We all probably learned it in school, but most likely don’t remember the specifics. The picture to the left was taken of my Redspire Pear today. Cold, windy and rainy.
For many years, scientists have tried to fully understand the changes that happen to trees in the Autumn.
As the days grow shorter and the nights grow longer, biochemical processes in the leaf begin to paint the landscape awash with glorious color.
These chemicals are responsible.
- Chlorophyll for greens
- Carotenoids for orange
- Xanthophyll of the Carotenoid group for yellow
- Anthocyanins for reds and purples
- Tannins for brown as a waste product
The image below was taken on Garden Bloggers Bloom Day, a balmy warm day in the 60’s with temperatures dropping at night to the mid 40’s. Only three days between these two images passed. Does this not make you wonder? Even the same roses are present.
So what happened besides the weather change?
Because combinations of these pigments give us a huge range of color variation, just like an artist mixing paint on a canvas.
It depends on the quantities of pigments, in this case chemicals, being mixed. It depends on the weather and time of the season too. The leaves were gathered only for the post Gone But Not Forgotten on November 10. One week ago. A lot has remarkably happened in a week.
Here is the Breakdown
The red pigments of the anthocyanins protect leaves from the sun, giving some species extra time to absorb their essential leaf nutrients. As chlorophyll starts to exit the leaves, anthocyanins are being created to get the leaves additional time to unload the excess nutrients. Anthocyanins are a result of excess sugars within the cells and in combination with bright light, produce red pigment. Most anthocyanins are present only in Autumn. So that is why I saw the tree turning red at first, only to go full-blown yellow as the days lengthened and the rains came. My pear needed more food supply, so it went into high gear.
The carotinoids are present in the chloroplasts of the leaf cells during the entire growing season. But they are covered up by the abundance of chlorophyll in summer. As the chlorophyll is prevented from being produced, all the other colors start revealing themselves.
Bright light you say, when the nights are getting longer. Hum. The brightest Autumn colors are seen when the summers are dry and Fall has bright and sunny days with cooling nights in the lower 40’s. This is when some trees start making the anthocyanins. Not all trees make this chemical though. Many older European trees do not. In other trees, it is bred into them.
And without excess fresh water to renew it, the chlorophyll begins to disappear. The veins in the leaves start to close and cut off the water supply and trap the sugars. See my leaf image above as the veins turn red in the green leaf. This is the tree prolonging nutrient uptake with the production of anthocyanins.
There is so much detail to see if you just have a look. I picked the green leaves off the tree a week ago. You can see them changing to red. Then the yellows and oranges begin to appear after some have changed to red. Therefore all colors are present on my tree at once.
Different trees change at different times during the season, though.
US Forestry Service
Certain colors are characteristic of particular species. Oaks turn red, brown, or russet; hickories, golden bronze; aspen and yellow-poplar, golden yellow; dogwood, purplish red; beech, light tan; and sourwood and black tupelo, crimson. Maples differ species by species-red maple turns brilliant scarlet; sugar maple, orange-red; and black maple, glowing yellow. Striped maple becomes almost colorless.
Leaves of some species such as the elms simply shrivel up and fall, exhibiting little color other than drab brown.
The timing of the color change also varies by species. Sourwood in southern forests can become vividly colorful in late summer while all other species are still vigorously green. Oaks put on their colors long after other species have already shed their leaves. These differences in timing among species seem to be genetically inherited, for a particular species at the same latitude will show the same coloration in the cool temperatures of high mountain elevations at about the same time as it does in warmer lowlands.
This list of trees and color characteristics is from here. Interesting stuff, a bit technical, but good to know. It is a much more detailed explanation as to why leaves change color from the US Forestry Service. A place you can believe.
Leaves Fall Off
The bottom cells in the separation layer of the leaf form a seal between leaf and tree. The cells in the top of the separation layer begin to decay and break up. They form a break-line, and eventually the leaf falls from the tree.
If subjected to early frost, the trees will end their colorful display.
How About Those Evergreens
And what about evergreens like my Sparten Juniper behind the pear? What is different? Well, one difference is in that the needles are covered with a wax-like substance that prevents water loss. In the case of my concolor, the tree has a small surface size of individual, long, thin needles. Less surface, less exposure. Also quantity plays a part. Needles have many neighbors, cutting down on desiccation by wind. The interior of the needle also contains their own type of antifreeze to prevent evaporation and allow water to move even in winter. Now you know the principle behind antidesiccant. Coat em’ like a candle.
The colors you see in my leaves gathered last week are orange, yellow, red and green.
We all know the green is a result of Chlorophyll. The Reds are from the protecting anthocyanins. Since carotenoid production is not dependent on light, the levels in the leaves are not diminished by the shorter days. Carotenoids can be orange or yellow but most of the pigments found in leaves are yellow. Leaves with decent amounts of both anthocyanin and carotenoid will appear orange. Leaves with carotenoid but little or no anthocyanin will appear very yellow. If these pigments are not present, other plant chemicals also can affect leaf color such as tannins. Tannins are responsible for the brownish color of some oak leaves, for example.
Why are My Leaves Different Shapes on the Same Tree
The different shapes of the leaves are probably a result of the dry season we had. The alternating between dry spells with rain produced leaves smaller, stunted and misshapen, only to revert back to the expected shape of the pear leaf once moisture became plentiful.