The mycorrhizal symbiosis is ubiquitous in boreal forests. Trees and
plants provide their fungal partners with photosynthetic carbon in
exchange for soil nutrients like nitrogen, which is critical to the
growth and survival of the plants. But plant carbon allocation to
mycorrhizal symbionts can also fuel nitrogen immobilization, hampering
tree growth. Here we present results from field and greenhouse
experiments combined with mathematical modelling, showing that
mycorrhizal fungi can be simultaneously mutualistic to an individual
tree and parasitic to the networked community of trees. Mycorrhizal
networks connect multiple plants and fungi, and we show that each tree
gains additional nitrogen at the expense of its neighbors by supplying
more carbon to the fungi. But this additional carbon supply eventually
aggravates nitrogen immobilization in the shared fungal biomass.
Individual trees may thus independently benefit from increasing their
carbon investment to mycorrhiza, while causing a decline in nitrogen
availability for the whole plant community. We illustrate the
evolutionary underpinnings of this situation by drawing on the analogous
the tragedy of the commons, and explain how rising atmospheric CO2 may
lead to greater nitrogen immobilization in the future.