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

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 versus heterospecifics remain unknown in spatially heterogeneous light environments. 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. 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 or host preference in the fungus. 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. 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.

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引发的非竞争性距离依赖和/或密度依赖的幼苗死亡率得以维持。然而，在空间异质性光环境中，近邻同种与异种个体的病原关联效应仍未明确。本研究将硬阔树种灯台树（Cornus controversa）与灰叶稠李（Prunus grayana）的种子，分别播种于6~7株目标成树下方，涵盖林下层（forest understory, FU）与皆伐周边木本植被形成的林隙（gap, Gap）两种生境。本研究调查了幼苗生长、死亡率、致死因子（如引发猝倒病与叶部病害的病原），以及丛枝菌根真菌（arbuscular mycorrhizal fungi, AMF）的根系侵染情况。研究发现，两种树种下方的土壤真菌群落特征均存在显著生境效应：土传猝倒病菌引发的幼苗死亡率在林下层高于林隙；而能提升幼苗相对生长速率的丛枝菌根真菌根系侵染率，则在林隙高于林下层。在林下层与林隙两种生境中，同种或异种成树下方的灯台树与灰叶稠李幼苗，由猝倒病菌引发的死亡率并无显著差异，表明这类真菌几乎不存在距离依赖性或宿主偏好性。在灯台树与灰叶稠李成树下方，最常见的叶部病害分别为气传病原真菌Haradamyces foliicola引发的环斑叶枯病，以及Phaeoisariopsis pruni-grayanae引发的角斑病。尽管这两类病原均可侵染两种幼苗，但同种幼苗（即“本土”个体）相较于异种幼苗（即“异地”个体），表现出更严重的叶部损伤、更早的落叶或更弱的细胞壁防御行为，表明这类叶部病害存在负距离依赖的侵染（即宿主偏好性）。因此，在林下层与林隙处理下，同种幼苗的死亡率均更高。综合分析表明，温带森林中的詹森-康奈尔假说主要通过气传叶部病害的强负调控作用介导，而非土传猝倒病菌。本研究发现，气传病害表现出距离依赖的宿主偏好性，无论光环境条件如何，均会造成同种幼苗更严重的损伤。