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Introduction to leaf fall


Other October pages: Tree by tree - the autumn sequence Berries, fruit, nuts and shrubs Flowers Birds Deer rut and insects • Weather

Put your cursor over any photo to see its caption, or click here to see more Autumn colours photos.

One remembers October as a time of intense autumn colours, with all the trees yellow and gold. But in fact it is generally in early to mid November that the colours are at their best. This happens when leaf tinting has got down to the lower levels in woodland. This is when you get the best photographs.

At the end of October, by contrast, tinting often remains patchy. Some species may be showing quite a lot of colour, some may just be a greeny-yellow, and some may be mottled with colour (one leaf green, another tinted). Isolated trees or foliage on the outer edges or in the top layers of woods may also be quite coloured, but in the heart of the wood foliage still looks disappointingly green.

The slow shedding of leaves

That is not to say that leaf fall is not occurring throughout October. Look on the ground even early in the month and you will see freshly fallen leaves even under trees and shrubs that are not showing much tint. This is because many have been quietly losing foliage since September - even August or late July in some cases. Leaves may fall while still green or you may see a few tinted leaves on the tree. It is easy to assume that a more general colouring is going to follow but often this is not the case: the tinted leaves fall and the ones that remain stay green. Contrary to expectations, small patches of tinted leaves in September or early October are usually not a signal that autumn will be particularly early.

The causes of vibrant colour

Several factors decide when tinting starts in earnest, but a key one is certainly two or three cold nights  – down to three or four degrees centigrade, say. There is a delay of maybe ten days before the trees visibly react to this, however. How many species or trees respond depends on how early or late in the season the cold shock is. (See Some recent years below).

The cold nights destroy chlorophyll, the green pigments in the leaf. In many species this simply reveals the underlying colours of the leaf material - the yellow and orange carotenes. These colours will be brighter if the weather is dry. In some species (eg ash or elder) the best the leaf manages may be a greens-yellow and some species (eg alder) never tint at all in this part of the world.

As autumn progresses a layer of corky material also forms across the base of the leaf stalk, preparing it to snap off. Even after this layer has formed, sugars may continue to be produced in the leaf, however, and trapped by the corky layer they become concentrated, creating red or pink pigments - anthocyanins. Some species are more prone to this than others and again the process is helped by dry weather - even more so by sunshine, which dries the leaves and helps concentrate the sugars. (New England has all these factors, which is why it produces better autumn colours than we do). However if temperatures fall below freezing at night, anthocyanin production is stopped.

Dry summers can also help all of these processes, but they sometimes weaken the trees and increase the amount of leaves that shed early in the autumn before they have time to turn colour (as seems to have happened in 2014). Mild or wet weather produces muddier colours and delays leaf fall.

Certain species produce better colours. The golden leaves of beech are a famous example, and so are the glorious red-orange tints of wild cherry and (sometimes) hornbeam. Maples – particularly the exotic varieties in parks, but also our native field maple - are also famous for their yellow-golden tints. Sycamore (which is also a maple) is often disappointingly subdued, though can produce wonderful bright yellows, and the same is true for lime, poplar, ash and hazel. Hawthorn and blackthorn bushes can produce a wonderful kaleidoscope of colours. Rusty browns (usually on oaks or beech) are produced if the leaves have died completely but remain on the tree. (For more details see Tree by tree - the autumn sequence).

Trees and bushes both tend to thin from the top down. At ground level, they might seem to have lots of foliage, but seen from a distance the tops look bare and wintry. It is often after seeing a wood from above that you realise that leaf fall is really getting into gear. Leaves on the exposed edges of a wood - or on lone trees - also have a tendency to turn before those in the centre because they are more exposed to the drying effect of the sun.

Different trees tint in different ways. Many – hawthorn, birch, lime, field maple – become mottled with tinted leaves scattered among green ones. Others – ornamental maples, beeches – seem to turn in big patches (a patch of tint among the green). Hornbeam turns from the top or one side, so it looks frosted with colour, something you can also see on beeches and maples. Hazel leaves are unusual in that they can tint from the outside in – a rim of yellow that spreads towards the centre. Both field maple and sycamore leaves can be a pretty mixture of yellow and green areas.

The end of leaf fall

Whenever the big turn comes, the transformation can be very abrupt. Within four days from 27 to 31 October 2007 the entire treescape seemed to turn from green to yellow and gold, and the same happened to a lesser extent on the same dates in 2010, and a few days earlier in 2011. In 2014 the remaining oak foliage went from largely green to brown in three or four days from 14-17 November and it happened in the week leading up to 15 November 2016.

Once leaves are turned, the leaves can also fall very quickly, as in 2006 when many trees were quite bare just a week after spectacular leaf colour on 15-16 November. But this is not always the case. In 2010 and 2011 despite mass tinting at the end of October, colour on some trees - oaks, birch, field maple, hazel and sycamore - lasted right up to the end of the November, while in 2015, despite most trees being bare by the end of the first week of November, hazel and oak lasted until the first week of December along with a few dribs and drabs on field maple, sycamore, weeping willow, lime, larch and alder.

Windy days obviously help to strip leaves from trees, but are not as decisive as you might think - even if the leaves are tinted, they will only be blown off if the tree is ready to let them go. In 2009 after an early start to tinting, November was characterised by very strong winds, but quite a lot of foliage still remained by the third week of that month. In 2010 a windy spell mid November did not stop oaks and other trees holding onto their colour till the end of the month, and the same was true of a very windy November 2015.

Some recent years

Leaf tinting increasing ten days after cold nights is a rule that seems to work most years:

In 2006 trees remained largely green well into November, before suddenly turning on the 14th. This was ten days after a couple of surprisingly sharp frosty nights on 4 and 5 November.

In 2007 temperatures fell abruptly to as low as 4 degrees overnight on 17 October and the following nights, and generalised leaf colour followed about ten days later.

In 2008 cold nights at the start of October 2008 caused some trees to turn entirely in the second week - including lime, poplars, sweet chestnuts and ash (with the latter going a fabulous shade of yellow). But other trees were unaffected and once the leaves on the tinted trees had fallen there was a two week hiatus in which the treescape remained largely green. Freezing temperatures (and some snow) on 28 October then caused a mass turning of leaves in the first week of November.

In 2009 some trees were caught out by cold nights around 12 and 17 October, but others remained untouched and leaf fall did not finally finish till the last week of November.

In 2010 there were cold nights from the 16th to the 21st, followed by a sudden rush of colour at the end of October. But colour on oaks, birch, field maple, hazel and sycamore then lasted until the end of November.

In 2011 cold nights from 15 to 20 October punctuated an otherwise remarkably mild month, with daytime temperatures in the high teens. The treescape went from nearly green to widely tinted within a few days around 5 November, but colour on oaks, birch, field maple, hazel and sycamore then lasted until the end of November.

In 2012 regular bursts of cold nights - on 6-7, 14-15 and 26-30 October, plus 6 and 11-12 November - produced widespread tint by the end of October and full tint by 13 November. Leaf fall was over by the 24th.

2015 was a very mild autumn, with temperatures in the mid teens throughout September and October, but nevertheless sunshine and cold nights in the last five days of September and the first four of October produced some partial turning of birch, lime, Norway maple, hornbeam, birch, ash and hawthorn in the first half of the month. The treescape remained largely green, however. More cold nights (but still mild days) followed on 8-10 and 14 October, and then in the last week of the month there was an abrupt turning, with most trees tinting and going bare by the end of the first week of November. The exceptions were hazel and oak, which were largely unaffected, and retained foliage until the first week of December.

In 2016 there were three nights down to 4-5 degrees from 9 to 11 October, and moderate tinting in the second half of the month, including some beeches, but mainly at hedgerow and understorey level. But the first two weeks of November then saw cold nights which produced the best autumn colour in the third week, with trees largely bare by the 25th.

2013 and 2014 saw exceptions to the cold nights rule, however. In 2013 a mild first half to October, with night time temperatures often in double figures (albeit with one dip down to 6 degrees on the 6th), nevertheless saw a sudden uptick in tinting around the 16th, with not just early-turning trees such as maple, hornbeam and lime affected, but also some beech, ash, field maple and oak. In 2014, another mild October, many trees - including beech and oak - had thinned out considerably by the third week, despite no obvious weather trigger.

It is possible that in both years a dry summer weakening the trees was a factor in this early start to leaf fall, but the remaining foliage then remained largely green. It was not till 19 November in 2013 and 14 November in 2014 that the remaining leaves turned en masse, in both cases ten days after particularly cold nights of just 2 or 3 degrees (from the 8th to the 10th in 2013 and from the 4th to the 6th in 2014). In both years hazel, oak and larch went on to keep some leaves into the first week of December, with birch also doing so in 2013, and field maple, lime, hornbeam and sycamore in 2014.

More October pages:


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