Analysis: it's all down to a bunch of chemicals, a process called senescence and the lifecycle of a tree

By John Casey, Teagasc

"A fallen leaf is nothing more than a summer's wave goodbye." Anon

Leaf senescence is when the normal green leaves of many deciduous trees and shrubs take on shades of yellow, orange, red, purple, and brown in the autumn. For plants to grow, they need sunlight, nutrients and water. The nutrients and water come from the soil. The sunlight is captured by the leaves. To capture the sunlight, the leaves use a chemical called chlorophyll, which is what makes leaves green.

Chlorophyll turns sunlight into food, which the trees need to grow, through a process called photosynthesis. During spring and summer, the chlorophyll's green color masks the colours of the other chemical pigments. In summer, a lot of photosynthesis takes place in the trees, helped by the increased amount of light and increased temperature in the air and the soil. The food they make is sugar, which they use to grow new leaves, flowers and seeds.

In full bloom: an Irish country scene on May 17th 2020

As autumn approaches, with daylight hours shortening and temperatures falling, a layer of special cells forms at the base of each leaf, known as the abscission layer. Water and mineral intake into the leaf is reduced. The amount of chlorophyll in the leaf begins to decrease as the chlorophyll is drawn back onto the main stem and the root system for storage.

As the plants break down the chlorophyll, the green colour disappears from their leaves. What is left behind is other chemicals are normally not seen. The most important of these are called Carotenoids - in fact, this chemical is what also makes carrots orange. 

Depending on which chemicals are found in the leaf, the Carotenoids, Xanthophylls, and Anthocyanins can turn different shades of orange, yellow and red respectively. These chemicals do not have any nutrients in them, so the plant does not bother to break them down, and instead retains them in the leaves.

The same scene five months later on October 14th 2020

When this happens, the hidden pigments of red, yellow and orange are revealed in all their magnificence. The depth of colour is influenced by the blend of the chemical processes and weather conditions. A combination of cold nights, dry weather and bright, sunny days makes for the most eye-catching autumnal displays.

Many of our conifer trees originally come from climates with very cold winters and have thick waxes and resins to protect their leaves from freezing and fracturing, much as antifreeze works to protect car engines in the winter. This partly explains why most conifers leaves (needles) do not change colour. However, deciduous species generally have thinner leaves that are susceptible to cold temperatures.

Since water expands when frozen, unshed deciduous leaf cells could burst during the winter, making them useless for photosynthesis the following spring.  In addition, for many deciduous trees shedding their leaves means that they are better able to tolerate winter storms as strong winds can move through the branches more easily.

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From Teagasc Forestry, John Casey on senescence and changing leave colours in autumn

As mentioned earlier, as the vessels that carry water to the leaf and sugars to the rest of the plant are closed off, the abscission cell layer starts to grow between the leaf stalk and the twig holding it. These cells slowly cut the leaf off from the tree twig without leaving an open wound. Instead, a callus or scar tissue is formed. Once the leaf is completely choked off, the layer becomes dry and flakey and, through decomposition, the leaf detaches and falls from the tree.

Both the leaves and the fruit of the tree fall to the ground, acting as a food resource for animals and insects over the winter and to provide seed for the next year. For example, the mighty acorn plays an important role in forest ecology and is an important part of the diet of pigeons, some ducks and woodpeckers (recently returned to Ireland!) as well as mice, squirrels and deer. A year in which a large number of acorns is produced is known as a "mast" year.

As the leaves decompose, their nutrients trickle into the soil and feed future generations of plant and animal life. Earthworms, mites, fungi and bacteria in the soil break down the leaves, grinding up the decaying material and mixing it with the underlying soil. Fungi break apart some of the more complex compounds, reducing them down into even smaller components.

Meanwhile, the tree sleeps during winter dormancy, storing sugars and amino acids in its roots, branches and trunk. Next next spring, fueled in part by the previous years’ leaves, the tree begins to grow anew.

John Casey is a forestry development officer with Teagasc, with more than 27 years’ experience of advisory, training and research in the forestry sector.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ