Data from: Roles of pathogens on replacement of tree seedlings in heterogeneous light environments in a temperate forest: a reciprocal seed sowing experiment

1. In forest communities, the Janzen–Connell (J-C) hypothesis proposes that species diversity is maintained by non-competitive distance- and/or density-dependent seedling mortality caused by host-specific natural enemies. However, the effects of pathogen associations from nearby conspecifics vs. heterospecifics remain unknown in spatially heterogeneous light environments. 2. Seeds of hardwood species Cornus controversa (Cornus) and Prunus grayana (Prunus) were sown beneath 6–7 Cornus and Prunus adults in both the forest understory (FU) and in gaps (Gap) created by felling all woody vegetation near the focal adults. Seedling growth, mortality, killing agents (e.g. pathogens that cause damping-off and leaf diseases), and root infection by arbuscular mycorrhizal fungi (AMF) were investigated. 3. We found strong habitat effects on the expression of soil fungi beneath both tree species. Seedling mortality caused by soil-borne damping-off pathogens was greater in FU than in Gap, and AMF infection, which enhanced relative seedling growth rate, was greater in Gap than in FU. Seedling mortality caused by damping-off pathogens did not differ between Cornus and Prunus seedlings beneath the adults of conspecific or heterospecific adults in both FU and Gap, suggesting little distance dependence (i.e. host preference) in the fungus. 4. Beneath the adults of Cornus and Prunus, the most prevalent leaf diseases were zonate leaf blight and angular leaf spot caused by the airborne pathogenic fungi Haradamyces foliicola and Phaeoisariopsis pruni-grayanae, respectively. Although these pathogens attacked the seedlings of both species, conspecific seedlings (i.e. home) showed more severe leaf damage, earlier leaf shedding and/or less defensive behaviour (cell wall defence) relative to heterospecific seedlings (i.e. away), suggesting negative distance-dependent attack (i.e. host preference) for these leaf diseases. As a result, greater seedling mortality was observed for conspecific seedlings under both FU and Gap treatments. 5. Synthesis. In the temperate forest, the J-C hypothesis is largely mediated through the strong negative influence of airborne leaf diseases rather than through soil-borne damping-off pathogens. We found that airborne diseases demonstrated distance-dependent host preferences, which led to greater conspecific seedling damage regardless of environmental light conditions.1. 在森林群落中，詹森-康奈尔（Janzen–Connell，J-C）假说提出，物种多样性由宿主特异性天敌对幼苗造成的非竞争性距离依赖型与/或密度依赖型死亡率得以维持。然而，在空间异质的光照环境中，邻近同种与异种个体的病原菌关联效应仍未明确。 2. 本研究选取硬阔树种灯台树（Cornus controversa，简称Cornus）与灰毛稠李（Prunus grayana，简称Prunus）的种子，分别在林下层（forest understory，FU）以及皆伐目标成年植株周边所有木本植被形成的林隙（Gap）中，于6~7株同种或异种成年植株下方进行播种。随后对幼苗生长情况、死亡率、致死因子（如引发猝倒病与叶部病害的病原菌）以及丛枝菌根真菌（arbuscular mycorrhizal fungi，AMF）的根系侵染状况展开调查。 3. 研究发现，两种树木根围土壤真菌的群落特征均受到显著生境调控。由土传猝倒病原菌引发的幼苗死亡率在林下层显著高于林隙；而可提升幼苗相对生长速率的丛枝菌根真菌根系侵染率，则在林隙显著高于林下层。此外，在同种或异种成年植株下方的林下层与林隙生境中，灯台树与灰毛稠李幼苗的猝倒病死亡率并无显著差异，表明这类真菌几乎不存在距离依赖型宿主偏好（即宿主特异性）。 4. 在灯台树与灰毛稠李成年植株下方，最普遍的叶部病害分别为气传病原菌Haradamyces foliicola引发的环纹叶枯病，以及Phaeoisariopsis pruni-grayanae引发的角斑病。尽管这两类病原菌可侵染两种宿主的幼苗，但同种幼苗（即“本土”个体）相较于异种幼苗（即“异地”个体），会表现出更严重的叶部损伤、更早的落叶现象，或更弱的防御行为（如细胞壁防御），表明这类叶部病害存在负向距离依赖型侵染（即宿主偏好）。最终，在林下层与林隙两种处理下，同种幼苗的死亡率均更高。 5. 综合分析表明，在温带森林中，詹森-康奈尔假说主要通过气传叶部病害的强负调控作用介导，而非土传猝倒病原菌。本研究发现，气传病害表现出距离依赖型宿主偏好，无论光照环境如何，均会造成同种幼苗更严重的损伤。