Why the tree branch lit up when exposed to UV light?

I had an old tree branch that was on the table, a party UV light was on, and the cuts of the branch lit up like a day-glow poster only white. Why is that?

Many materials such as minerals, marine life, and terrestrial life are naturally fluorescent. These naturally fluorescent products will respond to ultraviolet light differently than non-fluorescent items would​.

For example, some of our customers utilize our realUV™ LED Flashlight with BLACKOUT™ Filter Technology for nighttime scorpion photography or yooper-light stone hunting, which are both naturally fluorescent.