Light barely reaches the forest floor. The ground is damp. Fallen kauri trunks lie in the half-dark, their bark sloughing off in plates. On the undersides of these trunks, a yellow crust grows. It spreads in patches. Sulphur yellow. Pale at the edges. The surface is smooth, waxy, almost membranous. Touch it and nothing happens. The crust does not bruise. It does not break. It clings to the wood like paint. Roll it back and the wood beneath is white. Bleached. The fungus has been working for years. Threats are severe. Kauri dieback disease (Phytophthora agathidicida) has killed thousands of mature trees across Northland, Auckland, and the Coromandel. The Kauri Forest Crust depends on dead kauri wood. No living kauri dies naturally in large numbers. Dieback creates dead wood. This sounds like an opportunity. It is not. The crust needs wood that has been dead for decades. Wood from trees that fell in storms. Wood from old windthrow. Dieback kills trees standing. They rot from the roots. They fall within years, not decades. The decomposition rate changes. The crust cannot adapt. Diet is saprotrophic. Phlebiella sulphurea consumes lignin and cellulose simultaneously, causing a simultaneous white rot. The wood loses colour and weight. The crust spreads centimetres per year. A single log may host it for thirty years. The fungus does not fruit every season. It rests. It waits. It expands slowly beneath the bark. When conditions are right, the yellow crust appears. Sometimes it glows faintly in the dark. Not bioluminescence. The colour reflects whatever light reaches the forest floor. In the dimmest places, it seems to make its own. Range is restricted to mature kauri forest remnants. Confirmed populations exist in Waipoua Forest, Trounson Kauri Park, the Waitakere Ranges, and the Coromandel Peninsula. Historic records from the Hunua Ranges and Great Barrier Island. Recent surveys failed to find it in the Hunuas. It may be extinct there. Northland populations are the largest. The species does not occur in the South Island. No kauri, no crust. The relationship is absolute. The fungus follows the tree. When the tree retreats, the fungus retreats with it. Habitat is specific to the decay phase of kauri forest. Not living trees. Not freshly fallen logs. Wood in the intermediate stage of decomposition. Bark still intact but loosening. Sapwood still firm. The crust grows on the underside of logs that have not touched the ground. Airflow matters. The fruit body needs humidity, not wetness. Logs lying directly on soil rot too fast. The crust cannot keep pace. Fallen branches suspended by ferns or other debris provide the ideal surface. These microsites are rare. Mature forest provides them. Plantation forest does not.