Belowground carbon transfer across mycorrhizal networks among trees: Facts, not fantasy

The mycorrhizal symbiosis between fungi and plants is among the oldest, ubiquitous and most important interactions in terrestrial life on Earth. Carbon (C) transfer across a common mycorrhizal network (CMN) was demonstrated over half a century ago in the lab (Reid & Woods, 1969), and later in the field (Simard et al., 1997a). Recent years have seen ample progress in this research direction, including evidence for ecological significance of carbon transfer (Klein et al., 2016). Furthermore, specific cases where the architecture of mycorrhizal networks have been mapped (Beiler et al., 2015) and CMN-C transfer from mature trees to seedlings has been demonstrated (Orrego, 2018) have suggested that trees in forests are more connected than once thought (Simard, 2021). In a recent Perspective, Karst et al. (2023) offered a valuable critical review warning of over-interpretation and positive citation bias in CMN research. It concluded that while there is evidence for C movement among plants, the importance of CMNs remains unclear, as noted by others too (Henriksson et al., 2023). Here we argue that while some of these claims are justified, factual evidence about belowground C transfer across CMNs is solid and accumulating.