Thanks for contacting "ask extension" and providing such good pictures of the roots with nodules. These are the roots from your nearby Alder tree. Alders have the ability to naturally fertilize forests by converting atmospheric nitrogen into forms available for other plants. This process, called nitrogen fixation, is essential for natural ecosystems.  (Garden peas and other legumes also have the ability to fix nitrogen and have similar nodes on their roots.
Happy Gardening!