While you were fishing in the boreal forests the leaf bearing trees were working hard to keep you cool. To feed the shiny green leaves that make shade, trees use sunlight to convert water and carbon dioxide into sugar. This is called photosynthesis.

Now it's autumn, and those hardworking trees, on the other hand, need to take a break from all that photosynthesizing. When leaves change colour, from green to yellow, bright orange or red, you'll know trees are beginning their long winter's rest.

As the Earth makes its 365-day journey around the sun, some parts of the planet will get fewer hours of sunlight at certain times of the year. In those regions, the days become shorter and the nights grow longer. The temperature slowly drops. Autumn comes, and then winter.

Trees respond to the decreasing amount of sunlight by producing less and less chlorophyll. Eventually, a tree stops producing chlorophyll. When that happens, the colours already in the leaves can finally show through. The leaves become a bright rainbow of glowing yellows, sparkling oranges and warm browns.

The amount of rain in a year also affects autumn leaf colour. A severe drought can delay the arrival of fall colours by a few weeks. A warm, wet period during fall will lower the intensity, or brightness, of autumn colours. A severe frost will kill the leaves, turning them brown and causing them to drop early. The best autumn colours come when there's been:

    * a warm, wet spring

    * a summer that's not too hot or dry, and

    * a fall with plenty of warm sunny days and cool nights.