Why Do Leaves Fall and Change Color in Autumn?

Featured in the Carolina Tree Chronicle Fall Edition 2022

By Stephen Weil, Plant Health Care Manager

I don’t know about you, but fall is my favorite time of the year. The mild days and cool nights are a welcome change from the heat of summer. Perhaps like me, you also enjoy orchard walks, picking apples, hayrides, corn mazes, fresh donuts, apple cider slushies, and finding that perfect pumpkin.

And don’t get me started on the fall colors! Here in North Carolina, we are blessed with a large variety of deciduous trees that offer a tree-mendous palette of fall colors! We also have a lot of trees in general. North Carolina ranks 14^th in terms of forest cover at 60% as per the US Forest Service (2016 data). Maine is 1^st at a whopping 89%, while North Dakota is dead last at 2% (mostly owing to it being part of the Great Plains, but also due to intentional human-related removal).

Speaking of fall color, have you ever wondered how deciduous trees know when to drop their leaves and why they change color? We all probably covered this in school at one point or another, but here’s a brief refresher if it’s been a while.

All trees make their “food” in their foliage by photosynthesis – a process that has carbon dioxide, water, and sunlight as inputs. This process occurs in the foliage in something called a chloroplast, which contains a green pigment called chlorophyll.

Since chlorophyll is constantly being degraded, new chlorophyll must continually be produced to replace the chlorophyll that is being lost.

Besides chlorophyll, trees also have several other colored pigments in their foliage. These pigments include xanthophylls (yellow), carotenoids (yellow, orange, red), and anthocyanins (red). However, chlorophyll is typically the largest constituent, so the other pigments are not typically seen until a significant change occurs.

Fall is that change.

As the amount of available sunlight in each day decreases (decreasing photoperiod), a deciduous tree is triggered to begin the process of leaf senescence (aging) and eventually leaf abscission (loss). Essentially, the tree starts the process of getting ready to lose its leaves. At that point, chlorophyll production starts to decrease since the tree no longer needs to make as much “food” for itself.

Eventually, when chlorophyll levels decrease enough, the other pigments can start to be seen since the chlorophyll is no longer concealing them. The spectacular yellows, reds, and oranges we see in fall were actually there the whole time, we just couldn’t see them until the chlorophyll started going away!

Until next time,

Stephen