Ivy is a climbing vine that once introduced to the redwood forest, can aggressively spread across the forest floor and up the trunk of redwoods. While appearing harmless at first, once established, ivy can grow quickly and cover the tree with leaves that block light and woody vines that restrict tree growth. Once a tight network of ivy branches are wrapped around a tree trunk, the tree will no longer be able to grow in girth.
If you have ivy creeping up one of your redwood trees, I recommend removing the ivy before it gets too firm of a grip on the tree. Your redwood will appreciate it!