Autumnal leaf fall controlled by trees themselves
Published 17/09/2016 | 00:00
Autumn is a time of harvest, of tumbling temperatures and of falling leaves. Strong winds can and do, of course, tear leaves from trees at any time during the spring and summer but the annual leaf fall that is now beginning is controlled by the trees themselves.
Plants differ radically from animals in that most of them make their own food. In general, animals have to acquire their food ready-made.
Leaves are the plants' food-making factories. Via the process of photosynthesis, leaves use sunlight to power the conversion of carbon dioxide gas from the air and water from the soil into sugars, starches and other life-sustaining chemicals.
Photosynthesis works great during the long, calm, sunny days of summer. Things are now taking a turn for the worst; days are getting shorter so there are fewer hours of bright light for making food. Sunlight grows weaker, days shorten, temperatures drop and the likelihood of gales increases.
As a consequence of autumn closing in, many trees actively cease producing food. The green pigment that made all the leaves green during the summer is broken down and recycled. Anything of use is moved into the bark and rubbish is dumped in the leaf. As the green colour is broken down and as waste products are deposited in the leaves autumnal yellows and browns come to the fore.
Since the leaf is no longer of any use to the tree, the tree cuts off its food and water supply. Starved of nourishment, the leaf dies, killed by the tree. A layer of cork severs the link between tree and leaf so that when the leaf falls it will not leave an open wound that could be an entry point for infection.
Triggered by decreasing day length and perfected by hundreds of millions of years of evolution, the trees shed their dead leaves in the great autumn colourful happening that is 'the fall'.
There is evidence that some trees have evolved a pre-programmed life expectancy for their leaves. The downside of this is that these trees cannot capitalise on spells of unusually fine autumn weather. Furthermore, global warming is causing trees to bud earlier in spring and those with a set life expectancy are shedding their leaves unnaturally early in autumn.
Weather plays a part in the autumnal display. If the weather is settled, calm and bright, dead leaves hang limply on the trees displaying their colours. A change brings them tumbling down and draws a curtain on the display.