What are these on trees?

What are these on trees?

You have found a burl! Burls are produced after a tree undergoes some form of stress from a fungus, an insect, or the environment. These irregular structures are produced by an abnormal growth of the xylem in a tree. Xylem cells provide structure for plants and transport water and nutrients up from the root to stems and leaves. The abnormal growth of burls make these strange structures prized by woodworkers because of their beautiful, irregular wood grain pattern. There is no way to manage these growths, and removing them will cause more harm then help. Keep your eyes out for more interesting burls in the parks – there are many!

– Naturalist Karie Wheaton