by Ian Monger
As summer draws to a close in the temperate regions of the world, few sights can be as uplifting as the explosive display of reds, oranges and yellows of autumn leaves.
Autumn generates a huge interest in trees by the public, as people suddenly notice the trees around them, and large numbers descend on arboreta around the country to witness the brightest colours. Arboriculturalists should encourage this interest, educate the public and be ready to advise on the best way to bring dramatic colour into people's gardens, our parks and other open spaces.
The best arboretum displays draw thousands from around the world. A few carefully chosen specimens can give equal interest even in the modest garden.
What causes leaves to change colour?
Leaves contain a complex mixture of pigments which give a species its characteristic colour. The presence of each is determined by the plant’s genes and the environment that the plant is in. Most of the pigments in the leaf are masked during the growing season by the green pigment Chlorophyll, so that in most cases the colour we usually see during the spring and summer months is some shade of green.
Chlorophyll is hugely important to all plants, as it is the main pigment responsible for capturing sunlight and using the resulting energy to manufacture food for the tree – simple sugars (carbohydrates) - which are produced from water and carbon dioxide. During this process, the chlorophylls are broken down and used up, but are also being continually replaced by the tree, so that the foliage stays green.
Deciduous trees and shrubs loose their leaves in autumn through a process called 'senescence'. In late summer, as days become shorter and nights are cooler, chemical changes take place that gradually close off the veins that have been carrying fluids into and out of the leaf. Special cork cells form at the base of each leaf. As this cork layer grows, water and mineral supply is reduced and chlorophyll production deceases. Eventually, the leaf becomes sealed off from the branch and the remaining green chlorophyll fades.
So, autumn colour is mainly due to the slow breakdown of chlorophyll which enables other, more vivid, pigments to become visible. Also, other pigments are increasingly produced as the chemistry in the leaf changes.
The main pigments that become more visible in autumn are:
These yellow pigments, which include Xanthophylls (Zan-tho-fils) and Carotenes, are common in many living things, including carrots, sweetcorn, buttercups, egg yolks, bananas and canaries. They are also found in Ash, Maples, Birch and Alder. Carotenoids are present throughout the year within the solid structures of the leaf cells, and are there to help Chlorophyll absorb light, but are masked by it during the spring and summer. Once revealed, this pigment gives countless hues of yellow and orange.
These pigments produce reds, purples & blues. They are common in fruits such as apples, strawberries and plums, but are displayed at their best in autumn by Maples, Sweetgum, Spindle, and the American Oaks. Anthocyanins occur in the sap of the leaves, rather than the cell membranes. These are the pigments which give edges of newly unfolding leaves a red tinge in the early spring. Otherwise, these pigments only develop in the leaf later in the season. Complex chemical changes in the tree in autumn lead to increased production of these pigments, which is dependent on the acidity of the sap, the concentration of sugar in the leaves, and levels of sunlight.
Tannin is the slowest pigment to break down. It is the pigment of waste products that collect in the leaves and gives the brown colour of dead leaves. Trees with a very high tannin concentration turn straight from green to brown in the autumn.
Why are autumn displays better in some years than in others?
Aside from genetic differences between, and within, species, the main factor determining the range and vividness of colours in the autumn is the weather.
Low temperatures destroy Chlorophyll and reveal the underlying yellow carotenoids but, if they stay above freezing, promote anthocyanin production. Anthocyanins and xanthophylls are formed from various sugars, and so their strong colours are related to an accumulation of sugars in sap. Dry conditions increase sugar concentration in sap, as do high light intensity, low temperature, drought or low nitrogen supply.
The best and brightest colours occur when late summer is dry and autumn follows with dry, clear and bright sunny days with cool nights (although frosts will kill the cells outright). On the flip side, continual overcast weather or strong shade will not give good colours.
In 2006, after a long dry summer, with drought in many areas, leaves started changing colour in the UK in July. But instead of vibrant reds and yellows the leaves were brown and dried. The natural response mechanism of the tree to water stress is to drop their leaves by letting them dry up and fall off, revealing the brown tannin pigment.
Are bright autumn leaves just a happy accident?
Studies have suggested many reasons why deciduous trees should shed their leaves in autumn: to conserve water and minerals, to excrete waste products, to avoid various leaf diseases and to minimise mechanical stresses during bad weather.
The evolutionary function of autumn colours is less clear. Although some autumn colours are revealed almost accidentally as the green fades, others are actively produced by the tree during this period. There are two current theories to help explain why a tree might benefit from autumn colours: photoprotection and coevolution.
This theory (Lee and Gould, 2002, Why leaves turn red. American Scientist 90:524-531) suggests that autumn colours help the tree to maximise the nutrients that can be reabsorbed from its leaves before they are dropped. The research has found that anthocyanins 'shield' the leaf from the harmful effects of light at low temperatures, which would otherwise obstruct the reabsorption of nutrients (especially nitrogen) from the leaves to the branches.
Archetti (2000, The origin of autumn colours by coevolution. J. Theor. Biol. 205: 625-630) believes that the colours provide a warning to insects (eg. aphids) that attach themselves or their eggs to the trees over the winter. The colours visibly signal the quantity of chemical defense against the insects. The branches with the brightest or most intense colour deter most insects. The insects benefit by finding the least defended trees on which to lay their eggs. The trees with the reddest leaves minimise the numbers of parasites they carry. The theory is based on the wider branch of evolutionary 'signaling theory' which says that the trees must be defending themselves in this way, otherwise they wouldn't be putting so much effort and expense into doing it. There is some evidence that aphids avoid trees with red leaves. (Hamilton and Brown, 2001, Autumn tree colours as a handicap signal. Proc. R. Soc. B 268:1489-1493)
What are the best trees for autumn colour?
There are countless species of hardy tree and shrub available with excellent autumn leaf colour, and it's all a matter of taste. Leaves are not the end of the story through. Many species display bright fruits and seeds. As ever, the ultimate height and spread of the tree or shrub should be a primary consideration when choosing plants for a site.
•Cercidiphyllum japonicum – Katsura – Pink then lemon and orange, with the famous burnt sugar smell from summer onwards.
•Liquidambar styraciflua – Sweet Gum – Leaves like Maple but alternate, not opposite. Lemon-yellows, reds and saturated purples.
•Nyssa sylvatica – Tupelo – Reds and golds
•Parrotia persica – Persian Ironwood – Yellows and red starting at the branch tips earlier than nearly any other.
•Amelanchier lamarckii – Snowy Mespil – Rich orange and red in autumn.
•various Prunus spp. – rich ambers and crimson pinks.
•Euonymus europaeus – Spindle – huge array of reds and purples in even the most modest site. Also spectacular orange and magenta berries on this native tree.
•P. sargentii - early scarlet colours.
•Most Rowans, but especially Sorbus ‘Joseph Rock’ - crimson and purple autumn foliage. Primrose-yellow fruits hang amongst the bright leaves.
As tree people, it's easy to forget about shrubs, or to consider them 'someone else's bag'. But many shrub species are essential to any autumn garden. The best examples are:
•Acer palmatum ‘Osakazuki’ - the most brilliant of the Japanese maples – the green leaves turn the most vivid, fiery scarlet.
•Berberis thunbergii – an invaluable little deciduous shrub which is unsurpassed in its autumn foliage brilliance.
•Cornus sanguinea – Dogwood – Although more exotic members of this huge genus may be more colourful in autumn, our only native shrub species displays burgundy leaves and black fruits, and is an important shrub in habitat creation.
•Cotinus coggygria – Smoke tree – medium-sized shrub with rich yellows, oranges and reds, and Cotinus obovatus – Chittamwood – large shrub or small tree which, under the best conditions is one of the most brilliantly coloured of autumn shrubs.
•Enkianthus campanulatus – autumn foliage in every shade from yellow to red, and almost unrivalled.
•Euonymus alatus – one of the finest and most reliable of all deciduous shrubs for autumn colour, with leaves showing crimson-pink, accompanied by red/purple fruits opening to orange seeds.
Hamamelis spp. - Witch Hazels with a fascinating winter flower display, and very attractive coloured autumn foliage from yellow to rich red, depending on the species.
•Viburnum opulus – Guelder Rose – A native large, vigorous shrub with stunning bright red leaves, and translucent red berried which last well into winter. Especially happy in boggy conditions.
© Text and photographs by Ian Monger September 2007