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.