Autumn in Technicolour: the Science Behind Changing Leaves

There is truly nothing more indicative of Autumn’s arrival than the crunch of dry leaves beneath our feet. Everyone at some point in their lives, whether that be at five years old or fifty, has enjoyed moments of pure joy and satisfaction when jumping on piles of fallen leaves at the side of the road; if you haven’t ever done this, you’re missing out! But before those leaves give in to gravity and become part of the woodland carpet, they undergo a terrific transformation that results in a multi-coloured spectacle only seen for a short period of time each year.

Autumn this year has given us a particularly vibrant demonstration of the leaves turning. The deciduous trees have been showing such a variety of warm colours that all seem to be especially vivid, and the absence of strong winds this season has kept the branches full, allowing us all to enjoy the sights for longer. The question is: why has there been such a spectacular display? As you might expect, the weather plays an integral part in the answer.

Firstly, we need to understand how and why leaves change colour. In the colder months, it becomes more difficult for trees to access water because the ground hardens, and so they must prepare themselves for a form of hibernation. They do this by redirecting nutrients from the leaves into the rest of the tree, to build up their carbohydrate reserves which will last them until the spring. At the point where the stem of the leaf joins the branch, a layer of cells called an abscission layer builds up, preventing any more sugars to be passed on to the leaves. As the air temperature begins to decrease, the chlorophyll in the leaves starts to break down, leaving behind another chemical pigment called carotene which has a orange/yellow appearance and is unaffected by the cold (you might need to recall the process of photosynthesis from your school science classes!)

Autumn OakSo the leaves initially take on a yellowy hue in the early stages of autumn. If there are an abundance of sunny days during the season, another pigment called anthocyanin will be produced, which is a natural sun protection. You may already be familiar with this pigment if you enjoy blackberry picking; it’s responsible for that purplish stain left behind on your fingers! The remaining sugars in the leaves grow more concentrated and eventually convert into anthocyanin, producing the deep red colours we see later in the season. As the nutrients run out and the abscission layer grows, weakening the joint, the leaves will fall away.

That explains the process-in a nutshell-but why are we seeing such vibrant colours now? The reasonably mild weather we’ve been having has a lot to do with it. The brightness of the leaves is decided by a number of factors, including the amount of rain that falls during the summer and the nighttime temperatures in early autumn. Plenty of rainfall in summer means good growing conditions for trees with little risk of drought, and if this is succeeded by a dry autumn with sun during the day so that more anthocyanin production is encouraged, and cool but not freezing temperatures at night, the colours will be much brighter than if heavy rain and frosty nights were had.

Provisional rainfall percent of average map for Summer 2015 (June, July and August)

According to the Met Office, this summer was wetter than the past two, and levels of rainfall were higher than average in many parts of the country. Nighttime temperatures during September and October have been lower than average, at 8.3°C and 7.7°C respectively, but have remained above freezing. We have also seen quite a number of sunny days over the past couple of months; in years when we experience a more typically British autumn with thick, overcast cloud, our changing leaves are more likely to be dull and muddy. All of these elements have been instrumental in the creation of the fantastic chromatic display we’ve had.

Many of the leaves have already fallen now but there are still lots of trees out there sporting their colours with pride, so pull on your wellies and get out there to make the most of your last opportunity to enjoy the show this year.

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Emily Summers
I am a Zoology student, wildlife volunteer, blogger, artist and aspiring conservationist. You can read about my experiences with nature on my blog The Art of Nature

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