What caused the pen-like lines on this spruce tree stump?
These lines are called Spalting. They are a result of a fungal invasion of the wood. These lines are an early indication of the disease - eventually the fungus will grow and the rotted wood will be more apparent. There may have been other sections of this tree that had a more advanced rot, and caused the failure of the whole tree.
Woodworkers often prize wood that is in the early stage of decay because these lines add interest to the finished products.
Here are some photos of various tree woods, and the spalting that occurs in them: