Warning: This graphic requires JavaScript. Please enable JavaScript for the best experience. Not all trees drop their leaves in the fall. Even in the height of winter ...

This is the time of year when Nicole Hughes gets constantly distracted by the fall foliage as she drives around North Carolina. "I'm always looking," says Hughes, a biologist at High Point University.

The study also shows that nearly 41% of so-called naturalized tree species—species that do not naturally occur in a given area but now grow wild there—possess traits such as fast growth and small ...