Birch polypore (Piptoporus betulinus) growing on the trunk of a silver birch tree (Betula pendula). This parasitic bracket fungus is found only on birch trees. The visible part of the fungus is the fruiting body that produces the spores, which are held in pores on the underside of the bracket. The fungus uses a fine mycelium network to extract nutrients from the trunk of the tree. It is generally only found on weakened trees, and the fungus can kill a birch tree over time.