Forests have expanded across Europe over the last centuries as a consequence of farmland (agricultural and pasture) abandonment. Agricultural practices usually increase soil fertility and reduce the diversity and abundance of ectomycorrhizal (ECM) fungi, essential mutualists of many woody species in temperate and Mediterranean forests. The recovery of this biotic interaction after the cessation of human activities is, thus, crucial for the re-establishment of functional forest ecosystems.

Here we addressed the legacies of past land use and the recovery of the mutualism between ECM fungi and Fagus sylvatica trees in recent beech forests (< 50 years) in Northeast Spain. Soil and root samples were collected in 6 long-established and 8 recent beech forests to analyse soil abiotic properties and the ECM fungi associated with beech roots (Illumina DNA metabarcoding of ECM tips).

Up to 609 amplicon sequence variants (ASV) of ECM fungi were identified, with 220 ASV found in both forest types. Recent forests had a significantly lower soil organic matter and phosphorus content, which had a significant effect on the community structure of ECM fungi in beech roots. Moreover, beech trees in recent forests interacted with less fungal taxa but had a higher relative abundance of Ascomycota. Tarzetta spp. (Ascomycota, Pezizales) and Lactarius blennius (Basidiomycota, Russulales) emerges as the particular taxa associated with recent and long-established forests, respectively.

More specialized mutualistic networks with a lower species normalized degree were found in recent forests, which might result in a lower quality and resilience of the ectomycorrhizal mutualism.

Synthesis: Land-use history modulated the mycorrhizal symbioses in regenerating beech forests through changes in soil organic matter and nitrogen, which were the main drivers of the differences in fungal community composition and functional types associated with beech trees in recent forests.

Fagus sylvatica trees in recent beech forests (< 50 years) and in long-established forests in Northeast Spain were selected and sampled. Soil and root samples were collected in 6 long-established and 8 recent beech forests to analyse soil abiotic properties and the ECM fungi associated to beech roots (Illumina DNA metabarcoding of ECM tips).

In addition to the data provided in this Dryad and in the supporting information section of this article, the raw reads (DNA) have been deposited in the sequence read archive and are available under the BioProject ID PRJNA622562.