Collect some dead branches or saplings that show some fiber or "strings" peeking out of rotten spots in the bark. Tulip Poplar, Basswood, Cedar, Willow, Elm, Mulberry, and many other trees produce fibrous inner bark that can be used as a raw material for cordage. These inner barks can be harvested from dead branches and tree trunks that have rotted to just the right consistency. The bark can also be stripped from live tree trunks and branches, and rotted in water. This process is traditionally called retting (old word for rotting). Place the bark strips under water and leave them there for several days. Then check the fibers every day until they separate from the outer bark into "ribbons." Twist them into cord while they're wet, or after drying, or use the ribbons for quick tying jobs. FYI, retting fiber is a very smelly process, especially in warm weather.