ACT Reading — Natural Science — Drill 4

NATURAL SCIENCE: This passage is adapted from the article "The Language Beneath the Soil: How Trees Communicate Through Fungal Networks" by Ingrid Halvorsen (©2019, Forest Ecology Today). Beneath the forest floor, a network of extraordinary complexity connects tree to tree through a web of fungal filaments called mycorrhizae. These threadlike structures, thinner than a human hair, form symbiotic relationships with the roots of most tree species: the fungus receives sugars produced through photosynthesis, and in return extends the tree's effective root surface area by orders of magnitude, dramatically improving its access to water and soil nutrients. What researchers have more recently discovered is that this network does more than facilitate individual nutrition. Trees in a forest can transfer carbon, nitrogen, and water to one another through mycorrhizal connections, and the transfers are not random. Studies in temperate forests have found that large, established trees — sometimes called "hub trees" or "mother trees" — send disproportionate quantities of carbon to smaller, younger trees, particularly seedlings growing in low-light conditions where photosynthesis alone would be insufficient for survival. The mechanisms behind these targeted transfers are not fully understood. Some researchers propose that the transfers are a passive consequence of concentration gradients — nutrients move toward where they are most needed simply because of differential pressure. Others suggest that trees may actively regulate flows through biochemical signaling, though evidence for deliberate "communication" remains contested. What is clear is that the mycorrhizal network functions as infrastructure: a forest is not a collection of independent organisms competing for resources but an integrated system in which the fates of individual trees are materially linked. The practical implications for forestry are significant. Logging practices that remove hub trees disrupt not only the canopy but the underground network, potentially impairing regeneration across a much wider area than the cleared patch itself.