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.