Context‐dependent effects of below‐ground carbon transfer: Limited benefits from sunlit pines to shaded oaks

Despite gaining significant attention in recent years, it remains unclear whether mycorrhizal fungi distribute meaningful amounts of resources among trees in ways that increase the fitness of the receiving trees. To investigate this, we used pairs of shaded and unshaded Pinus halepensis or Quercus calliprinos saplings, growing in both inter‐ and intra‐specific combinations outdoors in forest soil. We examined the combined effects of indirect and direct below‐ground connections on tree performance and Non‐Structural Carbohydrate (NSC) pools. Although we did not observe any growth benefits, shaded recipient oaks exhibited higher levels of root and branch NSC compared to their control counterparts, which were not connected below‐ground. This finding suggests a potential benefit of establishing below‐ground connections. However, no such benefits were observed among the unshaded pairs or in the other inter‐ and intra‐specific pairs of pines and oaks. We monitored the carbon (C) flow from a 13CO2‐labelled donor pine tree to a below‐ground connected oak tree and demonstrated C transfer from pines to shaded oaks. We also identified the main fungal symbionts interacting with pines and oaks. Our findings indicate that the effects of below‐ground C transfer are context‐dependent, manifesting in nuanced alterations in plant NSC that are not readily apparent through conventional growth metrics. Read the free Plain Language Summary for this article on the Journal blog.