The mycorrhizal tragedy of the commons

Abstract

Trees receive growth-limiting nitrogen from their ectomycorrhizal symbionts, but supplying the fungi with carbon can also cause nitrogen immobilization, which hampers tree growth. We present results from field and greenhouse experiments combined with mathematical modelling, showing that these are not conflicting outcomes. Mycorrhizal networks connect multiple trees, and we modulated C provision by strangling subsets of Pinus sylvestris trees, assuming that carbon supply to fungi was reduced proportionally to the strangled fraction. We conclude that trees gain additional nitrogen at the expense of their neighbours by supplying more carbon to the fungi. But this additional carbon supply aggravates nitrogen limitation via immobilization of the shared fungal biomass. We illustrate the evolutionary underpinnings of this situation by drawing on the analogous tragedy of the commons, where the shared mycorrhizal network is the commons, and explain how rising atmospheric CO 2 may lead to greater nitrogen immobilization in the future.

Tree water uptake enhances nitrogen acquisition in a fertilized boreal forest – but not under nitrogen‐poor conditions

“…Hypothetically, the trade‐off for such an expanded root surface area would be a reduced inward water flux at the root surface, making soil microbes more competitive for soil N, which is in line with our model results. Previous field experiments have shown that immobilization of N by soil organisms, including ectomycorrhizal fungi, was alleviated under N fertilization (Näsholm et al ., 2013; Hasselquist et al ., 2015; Henriksson et al ., 2021). This observation is consistent with the current study, where total 15 N recovery per plot was twice as high in the fertilized stand as in the reference stand (Table 2).…”

Ectomycorrhizas and tipping points in forest ecosystems

“…In a strongly N-limited conifer forest (reflected in low needle N concentration and N : P ratio), the biomass of EM fungi is very high. Nitrogen immobilization in EM mycelium, described as the 'nitrogen trap' by Franklin et al (2014), can be so high that tree growth becomes negatively related to tree C supply to EM fungi (Henriksson et al, 2021). If N deposition increases, there will be an initial positive effect for both tree and fungus (Lindahl & Clemmensen, 2016).…”

Effects of Early, Small-Scale Nitrogen Addition on Germination and Early Growth of Scots Pine (Pinus sylvestris) Seedlings and on the Recruitment of the Root-Associated Fungal Community

“…Growing in association with already-established ECM is often positive for seedlings, and established ECM fungi can enhance survival at the plant community level [79]. However, it has been observed that under some environments, such as nutrient-poor soils, belowground interactions limit seedling growth due to their low competitive strength against adult trees [14,25,80,81]. Thus, although seedlings that germinate in undisturbed boreal forest soils have access to established mycorrhiza, from which they may gain valuable nutrients [82], seedling establishment in such undisturbed forests is often poor, and these seedlings are generally smaller compared to those that establish in scarified soils [25].…”

“…ECM are generally considered to enhance nutrient and water uptake by plants, seedling establishment and survival, and increased plant resilience against environmental stressors [8]. However, the ecological role of ECM can vary from mutualistic to parasitic [9][10][11], and in situations such as strong N limitation, mycorrhizal fungi can immobilize available N and aggravate plant N limitation [12][13][14].…”