Shared buddies: ectomycorrhizal fungal communities in pinus and quercus and their potential relevance for mixed-forest restoration.

Pines and oaks are ecologically and economically important taxa that co-occur in several forest and woodland systems across the northern hemisphere and that have been severely affected by diverse disturbances (e.g., wildfire, drought). Despite their relevance, the value of co-planting pines, that regenerate from seeds, and oaks, which are able to resprout, to improve forest restoration following disturbance has only recently been appreciated. An important consideration with this strategy is whether pines and oaks share root-associated fungal symbionts (ectomycorrhizas -EcM-, arbuscular mycorrhizas -AM- and dark septate endophytes -DSE-) that can be key to tree re-establishment following disturbance. However, the degree of overlap in fungal communities between pines and oaks is poorly understood. To address this knowledge gap, we conducted a systematic literature review of 948 studies published from 1969 to 2024. We hypothesized that both plant taxa would host DSE in addition to EcM, and that ectomycorrhizal fungal (EcMF) assemblages would differ in pines and oaks but that both host genera would share a limited subset of generalist taxa. We found that oaks and pines were mainly colonized by different EcMF species but also shared some taxa (26%), particularly members of the Russulaceae and Cortinariaceae. Few studies measured DSE or AM in addition to EcM, and the EcMF community of pines was better studied (~ 50% of described species) than that of oaks (~ 12%). We also conducted a greenhouse experiment using P. ponderosa and Q. gambelii as model species and the same mixed-forest soil as an EcMF inoculum source. We found that, at early developmental stages, both plant species formed associations with DSE but not AMF. For EcMF, P. ponderosa and Q. gambelii showed distinct communities with 30% of taxa in common. Importantly, when relative abundance of EcMF species was considered rather than occurrence only, the overlap between oaks and pines was more striking, approaching 90%. Taken together, these results support our hypothesis that Pinus and Quercus host distinct EcMF communities, but also share significant numbers of taxa. We discuss the potential relevance of this fungal overlap for understanding belowground linkages in mixed pine-oak systems and for future restoration-oriented research.