Autumn (or fall if you happen to be American) arrives in September in the northern hemisphere and in March in the southern hemisphere.
No matter where you are, you can expect to see leaves falling off deciduous trees. In areas home to certain species you can expect to see those leaves go through a gamut of colours, making our autumnal landscape so resplendent.
But why do leaves fall and what is the reason we can see the purples, reds, oranges, yellows, and browns, as trees prepare for the oncoming winter months?
As Autumn arrives in the northern hemisphere, in the colder countries, we can already see the leaves turning. Let’s ponder the questions about this changing of colour and eventual leaf fall, as we revel in the beauty of this season.
Why do leaves change colour?
The transformation from the verdant green of spring and summer to the reds, yellows and oranges of winter is due to chemical processes and environmental factors. The green leaves serve as food factories for trees and plants during spring and summer, manufacturing food through photosynthesis. The cells in leaves contain chlorophyll. A chemical that gives them their green colour. This chemical absorbs light from the Sun, using its energy to transform carbon dioxide and water into sugars and starches. These sugars and starches are food for plants and trees.
Here is the interesting bit. Leaves also have other pigments like carotenes (that give carrots their yellow/ orange colour) and xanthophyll, among others. However, for most of the year these colours are overshadowed by the large amounts of the green of chlorophyl.
How do leaves go from green to other colours?
As autumn arrives, the trees and plants sense the change in the length of daylight and in temperature. Thus begins the process where leaves stop making food via photosynthesis. Because the process is on temporary hold, chlorophyl breaks down and with it goes the leaves’ green colour. The oranges, yellows, and browns start making an appearance. At this time, other chemical processes also take place, which together with certain weather conditions, may result in the appearance of additional colours through the development of what are known as ‘red anthocyanin pigments’. These pigments bring about the reddish and purplish hues of certain trees, such as dogwoods and sumacs. Other mixtures give the sugar maples their orange colours, and oaks their browns. The reason for the difference in colours is the different amount of left-over chlorophyll, mixed in with the other pigments.
How weather effects leaf colour?
Duration of light, changes in temperature and water availability all affect the degree and duration of leaf colour in autumn / fall. When temperature is low but above freezing, more anthocyanin is formed producing the purples and bright reds. Dry weather ensures that sugars are concentrated in the leaves, which produces more anthocyanin, making leaves even redder. The appearance of early frost weakens the brilliance of the red colour, while rain or overcast days intensify the vibrancy of autumn hues. The best time of course is a clear, dry, and cool day. Then some amount of photosynthesis may still be going on. This increases sugar concentration and with it anthocyanin production, giving us bright red trees.
Why do leaves fall?
With the appearance of fall colours, other changes are also occurring within trees and plants.
The start of leaves dropping off is known as abscission. A layer of cells is formed between the end of the leaf stalk and the stem (where the leaf stalk joins the stem). This layer of cells is known as abscission layer and it forms during spring, when there is active new leaf growth. But in autumn, just like a human teenager, the leaf hormones start going through changes. One of the most prominent of these hormones is auxin. In spring, during active growth, auxin is produced at the same rate as growth in the rest of the tree or plant. So, if the auxin production rate and the rate of active growth of the tree are stable, the abscission layer remains connected, and the leaf remains attached to the stem. However, as days start to shorten in autumn and temperatures start to fall, auxin production in the leaves drops off, triggering cellular elongation inside the abscission layer. This elongation creates fractures, forcing the leaf to fall away from the tree or plant, leaving behind a scar. Sometimes, this process is helped along by wind or a strong breeze.
What are the benefits of leaf fall?
Falling leaves help trees and plants to not only retain moisture within their branches and trunks but also makes the plant or tree dormant, thus requiring less energy to survive during winter. Without leaves, trees are also able to withstand winter storms, allowing extremely strong winds to pass through without barriers.
Some years we may not see full leaf colour because the leaves may have fallen off before the tree reaches its full colouration, perhaps due to an early storm or strong winds.
Sometimes even the same species of tree can do different things and so, one tree of the species may be bare, while another one is full of colour.
Most northern climate broad-leaved trees shed their leaves in autumn or fall. Some however, like oaks and others, may retain their brown leaves until the beginning of the next growing period in spring. Southern trees have to deal with milder winters and there some broad-leaved trees are evergreen.
Conifers, such as pines, spruces, firs, hemlocks, and cedars (among others) are evergreen in the north and the south.
What is false autumn?
You may have noticed that in the summer of 2022, a drought occured in the UK. The weather was hot and dry and as early as July we could see leaves of certain trees, like horse chestnuts, turning brown and even falling off. Due to the unexpectedly hot weather and drought, the trees experienced stress and turned brown or shed their leaves to retain moisture within trunks and stems. This is what is known as false autumn. It is a survival mechanism where trees deal with the stress of unexpected weather events, such as drought, by changing leaf colour or even shedding leaves altogether. With climate change we can expect to see more and more of such events.
And now you know what happens to leaves in the autumn. Remember it when you go out to and see all the fall colours, wherever you are.