Kindled by lingering light and nurtured by waning warmth, September sends autumn into a crescendo and cascade of colour which fades away in November as murky mists move across the landscape and pave the way for winter. But the real action around leaf colour and leaf fall in autumn takes place during October, the middle month and height of the season, writes Dr Terry Mabbett.

October
On frosty morns with the woods aflame, down, down
The golden spoils fall thick from the chestnut crown ...

From ‘The Months’ (‘Basil and Edward’)
By Robert Bridges (1844–1930)

WHETHER Robert Bridges was talking about horse chestnut or sweet chestnut is unclear, although sweet chestnut is the classic woodland tree bearing leaves transformed into a glorious, golden brown. 

Characterisation of October as the month when the leaves all change colour and all fall down may be convenient but is far too simplistic. Each and every tree species takes its own particular autumn pathway on the journey from summer into winter. Whether we are talking about leaf colour or leaf fall, then no single species has the monopoly on how this proceeds.

Forestry Journal: Beech leaves take some beating for autumn colour in the ‘golden brown’ category.Beech leaves take some beating for autumn colour in the ‘golden brown’ category.

ALL CHANGE
With few exceptions like the copper beech tree and varieties of cherry plum, Japanese maple and Norway maple custom-bred for reddened foliage, the classic autumn-leaf colour change is from green and into a spectrum of colour including yellow, brown, orange and red. The end result is governed by the nature and range of pigment chemicals inside the leaves and is revealed when the overwhelming concentration of green chlorophyll pigment finally breaks down and fades away. 

Orange and red are the colours craved for by autumn watchers, but our native tree species are generally uncooperative in this respect, in contrast to many native North American tree species which light up the landscape with canopies of fire. The few British natives that can offer a glimpse of such colourful spectacles just happen to be some of the most retiring and reticent species and therefore may go unnoticed. Dogwood hidden in the hedgerow and wild service tree tucked away in dense, lowland woodland are two such examples.

Forestry Journal: Norway maples custom-bred for the amenity and landscape sectors throw up some interesting colour patterns in autumn.Norway maples custom-bred for the amenity and landscape sectors throw up some interesting colour patterns in autumn.

KALEIDOSCOPE OF COLOUR
The most ubiquitous and frequent native species prepared to give onlookers a taste of an orange and red autumn is the gean or wild cherry tree. However, the gean’s autumn leaves hang by the thinnest and most tenuous of threads, and are shed by the slightest breeze leaving any such spectacle sadly missed. Native wild pear is more than a colour match for the native wild cherry but you will now have to look far and wide to find this once common hedgerow tree.

Easier and more accessible viewings are offered by exotic North American tree species such as liquidamber, red oaks and sugar maples planted in the landscape and amenity sectors. But perhaps the most vivid autumn colour is courtesy of the red-leaved varieties of the Japanese maple tree. The purplish-red canopy seen in spring and summer is transformed into a stunning scarlet during the month of October.

READ MORE: Summer’s swansong and the aura of autumn

Sadly we are stuck with a motley crew of native and naturalised trees that go uniformly yellow (hazel, hawthorn, blackthorn, hornbeam, field maple, common lime, sycamore and English elm) or various shades of brown (English oak, common beech and sweet chestnut). Common ash and common elder bear leaflets that may grudgingly turn pale yellow before being shed, while common alder loses its leaves while they are still green. 

Autumn leaf and fruit colour is controlled by pigments called carotenes which emit shades of yellow, orange or red. Some trees possess another class of pigment called anthocyanins (and specifically cyanidin) which causes the claret-red or dark purple colour of elderberries, dogwood berries and sloes (fruits of the blackthorn), as well as contributing to the scarlet skins of hawthorn berries. That’s not to say that trees with yellow autumn leaves do not contain the orange carotene (beta-carotene) or the red carotene (lycopene) pigments or indeed the reddish purple anthocyanin pigment somewhere in the tree, and even in the leaves – but clearly at concentrations too low to make any large or lasting impact on autumn leaf colour. 

Take for instance the native field maple and sycamore trees. Sycamore is still generally regarded as an exotic species but with an increasingly vocal following claiming a true native status for the tree. The new young leaves of these maples exhibit a distinct red colouration as they unfurl and expand in spring, while the petioles remain reddish in colour throughout the summer months. Even the winged fruits (samaras) show flashes of orange and red during late summer and into autumn. But come October the once-green leaves become uniformly yellow and without any hint of orange or red in sight. The orange and red carotene pigments are clearly there in various parts of the tree but not in the leaf laminas or, if so, completely overwhelmed by the yellow-coloured carotene pigments called xanthophylls. A similar situation is seen with the blackthorn bush on which the current year’s shoot growth and leaf petioles (stalks) are claret-red throughout the summer months, although come October the leaves turn yellow before falling to the ground.

But autumn is not always a single-colour situation because leaves of some trees can exhibit a range of hues, apparently governed by biotype, aspect in relation to light regime and growing conditions including soil chemistry. For instance, the final autumn colour of hawthorn leaves is generally yellow, but the leaves on some shrubs in the hedgerow may display a kaleidoscope of colour with different shades of orange and even red. This is not surprising, since the obvious orange tinge of newly-emerged leaves in spring and the bright red hawthorn berries shows that these pigments are present and correct inside the cells and tissues of the hawthorn tree. 

Just occasionally, if prevailing autumnal conditions allow, the yellow leaves of blackthorn and common elder may show a hint of claret-red reminiscent of the French red wines of Burgundy and Bordeaux. Sloes (fruit of the blackthorn) are covered with a soft layer of wax which gives the appearance of a bright-blue coloured fruit. But gently rub off the wax to reveal the true blackish-blue-coloured fruit skin, and thus so because the covering of wax apparently absorbs a specific section of the light spectrum. 

Forestry Journal: October 2005, before horse chestnut leaf miner became well and truly established in southern England and ruined autumn for the white-flowering horse chestnut tree.October 2005, before horse chestnut leaf miner became well and truly established in southern England and ruined autumn for the white-flowering horse chestnut tree.

ONE YELLOW BRANCH DOESN’T MAKE AN AUTUMN
Just as a middle-aged man or woman may develop grey streaks in an otherwise colourful head of hair, so the mature lime tree develops the odd branch of totally yellow leaves in summer while the canopy is a uniform lively green. 

This was a striking feature of English elms but there are precious few trees left to authenticate the claim and decreasingly few people who can remember a mature English elm let alone specific autumnal changes. Texts written before Dutch elm disease arrived describe mature elms growing great patches of bright yellow between otherwise green foliage. They stood out in the sunshine and extended day by day until the very end when the feathery twigs of the English elm became completely visible. 

It was Anglo-Welsh poet Edward Thomas who spotted these peculiarities in the English elm tree before he was tragically killed at Arras (France) during the First World War:

The green elm with one great bough of gold
Lets leaves into the grass slip, one by one.

From ‘October’
By Edward Thomas (1878–1917)

Forestry Journal: English oak trees hang on to their leaves right up into December.English oak trees hang on to their leaves right up into December.

ALL FALL DOWN
Just as each and every deciduous tree species has its own particular pathway of autumnal leaf colour change, then leaf fall proceeds in the same vein – with individuality in timing, amount and velocity of leaf fall, and what happens to fallen leaves once they are on the ground, and certainly not complying with the concept of convenience of ‘all fall down’. 

Among the first leaves to fall in autumn are those of the common ash tree, although more accurately it is the individual ash leaflets which fall one by one from the rachis, to eventually leave a single leaflet attached to an otherwise naked rachis which too will eventually fall to the ground. That native common ash should be one the first to undergo leaf fall is perhaps a surprise since the species is one the last trees to re-foliate in spring. Indeed you could reasonably surmise that common ash, with a seemingly truncated growing and seed-ripening season (late May to mid-September, shorter than most other native trees), is singularly unsuited to its home environment.

However, this comparatively shortened season has never stopped the female common ash tree entering the autumn season without good extension growth and bountiful bunches of ash keys (fruits), irrespective of whether the tree is growing in the south of England or the north of Scotland. It took Chalara ash dieback disease to put a stop to all that. Leaf fall without fanfare is perhaps the best description for common ash. The leaves go grudgingly pale yellow through September and the leaflets start to fall by early October. 

Because they lack the tannin of other native and naturalised forest trees like English oak, common beech and sweet chestnut, the leaflets of the compound pinnate common ash leaf soon disintegrate. Once again it was Edward Thomas who spotted and wrote about the autumnal peculiarity, this time in common ash.

The leaflets out of the ash-tree shed
Are thinly spread
In the road, like little black fish, inlaid
As if they played

From ‘After Rain’
By Edward Thomas (1878–1917)

Forestry Journal: North American natives like red oaks generally provide a more scenic autumn than British native trees do.North American natives like red oaks generally provide a more scenic autumn than British native trees do.

WHAT'S UNDERFOOT?
English oaks as the woodland counterparts and contemporaries of common ash retain their leaf complement well into December. When the leaves finally fall after Christmas a crisp and crackly carpet several inches deep is formed across the forest floor, and subsequently preserved for posterity by the high tannin levels contained in the leaves. As the Cumbrian-born poet Norman Nicholson says:

The oak tree thrust its fist
Through the brown-paper wrapping of dry soil,
Letting light into the earth.

From ‘The Oak Tree’
By Norman Nicholson (1914–1987)

Beech is another prime example of a tree shedding a thick leaf litter that spells a death knell for grasses and most broad-leaved plants. Strong competition also comes from the beech tree’s shallow root system which efficiently and effectively exploits water and nutrients from the upper layers of soil, and a leaf display providing near perfect absolute shade. Even hardy plants like evergreen holly, brambles and bracken grow with difficulty under the beech tree. The ability of beech to hold its ground against virtually all other plants was noticed by early foresters and naturalists, including John Evelyn (1620–1706) who is regarded as the father of English forestry.
John Evelyn found the dry carpet of beech leaves, gathered before it became too frostbitten, made excellent stuffing for mattresses and far better than straw because apart from tenderness and loose packing they stayed sweet for eight years. In France beech-leaf mattresses were called ‘lits de parlement’ or ‘talking beds’ because of the noise they made when the occupant turned over.

Autumn leaves on individual trees tend to fall in sequence, with those higher up and on the outside of the canopy falling first, because it is these very leaves that are deprived first of water and nutrients form the roots as the canopies close down for winter. This effect is not altogether obvious in trees like oak and sycamore with irregular-shaped canopies but clear as a bell in others like Turkish hazel with its highly geometrical pyramidal canopy with leaves at the pinnacle of the pyramid falling first. The same phenomenon can be seen on varieties of Norway maple that have been bred for regular canopy shape (among other characters) and use in the landscape sector.

Forestry Journal: On frosty morns with the woods aflame, down, down; The golden spoils fall thick from the chestnut crown … (Robert Bridges)On frosty morns with the woods aflame, down, down; The golden spoils fall thick from the chestnut crown … (Robert Bridges)

PROS AND CONS OF PESTS AND DISEASES FOR AUTUMN LEAVES
Leaf fall is all about and safety and survival of deciduous trees through winter and into the following spring, but leaf fall in some tree species has now become a time of acute danger. Leaves of the white-flowering horse chestnut tree contain the pupal stage of the horse chestnut leaf miner moth which over the last two decades has destroyed the natural autumn canopy of this naturalised tree. White-flowering horse chestnut leaves used to age gracefully and fall during October, but are now reduced to dry, rolled-up masses resembling ‘brandy snaps’ as early as August. Leaf fall is an especially high-risk time for the rapidly declining and disappearing common ash because fruiting bodies of the fungal pathogen responsible for Chalara ash dieback are borne in the fallen leaf rachises, and primed to produce and release spores to infect previously healthy trees in the following years.

Foliar symptoms of tree diseases may sometimes add to autumn leaves. Like the tar spot fungus which decorates leaves of the English elm and sycamore with a striking contrast of black blotches on yellow autumn leaves in October. And the Guignardia aesculi fungal pathogen which punctuates leaves of white flowering horse chestnut with chestnut-brown blotches surrounded by yellow halos, beginning while leaves are still green in September and as they turn golden brown in October. Alas, the horse chestnut leaf miner has ruined all that by sending the foliar canopies of white flowering horse chestnut trees into an early autumn in the final days of August. 
The ultimate fate of fallen leaves depends on the presence or absence of natural preservative chemicals such as tannins. Leaves and leaflets of common ash, common lime and sycamore which appear to lack preservative chemicals decompose rapidly, while others like English oak, common beech, sweet chestnut and hornbeam remain intact on the forest floor right through winter and beyond. Perhaps the toughest and most resilient of all are the autumn leaves of the wild pear. Foolish are they who plant a pear tree in the lawn. Fallen pear leaves are scorned by earthworms and remain in perpetuity to kill the grass.