Nicole Hughes

In the fall, to recover nitrogen, trees take apart green chlorophyll.

This reveals yellow pigments that were there all along.

But the red pigments are different.

Trees make brand new red chemicals just days before a leaf is about to fall.

Nicole Hughes is a biologist at High Point University in North Carolina.

She says the red pigments do act as a kind of sunscreen, which may protect trees' ability to recoup that precious nitrogen.

But some researchers think the red could be a signal to insects that lay eggs in the fall.

One study found more aphids on trees with green and yellow leaves compared to trees that were red.

Nell Greenfield-Boyce, NPR News.