Re‐examining the evidence for the mother tree hypothesis – resource sharing among trees via ectomycorrhizal networks

Abstract

Seminal scientific papers positing that mycorrhizal fungal networks can distribute carbon (C) among plants have stimulated a popular narrative that overstory trees, or 'mother trees', support the growth of seedlings in this way. This narrative has far-reaching implications for our understanding of forest ecology and has been controversial in the scientific community. We review the current understanding of ectomycorrhizal C metabolism and observations on forest regeneration that make the mother tree narrative debatable. We then reexamine data and conclusions from publications that underlie the mother tree hypothesis. Isotopic labeling methods are uniquely suited for studying element fluxes through ecosystems, but the complexity of mycorrhizal symbiosis, low detection limits, and small carbon discrimination in biological processes can cause researchers to make important inferences based on miniscule shifts in isotopic abundance, which can be misleading. We conclude that evidence of a significant net C transfer via common mycorrhizal networks that benefits the recipients is still lacking. Furthermore, a role for fungi as a C pipeline between trees is difficult to reconcile with any adaptive advantages for the fungi. Finally, the hypothesis is neither supported by boreal forest regeneration patterns nor consistent with the understanding of physiological mechanisms controlling mycorrhizal symbiosis.

Environmental controls on seedling establishment in a boreal forest: implications for Scots pine regeneration in continuous cover forestry

“…Clearly, our data do not provide any support for these propositions. To the contrary, our results suggest that strong belowground competitive effects from adult trees suppress seedlings, in line with recent arguments questioning the validity of the mechanisms of common ectomycorrhiza networks as a resource transfer tool for adult trees to support seedlings (Henriksson et al 2023).…”

Arbuscular mycorrhiza increases the facilitation between adult plant and seedling but root‐knot nematode inhibits it

“…It was impossible to separate the relative contributions made by these nonexclusive mechanisms using the methods described in this study. But there is no doubt that promoting carbon and nutrient transfer or uptake may still be a potentially important mechanism for invasive plants to promote seedling growth (Henriksson et al, 2023; Montesinos‐Navarro et al, 2019a; Simard et al, 2012).…”

Comparison of Toxicity and Cellular Uptake of CdSe/ZnS and Carbon Quantum Dots for Molecular Tracking Using Saccharomyces cerevisiae as a Fungal Model

“…Mycorrhizae, the ancient symbiosis between plant roots and fungi, have risen to the knowledge of the general public as altruistic conduits through which nutrients are moved around between forest trees and weak or young trees are nurtured by the old and strong ones [ 1 , 2 ]. While nutrient transfer in mycorrhizal systems has been studied isotopically for a relatively long time [ 3 , 4 ], and some studies have indicated the potential movement of nutrients between plants connected via the mycorrhizae, several scientists have recently raised concerns about this narrative [ 1 , 2 , 5 ]. Indeed, based on a recent meta-analysis [ 5 ], previous results show little evidence of this kind of nutrient transfer.…”

“…Indeed, based on a recent meta-analysis [ 5 ], previous results show little evidence of this kind of nutrient transfer. One major reason is that testing the nutrient transfer with isotopic methods does not enable the exclusion of alternative pathways, as the transfer pathway is only deduced based on various control treatments instead of being directly observed [ 2 , 5 ]. In this respect, conjugating the nutrients to intrinsically fluorescent quantum dots (QDs) for nutrient tracking in mycorrhiza-plant systems could be a viable option to exclude the alternative pathways and determine whether such transfer is taking place [ 5 ].…”