Saprotrophic fungi degrade dead organisms or their remains, e.g. plant remains as dead leaves, stalks, wood or roots, or animal remains as dung. According to their mode of breaking down cellulose or lignin, we recognise brown rot and white rot fungi. Brown rot fungi can only break down cellulose (a carbohydrate chain polymer) and leave the lignin (a phenolic three dimension polymer) intact. The substrate becomes brown and brittle because of the colour and consistence of the remaining lignin. White rot fungi break down both cellulose and lignin. The substrate becomes pale, fibrous and soft. Saprotrophes also break down proteins and other complicated organic matters, releasing simple compounds containing nitrogen and phosphorus, which can be taken up by the green plants. In the Arctic, degrading processes proceed slowly due to the cold and dry climate. Therefore remains of plants and animals can be very old, a type of natural mummification. Fungi may stop their decaying activities during certain periods of the year, and continue their work as soon as the right temperature and humidity conditions are met. Saprotrophic fungi are found in phyla as Chytridiomycota, Mucoromycota, Ascomycota and Basidiomycota. Chytridiomycota, Mucoromycota and many Ascomycota can only be studied after culturing and isolating fungi from soil samples. Most saprotrophic Basidiomycota and some Ascomycota with large ascocarps can be easily monitored in field.
Wood inhabiting saprotrophic fungi are of course very rare in the Arctic, and they are dependent either on drift wood or on wood in man-made constructions. However, some of these fungi are reported to be destructive in buildings. On Svalbard, Hyphocniciellum molle is an example of a harmful indoor brown rotter.