As we head into mid-October, the fall season in many areas of the US is in full swing with crisp cool days and a dazzling display of autumnal colors. All around us - from our backyards to our highway roadsides - our trees are staging their final and most spectacular show before closing down for the winter, as they transition from their peaceful summer shades of green to fiery autumn tones of red, yellow and orange.
Why and how do trees change their leaf colors year after year? And how do they get to be so vibrant? The reasons and factors influencing these changes may be as intriguing as the visual effect of the colors themselves. In this article we will go behind the scenes and take a backstage view of our trees’ fall finale to learn some of the science - and the purpose - of this fascinating process.
Why do trees lose their leaves in the fall?
Not all trees drop their leaves with the advent of cold winter temperatures, most notably many conifer species and broadleaved evergreens. But deciduous trees that lose their leaves prior to winter do so for a number of environmental reasons that positively impact their survivability, protecting vulnerable tissues from injury caused by freezing cold and allowing trees to conserve energy and moisture resources that are sparse as water freezes in the frigid winter months. This winter period for deciduous trees, called dormancy, is somewhat similar to hibernation in animals, and, for many trees and plants, a winter rest provides an internal re-set for their growth cycles that is necessary for their health and survival.
The roles of leaf pigments
The colors and changes that we see in leaves during the fall are the result of substances contained within them that are called pigments. The most common pigments in tree leaves are:
- Chlorophylls – these are present during the growing season, producing a green color
- Carotenoids – these are also present during the growing season, producing yellow and orange colors, but are usually masked by green chlorophyll
- Anthocyanins – these produce reds and purple colors in the fall
- Tannins – these produce browns and russet shades
Leaf pigments not only appear as different colors, but they also play different roles that help the tree. And they become visible at different times of the year as they perform their separate functions. Let’s take a closer look at each of these pigments and discover their remarkable roles.
Chlorophyll
Chlorophyll is the green pigment that plays the most prominent role in photosynthesis, a process which enables a tree to capture the energy of sunlight and convert it into sugars that can be used by the tree. This pigment is made on a continual basis throughout the growing season in response to the amount of daylight the tree receives. However, this pigment is short lived, and as the day’s length of sun exposure shortens after midsummer, trees make less and less chlorophyll.
In the fall, the length of day continually shortens, and chlorophyll production slows and eventually ceases to the point where it breaks down and disappears. As this happens, valuable nutrient elements contained in chlorophyll, especially nitrogen and magnesium, are reabsorbed by the tree and stored for growth next year.
Carotenoids
Meanwhile, as chlorophyll fades away, other pigments are either uncovered or are produced in the green pigment’s absence. Carotenoids are responsible for the yellow and orange colors that we see in our trees during the autumn season. They are the same pigments that occur in many familiar orange and yellow vegetables such as carrots, pumpkins, and corn.
Carotenoids assist the tree’s energy production by harvesting additional light energy wavelengths not absorbed by the green chlorophyll pigments, making more light energy available to the tree. In the spring and summer, these pigments are masked by the abundance of chlorophyll, so they generally are not visible during the growing season. But in the fall, as chlorophyll pigments disappear, carotenoids are left behind, and these remaining yellow pigments are gradually unveiled, becoming more and more visible as the green pigments disappear.
Anthocyanins
Meanwhile, as light absorbing chlorophyll disappears, the leaves still remaining on the tree are now exposed to sunlight radiation. In response, to protect their leaves from harmful damage, many trees trap and convert the sugars still present in their leaves into another type of pigment which are known as anthocyanins.
These red and purple pigments are also found in red and purple fruits and vegetables such as blueberries, red cabbage, and eggplants. Anthocyanins act as a sunscreen protecting vulnerable leaves from the bright rays of the autumn sun, allowing the leaves to remain on the tree for a longer time as the tree continues to retrieve valuable nutrients for winter storage.
Tannins
Tannins are the last remaining brown pigments in leaves and also have an important function. Tannins have a bitter taste that can repel many leaf feeding insects and animals and even have antibiotic properties that ward off some disease-causing pathogens.
As other pigments degrade and remaining leaf cells die off, dying leaf tissues break down into essential nutrients – especially nitrogen and phosphorus – that are then contained in tannin pigments. As leaves finally brown and fall off the tree, these tannins then return valuable nutrients to the tree’s soil as the leaves are decomposed and recycled by soil microorganisms. This process of nutrient cycling is extremely important for maintaining soil fertility – and without brown tannin pigments, the fertility of soils would be lost during winter, depleting critical resources needed for spring growth the following season.
Does weather affect fall colors?
Climate and weather patterns definitely do have an impact on the intensity of fall color as well as the duration of fall leaf displays. In general, more vivid autumn colors are favored by an early fall period of dry warm sunny days followed by cool – but above freezing – nights, ideally between temperatures of 32 and 45 degrees. Extensive fall sunshine promotes the production of more sugars which during overnight chilly temperatures are converted into anthocyanin pigments responsible for the vivid red and purple hues.
Conversely, weather conditions can also have negative impacts on fall leaf color. Long periods of cloudy weather slow down sugar production which can mute fall foliage displays. Rainy fall weather can also inhibit vivid fall coloration by slowing down chlorophyll loss which can dampen the unveiling of other leaf pigments. Rainfall and wind can also cause earlier leaf drop, shortening autumn displays. And an early hard freeze can kill leaves prematurely which can severely impact fall coloration, if not forego it altogether.
Tree health impacts fall foliage displays
Tree health and growing conditions throughout the year can have positive - as well as negative impacts that can be showstoppers for fall foliage displays. Trees stressed by drought, disease, and poor soil conditions may lose leaves prematurely, leaving them bare in the autumn season. In fact, premature mid and late summer leaf discoloration is a typical sign of tree stress and warrants a visit and health evaluation by a qualified arborist.
Healthy trees, on the other hand, are more likely to have more vivid and longer-lasting fall displays. Soil health and proper water and nutrient availability are especially critical factors that contribute to a tree’s health and vitality. With less stress and more resources available, healthy trees can make more sugars and produce more pigments that result in gloriously vibrant fall colors.
Monster Tree Service offers a full range of tree and soil health care services that can keep your trees healthy and vibrant for the entire year, and, with a little help from Mother Nature, make them especially beautiful during the fall season. Give your trees the Tree Loving Care they deserve. Call your Monster Tree Service for a tree health service appointment today!