Data from: Competition and facilitation determine dwarf mistletoe infection dynamics

1. Interspecific interactions have a fundamental role in plant population dynamics, as they may set the conditions for species coexistence. Parasitic plants, like dwarf mistletoes, offer the opportunity to study competition for resources that are different from those consumed by most plants, allowing for a better understanding of the interaction. 2. We explored how interspecific interactions between two dwarf mistletoe species (Arceuthobium), co-infecting the same host species (even sharing the same individual tree of Pinus hartwegii) affect their infection dynamics at two different stages of population development (colonization of new hosts and subsequent growth), and if heterogeneity in resource availability (host density and size structure) affects these interactions. For that purpose, we integrated these processes into a spatially-explicit model of density-dependent population growth. 3. We found that self-regulation (density-dependence) was strong for both species; however the intensity and sign of interspecific interactions changed depending on host size and demographic process. Population growth in A. globosum was reduced by competition, except for smaller hosts where A. globosum growth was facilitated by A. vaginatum. A. vaginatum was facilitated by A. globosum regardless of host size. Colonization of new hosts by A. globosum was enhanced by previous infection by the other species, showing intraguild facilitation. 4. Demographic importance of interactions depended on stand structure: in homogeneous, low-density forests, facilitation predominates, increasing the population sizes of both species, whereas the opposite occurs in heterogeneous and dense forests. Both species achieved stable coexistence, fulfilling the invasibility criterion because each mistletoe species can invade a forest that is already infected by the other species. 5. Synthesis. Despite the fundamentally different mechanisms underlying the interactions between mistletoes compared with non-parasitic plants, our results reveal that their behaviour at the population level is similar. Stabilizing mechanisms, like strong self-limiting population growth, allow dwarf mistletoe coexistence. Interactions shift as populations develop, and they depend largely on environmental factors such as forest structure. Intraguild mutualism is shown as a relevant process for colonization of new spaces, highlighting the complexity of competitive/facilitative interactions between parasitic plants, a formerly unexplored subject. Interactions can only be fully understood when integrating all their components at the population level. Analysing these interactions may contribute to the understanding of plant-plant interactions in general, and convey interesting implications for forest management.1. 种间相互作用（interspecific interactions）在植物种群动态中具有核心作用，可为物种共存奠定基础。寄生植物如矮槲寄生（dwarf mistletoes）为研究不同于多数植物所利用资源的竞争提供了契机，有助于更深入地理解此类相互作用。2. 我们探究了两种同侵染同一宿主物种（甚至共享同一株哈特维吉松（Pinus hartwegii））的油杉寄生属（Arceuthobium）矮槲寄生物种之间的种间相互作用，如何在种群发育的两个不同阶段（新宿主定殖与后续生长）影响其侵染动态，以及资源可利用性异质性（宿主密度与径级结构）是否会对这些相互作用产生影响。为此，我们将这些过程整合至空间显式密度依赖种群增长模型（spatially-explicit density-dependent population growth model）中。3. 我们发现，两个物种均存在较强的自我调节（self-regulation）作用，即密度依赖（density-dependence）效应；但种间相互作用的强度与方向会随宿主大小以及种群统计过程发生变化。球果油杉寄生（A. globosum）的种群增长会受到竞争抑制，但在小型宿主上，其生长会受到腋花油杉寄生（A. vaginatum）的促进。腋花油杉寄生（A. vaginatum）的生长无论宿主大小如何，均会受到球果油杉寄生的促进。球果油杉寄生的新宿主定殖过程会因另一物种的先期侵染而得到增强，表现出同营养类群促进作用（intraguild facilitation）。4. 相互作用的种群统计重要性取决于林分结构：在均质低密度林分中，促进作用占主导，可提升两个物种的种群规模；而在异质高密度林分中则恰好相反。两个物种均实现了稳定共存，且满足可入侵性准则（invasibility criterion），即任一槲寄生物种均可入侵已被另一物种侵染的林分。5. 综合分析。尽管矮槲寄生间的相互作用机制与非寄生植物存在根本性差异，但本研究结果表明，二者在种群水平的行为模式具有相似性。诸如较强的种群自我限制增长这类稳定化机制，可促成矮槲寄生的物种共存。相互作用会随种群发育进程发生动态变化，且在很大程度上依赖于森林结构等环境因素。本研究揭示出同营养类群互利作用是新空间定殖的关键过程，凸显了寄生植物间竞争/促进相互作用的复杂性——这一主题此前尚未得到充分探索。只有整合种群水平的所有组分，才能全面理解此类相互作用。对这类相互作用的分析，可为一般性的植物-植物相互作用研究提供借鉴，并为森林管理带来颇具价值的启示。