Autumn leaves

One thing I like about working near Bloomsbury is that there are a lot of nice parks nearby where, once inside, its possible to surround yourself by trees and forget the urban chaos of London.

I didn't realise Central London had so many trees until the middle of summer, when their weighty green boughs hang over the road, almost obscuring the buildings behind them.

In summer, Russell Square became a thick green forest, a popular relaxing spot for students studying nearby or schoolkids on excursions.

Come autumn, my fascination has switched from admiring the immense greenness, to watching the leaves turn all sorts of colours before gently being picked up by the wind and fluttering to the ground.

Each time I walk through Russell Square, something is different. One day a tree will have turned a light amber, while the one behind it is a powerful shade of purple. Another day, a section of trees will be completely bare, while another group will have just started turning brown.

Why do some trees decay faster than others? And why on earth are British kids wearing hi-vis vests when they go on school excusions - is that happening in other countries too?

Naturally, I turned to the internet for answers. Wisegeek says trees turn different reds, ambers and browns because they already possess these colours, but in summer the chorophyll in their leaves mask the colours.

But this doesn't explain why some trees lose their leaves faster than others, especially in the same spot. Just Gardening says trees with a northern exposure often change colour and lose their leaves faster than other trees with a different orientation, because they are losing sunlight hours faster. Trees facing south get more exposure to the weakening sun.

And it turns out temperatures don't really play a role in the rate at which trees lose their leaves. Sunlight exposure is more important, so trees will start to lose their leaves when the duration of daylight shortens, even if temperatures have not fallen sharply.

And as for the hi-vis vests? Well plenty of internet sites sell them for as little as £1.99, claiming they are a must for taking a group of rugrats around in the dark. As the days shorten and the natural light dims, I can see why they're becoming popular!