If you google "redwood burl" you will find many links and images to companies selling burlwood products. They range from guitars, knife handles, bed headboards and even glasses frames. Burls are prized for their swirling patterns.
A redwood tree's primary way of reproducing is through a burl resprouting, although they produce seeds in small cones as well. Removal of the burls isn't just reducing redwood reproduction, however. The removal of the bark is allowing the inner wood to be exposed to disease and pests, and gets rid of its fire-resistance (redwood bark is naturally fire-resistant and allowed the trees to survive natural wildfires). Removal of burls has already been killing some trees.
Burls can be legally harvested from private land, and are first harvested from fallen trees. However, a park spokesperson said, "We are often asked, 'What is the price of a burl? A burl is priceless if it comes from a national park that is an International Biosphere Reserve and a UNESCO World Heritage Site."
Redwoods National Park is closing down some of their roads after sunset to try to halt the poaching.
A CNN article with a picture of a tree that had its burl poached:
A National Geographic article with more about burl use and redwood biology: