Did you ever wonder why the leaves turn colors in the fall and don't simply fall off of the trees? Did you ever wonder why the process takes so long? Did you ever wonder why some leaves turn yellow while others turn red? I bet you could guess the correct answers by just observing what is happening every day during the fall. Yes, the days are getting shorter. Yes, the temperatures are getting cooler and yes, these do have something to do with the process of leaves changing color by triggering a complex series of biological reactions to get the plant ready for winter survival. These biological processes are unseen for the most part and they involve the movement and storage of nutrients from the leaves down to the roots.
The process of nutrient movement, called translocation, is confirmed and can be seen via the harvesting of maple sap to make maple syrup, a yummy by-product. The harvesting of maple sap is actually done in the spring. The literature suggests that the sap is more starchy in the fall when it is translocated down the tree trunk and sap is more sugary in the spring when it is mixed with water as it rises up the trunk of the tree. The taps can be set up between the middle of February to the middle of March and the spring sap will flow until the trees buds break in May. Fun fact: It can take 40 gallons of sap to make one gallon of syrup.
The first phase of the nutrient conversion involves the breakdown of the material that makes green leaves green, chlorophyll. In the absence of chlorophyll, and specifically nitrogen, the leaves start to turn yellow because the green pigment is dismantled and the yellow pigment, which is also present in leaves, is left. But as we can observe, not all leaves turn yellow in the fall, some leaves turn red, orange, purple and brown. In these plants where the leaves turn red, it is the absence of the element phosphorus that allows the the green pigment to dissolve and the red pigment to remain. Pigment for the orange color and the brown color also exist in each leaf and also show through as the green color fades.
Fall color is very specific to plant species like in trees including maples, oaks and ash's, etc. For instance, the silver maple is always a light yellow color and the Norway Maple is always a golden yellow color. Fall color is very specific for varieties within a species too. A good example is the Red Maple, called that because its FLOWERS are red, can be either yellow or red with many variations depending on the variety. It is interesting to google search “Red Maple fall color images” to see how wide ranging the colors are, the fact that there are so many color variations is very uncommon in the tree community. The White Ash is another tree to check out close up because the fall color is purple on some leaves and yellow on other leaves, which you can see by searching “White Ash Fall color images”. Another interesting pattern of purple is found on the Sweet Gum, but the two most exciting trees to observe for fall color intensity are the Sugar Maple and the Sassafras.
Another attribute to pay attention to in the fall is the sequence of color changes among tree species and tree varieties. Just as there is an order to the sequencing in flowering trees and shrubs in the spring, there is the same sequencing in the fall. Among the first trees to turn are the red maples, which are also one of the first to flower in the spring. One of the last to turn is the Callery pear, which is one of the latest to flower in the spring. Again the driving force is the light cycle of shorter days and longer nights and the lowering of day-night temperatures. Even within species, like a Red Maple, there are a lot of different varieties, like October Glory and Red Sunset, which present their own, unique color palette. That's what makes Nature so incredible, no reminders are necessary, no time clocks need to be wound; external forces converge and the process kicks off. Beautiful.
It is for this reason there is a “peak” to the fall season. The peak is a range set in the middle of the season from the time leaves first start to turn color to when they have all fallen off of the tree. The middle of the season guarantees that all of the trees in a region have started the process of turning but before the majority have dropped their leaves. The peak is a bit of a moving target because, like the cherry blossom peak in Washington DC, it can be altered by rises and drops in temperatures.
The purpose of the process is to, obviously, get trees ready for surviving the winter cold. It is commonly thought that trees are dormant, that nothing is happening during the winter but this is not really the case. While food is translocated down the trunk of the tree to the roots for storage, winter is a time when tree root systems do a lot of growing. In fact the roots of lots of plants, not just trees, grow during the winter months. There are also other plant systems that continue to function during the winter, responding to the rising and lowering of daily temperatures. Trees continue to respire through lenticels on their trunks that can open and close. The trees that lose their leaves when temperatures drop are called “deciduous”. There are other plants that keep their leaves during the winter and these trees are called “evergreen”. The leaves on evergreens are constructed in a way that protect them from freezing temperatures, which is especially significant in the far north or far south approaching the arctic and antarctic circles.
If you have any concern about oxygen levels in the winter when the photosynthesis levels are reduced due to this dormancy, as opposed to oxygen levels in the summer when these same areas are in full photosynthesis production, worry not! Literature references vary, but it is generally estimated that the majority of the world's oxygen is produced by ocean algae. Scientists estimate that between 50% to 90% of the earth's oxygen comes from this resource. Don't you just love nature!