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 and Woods 1969), and later in the field (Simard et al. 1997). 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. Mycorrhizas are fungal associations between plant roots and beneficial fungi (mushrooms). In forests, some of these belowground associations can include more than one tree, creating a common mycorrhizal network (CMN). It has been shown in multiple studies that CMNs can serve as transport pathways of carbon among different trees. Recently, Karst et al. (2023) offered a valuable critical review questioning the importance of CMNs, as noted by others too (Henriksson et al. 2023). Here we argue that factual evidence about belowground C transfer across CMNs is solid and accumulating and discuss current questions in CMN research.
