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Beneath the visible world of tree trunks and forest canopy lies an extensive underground network formed by mycorrhizal fungi, which establish symbiotic relationships with the roots of most tree species. In this partnership, fungal filaments called hyphae extend far beyond a tree's root system, absorbing water and essential nutrients such as phosphorus from the soil and delivering them directly to the tree. In exchange, the tree supplies the fungus with sugars produced through photosynthesis, sugars that fungi cannot generate on their own since they lack chlorophyll. This mutually beneficial arrangement has existed for hundreds of millions of years and is now understood to be essential to the health of most forest ecosystems.

Research over the past few decades has revealed that these fungal networks do far more than simply nourish individual trees. Because a single fungal network can connect the root systems of multiple trees, sometimes spanning dozens of individuals across a forest, nutrients and chemical signals can travel between trees that are not directly touching one another. Ecologist Suzanne Simard demonstrated that carbon produced by one tree could be detected in a neighboring tree connected through the same fungal network, suggesting that trees can effectively share resources with one another. Some researchers have proposed that older, larger trees, sometimes called hub trees, may direct a disproportionate share of resources toward younger seedlings growing in their shade, potentially improving those seedlings' odds of survival during their most vulnerable early years.

These discoveries have led some scientists to describe mycorrhizal networks using the informal term "wood wide web," a nod to the way information and resources flow between trees much like data flows between computers. [A] Chemical signals traveling through fungal networks appear to warn neighboring trees of insect attacks, prompting the recipients to begin producing defensive compounds before the threat actually arrives. [B] Not all researchers agree on how significant these networks are for forest-wide behavior, and some caution against overstating the extent of cooperation implied by the term. [C] Nonetheless, most scientists agree that mycorrhizal networks play some role in how forests respond collectively to environmental stress. [D] Understanding these networks has become increasingly important for forest management practices aimed at preserving ecosystem resilience.