Fall 2022 is Here - The Leaves they are a Changing and other Bits and Bobs

Reach back and recall your grade school days and the simple science behind deciduous trees changing color every fall. If you said it's a result of shorter days and less sunlight which allows tree leaves to take a winter break from all that work they do converting water and carbon dioxide into sugars, technically known as photosynthesis, then you pass with flying colors ... pun intended! 

 

There are other processes going on as well ... it all has to do with leaf pigments.

 

In order for the whole photosynthesis process to happen, leaves require help from chlorophyll which is what gives leaves their green color. The natural substances that make up leaf cells (pigments) become more noticeable each fall as chlorophyll production wanes from less sunlight. 

 

Environmental and plant genetic factors affect the pigments in leaves too which play an important role in fall leaf color intensity and duration.

 

What's Weather Got to do with it?

Many warm, sunny days and cool (not freezing nights) allow the anthocyanins (colored molecules produced by leaves) to shine through with all those brilliant shades of crimson, purple and red.

Yellow, gold and orange leaves are fairly consistent from year to year due to other pigments (specifically carotenoid) that hang around in leaves despite the weather.

Fall moisture helps leaves stay colorful for a longer period of time. Drought conditions while leaves are losing their chlorophyll pigments lead to brown leaves and early drop.

Which Trees drop their leaves the soonest in fall? 

This is a fairly easy answer, take a look around where you live, even your own yard. Trees that are environmentally stressed or simply unhealthy from drought stress, lack of consistent watering, experiencing pest insect or disease issues often start losing their leaves the earliest, even in the summer months. 

It's easy to spot trees and shrubs dropping leaves that are stressed by leaf scorch. 

Trees that are growing in a northern part of the landscape will lose their leaves earlier than same types growing in a sunnier part of the yard. 

What's the Deal with Trees that don't drop their leaves in the fall?

Oak tree leaf loss, specifically with white, pin, English and red oaks have their own fall leaf story to tell. In a nutshell, these oaks (along with beech and hornbeam trees) hold onto their leaves during the winter and then shed them in the spring. This process is called marcescence (pronounced "mar-CESS-enss"). 

The reason for this phenomenon is all about the cells (called the abscission zone) in leaves separating from the end of the leaf stem and where it's attached to the twig. Most trees have the type of cells that drop their leaves in the fall while marcescent trees don't utilize the abscission cells to drop their leaves until the spring when the new buds form.

Why this happens is still not clearly understood by scientists. Read more on this topic here.

Happy Fall, enjoy the leaf colors!

Resources:

Aspen Fall Colors

Science of Fall Colors

When Oak Leaves Fail to Fall Excellent photos