Global warming has been shifting climatic envelopes of many tree species to higher latitudes and elevations across the globe; however, unsuitable soil biota may inhibit tree migrations into these areas of suitable climate. Specifically, the role of mycorrhizal fungi in facilitating tree seedling establishment beyond natural species range limits has not been fully explored within forest ecosystems. We used three experiments to isolate and quantify the effects of mycorrhizal colonization and common mycorrhizal networks (CMN) on tree seedling survival and growth across (within and beyond) the elevational ranges of two dominant tree species in northeastern North America, which were associated with either arbuscular mycorrhiza (AMF, Acer saccharum) or ectomycorrhiza (EMF, Fagus grandifolia). In order to quantify the influence of mycorrhiza on seedling establishment independent of soil chemistry and climate, we grew seedlings in soils from within and beyond our study species ranges in a greenhouse experiment (GE) as well as in the field using a soil translocation experiment (STE) and another field experiment manipulating seedling connections to potential CMNs (CMNE). Root length colonized, seedling survival and growth, foliar nutrients, and the presence of potential root pathogens were examined as metrics influencing plant performance across species' ranges. Mycorrhizal inoculum from within species ranges, but not from outside, increased seedling survival and growth in a greenhouse setting; however, only seedling survival, and not growth, was significantly improved in field studies. Sustained potential connectivity to AMF networks increased seedling survival across the entire elevational range of A. saccharum. Although seedlings disconnected from a potential CMN did not suffer decreased foliar nutrient levels compared with connected seedlings, disconnected AM seedlings, but not EM seedlings, had significantly higher aluminum concentrations and more potential pathogens present. Our results indicate that mycorrhizal fungi may facilitate tree seedling establishment beyond species range boundaries in this forested ecosystem and that the magnitude of this effect is modulated by the dominant mycorrhizal type present (i.e., AM vs. EM). Thus, despite changing climate conditions beyond species ranges, a lack of suitable mutualists can still limit successful seedling establishment and stall adaptive climate-induced shifts in tree species distributions.

中文翻译：

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菌根真菌作为物种范围扩展期间树苗建立的关键生物过滤器


全球变暖已将许多树种的气候范围转移到全球更高的纬度和海拔地区;然而，不合适的土壤生物群可能会抑制树木迁移到这些气候适宜的地区。具体来说，菌根真菌在促进超出自然物种范围限制的树苗建立方面的作用尚未在森林生态系统中得到充分探索。我们使用三个实验来分离和量化菌根定植和共同菌根网络 （CMN） 对北美东北部两种主要树种海拔范围内（内部和外部）树苗存活和生长的影响，这些树种与丛枝菌根 （AMF， Acer saccharum） 或外生菌根 （EMF， Fagus grandifolia） 相关).为了量化菌根对幼苗建立的影响，独立于土壤化学和气候，我们在温室实验 （GE） 中以及使用土壤易位实验 （STE） 和另一个操纵幼苗与潜在 CMN （CMNE） 的联系的田间实验，在研究物种范围内外的土壤中培养幼苗。根系定植长度、幼苗存活和生长、叶面养分和潜在根系病原体的存在被检查为影响不同物种范围内植物性能的指标。菌根接种物来自物种范围内，而不是来自物种范围外，提高了幼苗在温室环境中的存活率和生长能力;然而，在田间研究中，只有幼苗存活率，而不是生长，得到了显著改善。与 AMF 网络的持续潜在连接提高了 A. saccharum 整个海拔范围内的幼苗存活率。 尽管与相连的幼苗相比，与潜在的 CMN 断开连接的幼苗的叶面养分水平没有降低，但断开连接的 AM 幼苗（而不是 EM 幼苗）的铝浓度明显更高，并且存在更多的潜在病原体。我们的结果表明，菌根真菌可能促进在这个森林生态系统中超越物种范围边界的树苗建立，并且这种影响的大小受存在的主要菌根类型（即 AM 与 EM）的调节。因此，尽管气候条件的变化超出了物种范围，但缺乏合适的共生主义者仍然会限制幼苗的成功建立，并阻碍适应性气候诱导的树种分布变化。