Trees and fungi are capitalists.
The two plants act like buyers and sellers, with fungi even playing tree clients off against each other
When less nitrogen was available in the soil, fungi gave up less nitrogen to the trees. But when nitrogen was abundant, they built up their stores like speculators cornering the market in a commodity, effectively forcing the trees to get their nitrogen from the fungi no matter what.
Having multiple symbiotic trading-partners generates competition among both the fungi and the plants, where each individual trades carbon for nutrients or vice versa to maximize profits, not unlike a capitalistic market economy.