Why do trees lose their leaves in Autumn?

Winter Trees

BY William Carlos Williams

All the complicated details
of the attiring and
the disattiring are completed!
A liquid moon
moves gently among
the long branches.
Thus having prepared their buds
against a sure winter
the wise trees
stand sleeping in the cold.

Like some animals, deciduous trees go into a sort of hibernation state in colder weather. Producing and maintaining leaves is an expensive business, requiring a good deal of sunshine and moisture. Leaves contain nutrients that a tree needs to survive. These nutrients are derived through photosynthesis, so the tree can't get them from the soil. 

Trees know their environment well and pre-empt the upcoming winter (or, interestingly, the dry season in warmer climates), absorbing the nutrients from their leaves as the weather starts to get cooler or dryer, but before the leaves can be damaged by the unfavourable weather.

The main nutrient the tree sucks up is Chlorophyll. This is the nutrient that gives a leaf its green colour. Once it's gone, the other nutrients in the leaf become visible, such as caroteins, which are orange and yellow, and phycoerythrins, which are red. Filled up on Chlorophyll, the tree detaches its leaves and goes into a winter slumber. Thus, the golden and amber leaf drop of Autumn!

In a New Zealand winter, there is a lot of rain and the temperature barely drops below zero, unless you're up a mountain. This might explain why our native trees are mostly evergreens. They get enough rain and light to keep their leaves through the cold weather. 

In other climates that have very cold winters, such as North America, some trees have adapted to become evergreen so they can survive through a longer winter. They do this with needle-shaped leaves, which retain water that on a broader leaf would be lost to wind and evaporation from the sun. These trees stay evergreen so they have more chance to carry out photosynthesis all year round, and thus survive.

Clever old trees, so different from us humans. I tend to lose weight in the summer, when it feels too hot for stodgy food, and pack it on when the weather is cold. This makes sense too, since I don't hibernate and have to carry on through the wind and rain. A little extra weight to help me through the colder months? It's only natural. Yeah, I like that excuse. Pass the Tim Tams..

What's my Why?

It's been a busy week, too busy to wonder why, so I thought I would leave this one up to the readers. Can you suggest a Why for my next post in the comments? I need to take a weekend off to work on another project, so the next post will be in two weeks time. Gives you plenty of time for wondering...