The branches of the silver birch often have tangled masses of twigs known as witch's brooms growing among them, caused by the fungus "Taphrina betulina". Old trees are often killed by the decay fungus "Piptoporus betulinus" and fallen branches rot rapidly on the forest floor. This tree commonly grows with the mycorrhizal fungus "Amanita muscaria" in a mutualistic relationship. This applies particularly to acidic or nutrient-poor soils. Other mycorrhizal associates include "Leccinum scabrum" and "Cantharellus cibarius". In addition to mycorrhiza, the presence of microfauna in the soil assists the growth of the tree, as it enhances the mobilization of nutrients.