WE have arrived at the time of year when "this nipping air", as Wordsworth puts it, "a fore taste yields of bitter change - and bids the flowers adieu." But Longfellow looked more on the brighter side:
There is a beautiful spirit breathing now,
Its mellow richness on the clustered trees,
And from a beaker full of richest dye
It pours new glory on the autumn woods.
The dye in question, as we know, comes in various shades of yellow, brown and red. The precise trigger for this yearly colour change varies with the different species, being a combination of declining day length and falling temperature, and influenced by individual genes and the prevailing climate.
In Europe, for example, the horse chestnut, Aesculus hippocastanum, is among the first to turn yellow as early as mid September, while oaks and alders may remain green for another month or more. A late spring or a summer drought, by slowing growth, may delay the onset of the autumnal change for several weeks, while a moist, mild spring, combined with vigorous growth throughout the summer months can often promote an early tendency for falling leaves.
The green summer foliage owes its colour to a substance known as chlorophyll, Its most important role is as a vital catalyst for photosynthesis, the process whereby hydrogen and carbon, obtained respectively from water and from CO2 are converted by the energy obtained from sunlight into the tissues that provide for growth. But the abundant chlorophyll act's also as a pigment, and the resultant green dominates all the other colours that vegetation might otherwise display.
As the temperature begins to fall with the approach of winter, however, deciduous trees act on the reasonable assumption that growth for several months is likely to be much curtailed. They conserve resources by closing down the factory: the chlorophyll disintegrates with great rapidity, its green predominates no more, and other pigments - the oranges and reds and browns - are left for a brief but glorious period to reign supreme until the weakened leaves are swept away by autumn's winds.
The colours of high autumn have been celebrated by poets and songwriters almost since time began, but in parts of the world nowadays they are also a major tourist attraction. In the New England states of the US, for example, a "fall foliage hot line", gives day to day details of the most spectacular displays, and the whole event generates over a billion dollars in extra tourism for the region every year, most of it in a short period of a little over three weeks.