Data from: Ectomycorrhizal fungal richness declines towards the host species’ range edge

Plant range boundaries are generally considered to reflect abiotic conditions; however, a rise in negative or decline in positive species interactions at range margins may contribute to these stable boundaries. While evidence suggests that pollinator mutualisms may decline near range boundaries, little is known about other important plant mutualisms, including microbial root symbionts. Here, we used molecular methods to characterize root-associated fungal communities in populations of two related temperate tree species from across the species’ range in the eastern United States. We found that ectomycorrhizal fungal richness on plant roots declined with distance from the centre of the host species range. These patterns were not evident in nonmycorrhizal fungal communities on roots nor in fungal communities in bulk soil. Climatic and soil chemical variables could not explain these biogeographic patterns, although these abiotic gradients affected other components of the bulk soil and rhizosphere fungal community. Depauperate ectomycorrhizal fungal communities may represent an underappreciated challenge to marginal tree populations, especially as rapid climate change pushes these populations outside their current climate niche.

植物分布范围边界（plant range boundaries）通常被认为由非生物环境条件（abiotic conditions）所决定；然而，分布边缘处物种相互作用由正转负或正相互作用衰减，或许是促成这些稳定边界形成的重要因素。尽管已有证据显示传粉者共生关系（pollinator mutualisms）在分布边界附近可能出现弱化，但学界对其他重要的植物共生关系——包括根系微生物共生体（microbial root symbionts）——的了解仍十分有限。本研究针对美国东部全境范围内两种近缘温带树种（temperate tree species）的种群，采用分子方法（molecular methods）对其根系相关真菌群落（root-associated fungal communities）开展了表征分析。研究结果表明，宿主植物根系上的外生菌根真菌（ectomycorrhizal fungi）丰富度随其与宿主物种分布中心的距离增加而逐渐降低。这一生物地理分布模式在根系非菌根真菌群落（nonmycorrhizal fungal communities）以及非根际土壤（bulk soil）的真菌群落中均未体现。尽管气候与土壤化学变量会对非根际土壤和根际（rhizosphere）真菌群落的其他组分产生影响，但它们无法解释上述观测到的分布模式。外生菌根真菌群落的物种贫乏化或许是边缘林木种群面临的一项未被充分认知的生存挑战，尤其是在快速气候变化正迫使这些种群脱离当前气候生态位（climate niche）的背景下。