Why Do Trees Lose Their Leaves?

Snowcreek (11/2/15) Alicia Vennos

It’s survival not just of the fittest, but of the wisest.

Deciduous trees drop their leaves in order to survive.  As days grow shorter and colder, deciduous trees shut down veins and capillaries (that carry water and nutrients) with a barrier of cells that form at the leaf’s stem.

Called “abscission” cells, the barrier prevents the leaf from being nourished. Eventually, like scissors, the abscission cells close the connection between leaf and branch and the leaf falls.

Had the leaves remained on branches, the leaves would have continued to drink and, once temperatures drop to freezing, the water in the tree’s veins would freeze, killing the tree.

Further, with leaves fallen, bare branches are able to carry what little snow collects on them, protecting them from being broken under the weight of the snow. So, by cutting off their food supply (leaves), deciduous trees survive winter.

The fallen leaves continue to benefit the tree through winter, spring and summer by creating a humus on the forest floor that insulates roots from winter cold and summer heat, collects dew and rainfall, and decomposes to enrich the soil and nurture life.

It’s a cycle of survival, planned wisely.

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Why Do Leaves Change Color?

Chlorophyll Molecule (Wikipedia)

Leaves on deciduous trees change color in autumn from green to various hues of lime, yellow, gold, orange, red and brown because of a combination of shorter days and colder temperatures.

Throughout spring and summer, green chlorophyll (which allows trees to absorb sunlight and produce nutrients) is made and replaced constantly. However, as days grow shorter, “cells near the juncture of the leaf and stem divide rapidly but do not expand,” reports Accuweather.com, “This action of the cells form a layer called the abscission layer. The abscission layer then blocks the transportation of materials from the leaf to the branch and from the roots to the leaves. As Chlorophyll is blocked from the leaves, it disappears completely from them.”

That’s when vivid yellow xanthophylls, orange carotenoids and red and purple anthocyanins emerge.

Orange is found in leaves with lots of beta-carotene, a compound that absorbs blue and green light and reflects yellow and red light, giving the leaves their orange color.

Yellow comes from Xanthophylls and Flavonols that reflect yellow light. Xanthophylls are compounds and Flavonols are proteins.  They’re what give egg yolks their color.

Though always present in the leaves, Carotenoids and Xanthophylls are not visible until Chlorophyll production slows.

Red comes from the Anthocyanin compound. It protects the leaf in autumn, prolonging its life. Anthocyanins are pigments manufactured from the sugars trapped in the leaf, giving term to the expression that the leaves are sugaring up.

The best fall color occurs when days are warm and nights are clear and cold. California’s cloudless skies and extreme range of elevations (sea level to 14,000′) provide ideal conditions for the development of consistently vivid fall color, as seen in these reports.

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Darrell Sano Scores Several Firsts

Sunol Regional Wilderness (8/20/17) Darrell Sano

Color spotter Darrell Sano (dksfoto.smugmug.com) began his search for fall color “a bit early this year,” by hiking into the Sunol Regional Wilderness and along Alameda Creek this past Sunday, where he saw the first reported “hints of fall color.”

Sunol Regional Wilderness (8/20/17) Darrell Sano

Sunol Regional Wilderness (8/20/17) Darrell Sano

Darrell said the display wasn’t obvious. He had to search for it and hiked “off the trail to really see it.” Nevertheless, despite that temperatures were pushing 90 degrees, Darrell said he “still could sense the change of light,” and that autumn was approaching.

Should you visit this preserve in San Francisco’s east bay, you’ll find alder, willow and sycamore, though mostly evergreen coastal live oak and gray pine, plus blue oak, elderberry, madrone and … rattlesnakes.

Northern Pacific Rattlesnake, Sunol Regional Wilderness (8/20/17) Darrell Sano

Sunol Regional Wilderness (8/20/17) Darrell Sano

The well-fed snake seen in this photograph (far left) crossed a shaded trail toward Darrell.

Congratulations to Darrell who scores several color spotter firsts: the first to file a report in 2017, the first to report from the Sunol Regional Wilderness and the first to set a record for the hundred yard dash.
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Solar Eclipse – Leaves As Pinhole Lenses

The sun’s rays passed through blue oak leaves today to cast crescent images of the solar eclipse.

Today’s solar eclipse was seen by millions across America. Most looked up while wearing protective solar eclipse glasses, though many others created pinhole cameras to watch the moon block out all or part of the sun.

Another way to see the eclipse was to stand beneath trees and look down. Light passing through small openings between leaves similarly cast a crescent reflection of the sun on the ground, as seen in these photos.

Solar eclipse as sunlight passes through tree leaves.

Acorns and leaves have been falling as autumn approaches.

 

 

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