Mycorrhizal fungi are the supply chains of the soil. With filaments thinner than hair, they shuttle vital nutrients to plants and tree roots.

In return, the fungi receive carbon to grow their networks. In this way, 13 billion tons of atmospheric carbon dioxide — one-third of fossil-fuel emissions worldwide — enter the soil each year.

These fungi cannot live on their own; they need the carbon from plants. In turn, 80 percent of the world’s plants rely on fungal networks to survive and thrive. The two are dependent trade partners.

These fungi make uncannily smart choices, even without a brain or central nervous system. Scientists describe them as “living algorithms.”

The trade algorithms reward efficiency: Build the most lucrative pathway possible for the lowest construction cost.

Fungal networks appear to assess demand and supply. Which plants need its nutrients the most? Which offer the most carbon? Where is the optimal payoff? This analysis shapes how the networks expand, as scientists learned when they mapped the growth in real-time.

“Fungi are super clever,” said Toby Kiers, an evolutionary biologist and director of the Society for the Protection of Underground Networks, a research organization. “They’re constantly adapting their trade routes. They’re evaluating their environment very precisely. It’s a lot of decision-making.”