1. Trees and Forests – Falling Leaves
Science 6

2. Why do leaves fall?
Historically, it was thought that the drop in temperature during autumn caused leaves to wither and fall.
Scientists discovered that the falling of leaves was related to light.
The shorter periods of daylight affect a special layer of cells at the base of each leaf where it joins the twig from which it grows.
In the fall, as the days grow shorter, this cell layer weakens and the leaf turns brown and drops off the tree.

3. Why do leaves turn colour in the fall?
Leaf colour depends on the kinds of pigment a tree has inside its leaves.
In fall, cool temperatures and shorter days cause some pigments to break down and new ones to form.

4. Why do leaves turn colour in the fall?
Leaves look green all summer because of chlorophyll.
Chlorophyll helps the tree make food during the growing season.
In late summer, when nights get cooler, the chlorophyll in each leaf breaks down and disappears.
Soon the leaf is no longer green.

5. Why do leaves turn colour in the fall?
Yellow and orange pigments are in leaves all year, but you can’t see them in summer because they are covered up by all the green chlorophyll.
Once the chlorophyll breaks down in late summer, the yellow and orange show up. (These pigments are the same ones that make carrots orange and egg yolks yellow)

6. Why do leaves turn colour in the fall?
The bright red and purple colours of fall show up only in leaves of certain trees.
Unlike the green, yellow, and orange pigments, these bright colours are made by trees only in the fall. That’s because they need cool temperatures at night and lots of sugar to form.
Red maples for example, make a lot of sugar and have many of these red and purple pigments.

7. Why do leaves turn colour in the fall?
There are certain trees, such as oaks, that don’t change to bright colours in the fall.
Their leaves look brown and drab.
These trees have a special chemical inside their leaves called tannin.
The tannin mixes with the yellow and orange and makes the leaves turn brown.

8. What type of weather will cause the brightest fall leaves?
A spectacular show of fall colours depends on the right amount of sunshine, rain, and cool temperatures.
Too much rain causes colours to fade.
Bright, sunny fall days with cool nights are the best.

9. What type of weather will cause the brightest fall leaves?
Sometimes one half of a tree will be a different colour from the other half. That’s because one side gets more sunlight, which causes brighter colours.
Temperature is important too. Slowly cooling temperatures that don’t dip below freezing cause the brightest colours.