Why nonnative invasive plant species commonly co‐occur, despite their competitive superiority and propensity to displace native species, remains a paradox in invasion biology. Negative interactions among competitively dominant invaders are potentially alleviated by two understudied mechanisms: seasonal priority effects, where phenological separation weakens the effect of competition on species with early phenology; and indirect facilitation, where competition between two species is mitigated by a third species. Although phenological separation has been speculated as a mechanism for explaining co‐occurrence patterns of invasive plants, it has never been directly tested. In a greenhouse experiment, we tested the effect of phenological separation on direct and indirect interactions between three co‐occurring invasive plant species found in the riparian forests of North America. These species have distinct natural phenological separation with reproduction in early spring (Ficaria verna), mid‐spring (Alliaria petiolata), and late summer (Microstegium vimineum). When phenology was experimentally synchronized, direct pairwise interactions among invasive species were overwhelmingly negative, asymmetric, and unlikely to promote co‐occurrence. However, increasing phenological separation generated seasonal priority effects, which weakened the effect of competition on species with early phenology. Furthermore, the addition of a third species generated indirect facilitative effects, which balanced competitive outcomes among the two weakest competitors. Based on these findings, we conclude that phenological separation modulates the strength of both seasonal priority effects and indirect facilitation within species interaction networks and may promote the co‐occurrence of three common invasive species within this study system. We articulate how future studies can test the external validity of these findings in more complex environmental conditions and with a larger range of invasive plants.

中文翻译：

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物候学介导共存入侵植物物种之间的直接和间接相互作用


尽管非本地入侵植物物种具有竞争优势并倾向于取代本地物种，但为什么它们通常会同时出现，这仍然是入侵生物学中的一个悖论。竞争性优势入侵者之间的负面互动可能通过两种未被充分研究的机制来缓解：季节性优先效应，其中物候分离削弱了竞争对具有早期物候的物种的影响;和间接促进，即两个物种之间的竞争被第三个物种缓解。尽管物候分离已被推测为解释入侵植物共现模式的一种机制，但它从未被直接测试过。在温室实验中，我们测试了物候分离对北美河岸森林中发现的三种同时存在的入侵植物物种之间的直接和间接相互作用的影响。这些物种具有明显的自然物候分离，在早春 （Ficaria verna）、仲春 （Alliaria petiolata） 和夏末 （Microstegium vimineum） 繁殖。当物候学在实验上同步时，入侵物种之间的直接成对相互作用绝大多数是负面的、不对称的，并且不太可能促进共现。然而，物候分离的增加产生了季节性优先效应，这削弱了竞争对具有早期物候的物种的影响。此外，添加第三个物种产生了间接的促进效应，平衡了两个最弱竞争者之间的竞争结果。 基于这些发现，我们得出结论，物候分离调节了物种相互作用网络内的季节性优先效应和间接促进作用的强度，并可能促进本研究系统中三种常见入侵物种的共现。我们阐明了未来的研究如何在更复杂的环境条件和更广泛的入侵植物中测试这些发现的外部有效性。