IELTS Academic Reading passage: The Hidden Networks Beneath Forests

The Hidden Networks Beneath Forests

A. A forest is in some ways easier to understand from below than from above. Beneath the leaf litter of almost any woodland lies a dense fabric of fungal threads, each far finer than a root hair, growing through the soil and wrapping themselves around, and sometimes inside, the roots of trees. The association is called a mycorrhiza, from the Greek words for fungus and root, and it is not a curiosity of particular forests but the ordinary condition of terrestrial plant life: something in the region of nine out of ten land plants form one. The quantities are hard to picture. A single teaspoon of healthy forest soil may contain several kilometres of fungal filament, and the fungus attached to one mature tree can reach through hundreds of cubic metres of ground.

B. The relationship was correctly described earlier than is often supposed, and then ignored. In 1885 the German botanist Albert Bernhard Frank, asked by his government to improve the cultivation of truffles, reported that the roots of forest trees were routinely sheathed in fungal tissue, coined the term mycorrhiza, and proposed that the two organisms were feeding one another. His contemporaries were unpersuaded. The prevailing assumption was that a fungus on a root was an infection, and the notion that a plant might depend on one for its nourishment struck most botanists as implausible. Frank's interpretation was not widely accepted until well into the twentieth century, and the machinery of the exchange he had guessed at was not demonstrated until radioactive tracers became available after the Second World War.

C. What each partner supplies is now reasonably well established. The tree, which can make sugars from air and sunlight but cannot mine the soil efficiently, delivers a portion of its photosynthetic production underground; estimates of the share vary, but a figure of up to a third of the carbon a tree fixes is often quoted. The fungus, which cannot photosynthesise at all, returns mineral nutrients, above all phosphorus and nitrogen, together with water. Its advantage is geometric. Hyphae are slender enough to enter soil pores that a root cannot penetrate, and by extending far beyond the root tip they enlarge the volume of soil a tree can draw on by orders of magnitude.

D. The finding that captured public attention came later, and concerned the trees rather than the fungi. Because a single fungal individual may connect the roots of many trees, the network offers a route by which material can move from one plant to another. Working in the forests of British Columbia in the 1990s, Suzanne Simard labelled trees with carbon isotopes and detected the label subsequently in neighbouring trees of different species, including seedlings growing in deep shade that could not have fixed the carbon themselves. Mapping the connections revealed that they are not evenly distributed: a few large old trees, sometimes called hub trees, are linked to far more individuals than the rest, and their removal disconnects the network disproportionately.

E. The popular account of these findings has run some way ahead of the evidence. The image of a forest as a cooperative in which strong trees feed weak ones is appealing, but a fungus is not a charity, and there is no obvious reason why one should ferry carbon to a plant that is not paying for it. Much of the carbon detected in a neighbouring tree may never have entered that tree at all, remaining in the fungal tissue wrapped around its roots, where the labelling method cannot distinguish the two. Sceptics also note that a large share of the work has been done on seedlings in pots rather than on mature trees in forests, and a review published in 2023 concluded that several of the claims repeated most confidently in the press rest on only a handful of field studies, some of which have never been replicated.

F. For forestry the practical implications are less disputed than the biology. A network built over centuries is destroyed in an afternoon by heavy machinery, and a clear-felled site replanted with nursery stock may hold no fungal partners suited to the species being planted, which is one reason why replanting fails on ground where the same species grew perfectly well before. Foresters increasingly retain scattered veteran trees when harvesting, not for their timber but as reservoirs of fungal life from which the network can regrow, and there is interest in inoculating nursery seedlings with fungi collected on the site itself. The measures are cheap. The difficulty is that their benefit appears over decades, on a timescale that few forestry accounts are built to measure.