Along any forest trail, you're likely to see fungus on a few tree trunks. Some fungi help trees, others can take a toll. This little brown shelf is evidence of heart rot, a disease that affects hardwood trees all over the world. It begins when broken tree bark exposes the wood of a tree to the fungus. This particular fungus is Polyporus schweinitzii, a common fungus in the watershed area.
Inside the tree, the fungus does damage to the wood, rotting it out. Initially, the rot may only occur in a small area inside the tree. But over time it can spread, affecting the structural integrity of the tree--to the point where a strong breeze can blow the tree over.
Up close, you can see a pattern characteristic of the fungus Polyporus schweinitzii. Notice how the wood is beginning to break apart in square chunks. This is why foresters call the rot caused by this fungus "brown cubical rot."
At the base of the fallen tree found along the trail is this pile of debris, chunks of rotted wood with the distinctive cubic shape. The unfortunate tree here is a Douglas fir, but this fungus can affect many kinds of trees, including maples, oaks, spruce and pine.