What triggers leaf drop in fall
Introduction
Autumnal leaf drop is a form of self-protection. Unlike evergreen plants, deciduous species generally have thin leaves that are susceptible to freezing. The tender leaf cells would rupture during the winter, making them useless for photosynthesis. As such, some plants evolved to drop them.
The green
Chlorophyll is the chemical that produces a green color. It actually is reflecting the green wavelength in a light ray (instead of absorbing it) - That is why you see the leaf green!
This pigment is used in photosynthesis to make sugar. There are other colored substances in leaves as well - but you can’t see them because there is so much chlorophyl it blocks them out !
During the growing season trees store energy in the form of sugars and starches. With temperatures falling in autumn, production of chlorophyll stops and the levels of this chemical within leaves reduces. This in what allows us to see the other substances and colors within the leaves.
Yellow and other colors
As chlorophyll reduces, there is an abundance of carotenes left in the leaf – these are crystals inside of cells that produces yellow, orange, and brown colors. Same as in corn, carrots, and daffodils – oh and bananas!
See these Gingko leaves - this yellow has been there all along – just superseded by the green. An invisible yellow !
Anthocyanins
These are water-soluble chemical compounds that are made from sugar. They are red to purple. Leaf cells use them as sunblock to protect chlorophyll which is super sensitive and quite reactive to sunlight.
By adding the reds and purples the leaf can disassemble and store its extra chlorophyl back into the stems and roots without being damaged by the sun.
You may have noticed this on a common NYC street tree (ornamental pear) turning this color in the fall.
Fall colors
What you see usually is a broad mix of greens, yellow, purples, and reds that are constantly changing. There are several reasons for this.
For one, the process is not uniform, and it varies by leaf age and position on the tree. It also depends a lot on the local environment - the drier and sunnier the autumn, the more anthocyanins are produced, and the redder the leaves will be.
Lastly - species have differing amounts of all these pigments and substances in their cells. That is why that Gingko is bright canary yellow – it is FULL or carotene crystals !
Abscission layer
In autumn, as the leaf begins producing less sugar it also reduces the amount of an auxin (hormone) it makes. This hormone in turn stimulates the production of another substance – abscisic acid.
The acid directs a band of cells in the petiole near where the leaf connects to the stem – to develop a wax (suberin) and also to form cork cells (lignin). This creates a barrier and in turn shuts off the vessels that carry water and substances to and from the leaf.
Suberin and lignin together create a durable and waterproof layer for the plant once the leaf gets detached – it’s that leaf scar you see. on the stem to the left.
The cork actually seals the stem from the outside air - and eventually the wind will blow the leaf away.
How odd
Dormancy is brought on by a change in the levels of plant hormones. Short day length is the most important environmental cue that stimulates the onset of dormancy and leaf fall. For that reason, in cities, trees close to street lamps are often the last to lose their leaves.
Drought will also hasten the onset of leaf fall, causing trees to shed their leaves earlier following a dry spring and summer.
On the other hand, superabundant watering or hard pruning stimulates vigorous growth which delays the onset of dormancy.
If an abnormal freeze hits before most trees drop their leaves, leaf tissues can be killed before sufficient enzymes are produced to cause a natural leaf drop. As a result, many trees may enter winter still carrying some of their leaves.
The depth of color (hue) is influenced by the blend of chemical processes and weather conditions.
If temperatures stay above freezing, anthocyanin production is enhanced, and the leaves take on a red color.
If it’s a dry fall – sugars become concentrated in the leaves, more anthocyanin is produced, and consequently leaves are redder.
Exceptions
The retention of leaves that would normally fall is called marcescens. Certain species exhibit this trait, like beeches, oaks, hornbeams and witch hazel.
More often, it is seen on the young trees of these species, or those in the forest understory where they are somewhat sheltered from weather and wind.
With marcescens, the abscission layer doesn’t fully form. Leaves hang on until severe weather or new leaf growth in the spring causes them to drop.
Some say it’s an evolutionary advantage to deter browsing animals, others that it’s to attract birds (and thus their droppings) – but it’s a bit of a mystery as far as I am concerned.
And now you know !
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Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA
All photographs used with permission @SHUTTERSTOCK
