Although now might be the best time of year for tree planting, they may not be looking at their best, a sure sign that winter is most definitely coming!
With this seasonal transition comes the beauty of leaf colour across the northern hemisphere. Footpaths become littered with leaves, pumpkin-spice seems to make its way into everything, and that autumn/winter wardrobe is dusted off again.
Although it may be getting darker, it’s not all doom and gloom – in fact, a recent Forestry Commission survey found that an astonishing 96% of people said that beautiful autumn colours improve their mood!
So to celebrate their wonderful colours this National Tree Week, let’s take a look at some of the science behind leaf colour, and why autumn’s kaleidoscope continues to amaze us, year upon year.
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Why are leaves green?
Before we look at why leaf colour changes, we must first appreciate why leaves are green! Plants make food via a process called photosynthesis, using sunlight to synthesise nutrients from carbon dioxide and water. Sugars made by photosynthesis are then transported around the plant and stored in leaf tissue.
As well as generating oxygen as a by-product, this fascinating process involves the leaf pigment chlorophyll – a chemical compound contained within stacked structures known as chloroplasts. These absorb both blue and red light. What we see is the remaining light, primarily green in colour. Without green leaves and photosynthesis, life on Earth would simply not exist today.
Green leaves enable photosynthesis, the energy from which is transported to the rest of the plant. Jess Wood (CC BY 2.0)
So why do leaves go yellow?
Changes in daylight lengths and lower temperatures are a signal for leaves to stop their process of photosynthesis. After all, dark and cold aren’t the best of working environments!
As a result, chlorophyll breaks down, and so the green pigment gradually disappears. This is an important process for redistributing nutrients from leaves to the main body of the plant in preparation for winter.
Carotenoids in the leaves of this Quercus rubra are responsible for its fiery orange colours. © Lara Nouri, 2019
The reason leaves appear yellow is due to a chemical called carotene – the main pigment in carrots – which is also found in leaves during the normal growing season. However, carotene isn't visible until chlorophyll production slows. As the green fades, the yellows of carotene shine through and affect our perceived colouration.
But, what about red leaves then?
As you can imagine, a whole range of other chemical changes also occur in the leaf over autumn. Some of these changes are associated with anthocyanins and flavonoids, a large group of chemical compounds responsible for the colours of our blackcurrants, blueberries, and red cabbage.
This Acer sp. puts on a show while the Quercus robur (background) is still in its green colours. Antti-Jussi Kovalainen (CC BY-ND 2.0)
Unlike the process behind yellow autumnal leaves, most red leaves result from the synthesis of new anthocyanin pigments. This is dependent on light levels, cool temperatures, and mild drought conditions.
The variability in conditions results in a range of reddish and purplish colours being produced. Though the process is slightly more complicated, it is thought that anthocyanins protect leaves from excess sunlight damage.
Going for gold this autumn
Across the UK, our native horse chestnuts are becoming more golden, with ash and sycamore not too far behind. Within our app, we’ve got a host of collections to inspire your autumnal feels. Have a look at our 'Top of the Crops Autumnal Plants' under the knowledge tab for some new ideas.
Some of my own personal favourites are the lobed leaves of Sweet Gum (Liquidambar styraciflua). They show a stunning range of brilliant oranges, reds, and purples through autumn. A number of oaks, such as Red Oak (Quercus rubra), are also firm favourites.
If it’s something smaller you’re after, Vaccinium parvifolium may well be your next hardy plant purchase. You’ll find all of these glorious species in our ever-expanding Knowledge Base!
A true autumnal spectacle at Groton State Forest, New England, USA. Ania Tuzel Photography (CC BY-NC-ND 2.0)
One of the most impressive places to go 'leaf-peeping' (yes, there’s a term for that!) is the colourful journey from Manhattan Island to Boston in New England, USA.
However, there’s plenty that our own British landscape has to offer. Whether it’s a copse of large beech trees or big veteran oaks, we’d love to hear how autumn is unfolding where you are!