For tree seedlings in boreal forests, ectomycorrhizal (EM) fungal networks may promote, while root competition may impede establishment. Thus, disruption to EM fungal networks may decrease seedling establishment owing to the loss of positive interactions among neighbors. Widespread tree mortality can disrupt EM networks, but it is not clear whether seedling establishment will be limited by the loss of positive interactions or increased by the loss of negative interactions with surrounding roots. Depending upon the relative influence of these mechanisms, widespread tree mortality may have complicated consequences on seedling establishment, and in turn, the composition of future forests. To discern between these possible outcomes and the drivers of seedling establishment, we determined the relative importance of EM fungal networks, root presence, and the bulk soil on the establishment of lodgepole pine and white spruce seedlings along a gradient of beetle-induced tree mortality. We manipulated seedling contact with EM fungal networks and roots through the use of mesh-fabric cylinders installed in soils of lodgepole pine forests experiencing a range of overstorey tree mortality caused by mountain pine beetle. Lodgepole pine seedling survival was higher with access to EM fungal networks in undisturbed pine forests in comparison with that in beetle-killed stands. That is, overstorey tree mortality shifted fungal networks from being a benefit to a cost on seedling survival. In contrast, overstorey tree mortality did not change the relative strength of EM fungal networks, root presence and the bulk soil on survival and biomass of white spruce seedlings. Furthermore, the relative influence of EM fungal networks, root presence, and bulk soils on foliar N and P concentrations was highly contingent on seedling species and overstorey tree mortality. Our results highlight that following large-scale insect outbreak, soil-mediated processes can enable differential population growth of two common conifer species, which may result in species replacement in the future.

中文翻译：

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外生菌根真菌网络对幼苗生长的影响取决于树种和过高死亡率的严重性。

对于北方森林中的树木幼苗，外生菌根（EM）真菌网络可能会促进，而根系竞争可能会阻碍其建立。因此，由于邻居之间正向相互作用的丧失，对EM真菌网络的破坏可能会减少幼苗的建立。广泛的树木死亡率可以破坏EM网络，但是尚不清楚幼苗的建立会受到正向相互作用的损失限制还是因与周围根系负向相互作用的损失而增加。取决于这些机制的相对影响，广泛的树木死亡可能对幼苗的建立以及未来森林的组成产生复杂的后果。为了区分这些可能的结果和幼苗建立的驱动因素，我们确定了EM真菌网络，根存在，沿着甲虫诱导的树木死亡率梯度，在松树和白云杉幼苗的建立上散装土壤。我们通过使用安装在黑松树林土壤中的网状圆柱体来操纵幼苗与EM真菌网络和根的接触，这些网眼圆柱体因山地甲虫而导致一系列高树死亡。与未经甲虫杀死的林分相比，在未受干扰的松林中使用EM真菌网络获得的小球松幼苗的存活率更高。就是说，过高的树木死亡率使真菌网络从对幼苗存活的好处转变为成本。相反，过高的树木死亡率并没有改变EM真菌网络，根系的存在和大量土壤对白云杉幼苗存活和生物量的相对强度。此外，EM真菌网络，根部存在和大量土壤对叶面氮和磷含量的相对影响在很大程度上取决于幼苗的种类和过高的树木死亡率。我们的结果表明，在大规模昆虫暴发后，土壤介导的过程可以使两种常见针叶树种的种群增长有所不同，这可能导致将来物种的替换。