Old man's beard – those pale, hair-like strands that hang from tree branches in damp, clean forests – are not plants, not fungi, but a partnership: a fungus and an alga living together, the fungus providing structure, the alga providing food through photosynthesis. They are the canaries of the forest, the first to die when the air turns foul. And there was once a beard lichen that grew longer than any alive today – strands that hung 2 metres from the branches, thick as rope, a living curtain in the canopy. It was the giant honey lichen, and it is gone. Length and sensitivity made it special. A 2-metre beard lichen is a spectacular thing – a cascade of pale greenish-yellow strands, hanging from the highest branches, swaying in the wind. It was a home for insects, a nest for small birds, a filter for the forest air. Its dense strands captured mist and fog, dripping water onto the forest floor below. It cleaned the air. Lichens are bioindicators – they absorb everything from the atmosphere, including pollutants. They cannot escape or filter what they take in. In clean air, they thrive. In polluted air, they die. The giant honey lichen was the most sensitive of all, requiring the purest air to grow. Its presence was a sign of a healthy, ancient forest. It also fixed nitrogen. The alga inside its strands converted nitrogen from the air into a form that the fungus – and, eventually, the forest – could use. It was a natural fertiliser, hanging from the branches, feeding the trees below. Lichens reproduce by fragments and by spores. A piece of the lichen can break off, blow in the wind, and grow into a new colony. The giant honey lichen grew slowly, adding centimetres each year, taking decades to reach its full length. That strategy works when the forest is stable and the air is clean. It fails when the trees are logged and the air turns sour. Logging and air pollution destroyed it. When Europeans arrived, they logged the old-growth forests. The giant honey lichen needed ancient trees with thick, rough bark to cling to – the kind of trees that only exist in mature, undisturbed forest. When the trees were felled, the lichen fell with them. At the same time, coal smoke and industrial pollution filled the air. The giant honey lichen was exquisitely sensitive to sulphur dioxide – a common pollutant from coal burning. It died where smaller, tougher lichens could survive. By the 1910s, it was gone. The last specimens were probably collected by a botanist who had no idea he was holding the final individual. He pressed them, dried them, put them in a drawer. And the canopy fell silent. The smaller beard lichens survived. They are tougher, more adaptable, able to grow in slightly polluted air. They are the survivors, the short-haired ones, the lichens that kept their heads down. But the giant honey lichen is extinct. A few pressed specimens in a herbarium, a few fragments of its DNA preserved in alcohol, and the memory of a lichen that used to hang from the canopy, 2 metres long, pale and ghostly. We logged its trees. We fouled its air. Then we wondered why the forest felt so bare.