Abstract

  • trees trade C for N with mycorrhizal network
  • this network is the “commons”, part of the tragedy
  • trees gain additional N et the expense of neighbours by supplying more C to fungi
  • this can lead to increased N immobilization (in fungal biomass)

Introduction

  • ectomycorrhizal fungi (EMF) hold N back from trees when they are in need
  • when N supply is amended by fertilization, also EMF give away greater proportion
  • further supply of EMF with C makes them diminish N returns
  • fungus competes with other EMF symbionts of the same plant, can gain more C by exporting more N, until personal N matches its received C
  • receiving more C makes EMF use more N for itself
  • enhanced EMF growth initially increases N uptake, but eventually N immobilization in fungal biomass; negative feedback on plant’s N uptake

  • How can this negative feedback made it through evolution?

  • dual scale of ectomycorrhizal symbiosis: one tree, many fungi and many trees, one fungus
  • trees have optimize individual benefit, this has consequences for other trees connected through the fungal network
  • combined efforts of trees to increase individual N gain aggravates N immobilization in fungal biomass

Hypothesis

  • fungus shares greater N proportion with trees that deliver more C
  • tree delivers more C to fungi that supply more N
  • tragedy from tree viewpoint: total C export to all fungi so high that it leads to N immobilization
  • tragedy from fungus viewpoint: exporting more N would reduce own growth, exporting less N reducec competitiveness for plant C
  • plants share common N pool, optimizing for individual gain hampers comunity gain
  • experiments with plant strangling and shading

Materials and methods

Seedling experiment

Field experiment

Model description

Results

  • shaded less biomass than sun, strangled no effect
  • shaded plants more N than sun plants, strangled plants way less: decreased C export to fungi mobilizes soil N to the plant and vice versa
  • maximum network-scale N mobilization occurs at intermediate level of network-scale C export
  • at individual tree level: 1 more C supply leads to 0.95 more N return, hence there is competition among trees which will eventually make network-scale N mobilization drop

Discussion

  • tragedy of the commons: individual gain reduces community gain
  • there is one common N pool all trees draw from (common EMF network, in which multiple fungi connect the host plants)
  • elevated atmospheric CO$_2$ levels should lead to higher N immobilization, more fungal biomass growth, maybe less tree growth