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.

植物分布区边界通常被认为是非生物因子的反映；然而，分布区边缘的物种负相互作用增强、正相互作用减弱，或是这类稳定边界形成的潜在驱动因素之一。尽管已有证据表明传粉者互利共生作用在分布区边缘可能减弱，但学界对其他重要的植物互利共生关系——包括根系微生物共生体——的认知仍较为匮乏。本研究采用分子生物学方法，对美国东部地区两种近缘温带树种全分布区种群的根系附生真菌群落进行了表征分析。研究发现，植物根系上的外生菌根真菌（ectomycorrhizal fungal）丰富度随宿主物种分布区中心距离的增加而显著降低。该模式在根系非菌根真菌群落以及非根际土壤（bulk soil）真菌群落中均未显现。尽管气候与土壤化学变量构成的非生物梯度会影响非根际土壤和根际真菌群落的其他组成部分，但无法解释上述生物地理格局。物种贫乏的外生菌根真菌群落，可能对边缘树种种群构成此前未被充分重视的挑战；尤其在快速气候变化迫使这些种群脱离当前气候生态位的背景下，这一问题将更为凸显。