Although gaining much attention in recent years, it is unclear whether mycorrhizal fungi distribute meaningful amounts of resources among trees in ways that increase the fitness of the receiving trees. To this end, we used shaded and non-shaded pairs of inter- and intra-species Pinus halepensis and Quercus calliprinos saplings growing outdoors in forest soil. Carbon transfer was measured using pulse labeling with 13CO2 and the mycorrhizal community of each tree was identified by DNA barcoding. The effects of belowground connections were examined by tree performance and Non-Structural Carbohydrates (NSC) pools. Although we did not observe any growth benefits, shaded recipient oaks exhibited higher levels of root NSC compared to their control counterparts, which were not connected belowground. This finding suggests a potential benefit of establishing below-ground connections. We also show that non-shaded pines connected to shaded oaks were depleted of their starch pools, suggesting a possible cost of the tree-fungi-tree interaction. Additionally, we monitored the carbon (C) flow from a 13CO2 labeled donor pine tree to the final recipient oak tree and were able to demonstrate C transfer from pines to shaded oaks. Finally, we were able to identify the main fungal symbionts interacting with pines and oaks. Our results link specific mycorrhizal species to belowground C transfer and suggest C-driven fitness costs and benefits to the trees.

Seedling establishment under natural conditions is often limited by numerous interacting factors. We tested the combined effects of drought, herbaceous competition, and ectomycorrhizal fungi (EMF) on the growth and shape of Aleppo pine seedlings and their associated EMF community. EMF inoculum significantly increased seedlings’ height, biomass, and the number of side branches. However, under either competition or drought, the effect of EMF on seedling biomass and height was greatly reduced, while the effect on shoot branching was maintained. All plants were strongly dominated by Geopora, a fungal genus known to inoculate seedlings in dry habitats. Under a combination of drought and competition, EMF had no influence on plant growth and shape. This discrepancy highlights the complexity of the benefits provided to seedlings by EMF under ecologically relevant conditions.