳理其全球分布变化、制定高效管理策略时，需纳入独特的考量维度。