Hello! Julie here!
In this episode we're exploring trees: Why their leaves fall off, why they change colour and how they move all their water and food up-and-down between the roots and leaves.
It all started with a question from 5-year old Nova, who, last Autumn asked us: why don’t all leaves fall off the trees at the same time?
Let’s start with why they fall off in the first place. It’s because the trees don’t need their leaves in the winter. Trees have a way of telling when winter is coming; they can feel when the weather is getting colder and wetter. So, they start preparing. A bit like some animals getting ready for hibernation.
Nick Rose, an arborist and our expert for this episode, says that the leaves don’t all fall off at the same time because it takes trees some time to do all this prep work for winter. Depending on their local environments, different trees experience their preparation in different ways. Some trees will be faster at sucking their nutrients out of their leaves. Others will be able to do this quickly.
The trees suck all their nutrients from the leaves and store them throughout the tree for winter. This is what turns the leaves brown (and other colours), and once they’re "empty" of nutrients, they can fall off. In this way, when the winter comes the trees have enough food to survive it, plus they’ve also got some ready for the growing season the following spring.
The tubes that all these nutrients flow through from leaves to roots (and back up again) are called the phloem. And water flows up-and-down through the tree in xylem tubes. Both of these tubes exist in a part of the tree called the cambium.
But how does the water flow up the tree, from roots to the leaves? This is due to a pressure difference that is created between the leaves and the roots. The leaves allow water to evaporate out through them, and as this happens there is a decrease in water pressure at the leaf, which pulls more water up to the leaves.
We’ve got a little demo to show how this works. The challenge is: can you make water come up through a straw without sucking?
What you’ll need:
1) A small cup or saucer of water
2) A paper straw
3) A rules
5) Sticky tape
What you need to do:
1) Using scissors and a ruler (and help from a grown-up if needed), cut one 5cm and 3cm piece from your paper straw
2) Tape the pieces together at right angles, so that they look like a corner of a square. But make sure the straws don’t overlap
3) Don’t stick the tape over the ends of the straw pieces
4) Stand the smaller straw in the water
5) Blow through the longer straw.
Why does this happen?
And what does this have to do with trees?
Well, as you blow over the top of the smaller straw, the air moves faster and so the air pressure drops. The air pressure is unchanged just above the water, so what you’ve created is a pressure difference. This means that the water gets sucked up through the smaller straw, just like in the tree!
Now, the leaves might not be needed up in the canopy, but they are needed by the tree…as a blanket! (how cute is that?) During the winter months the ground frost could damage the smallest roots of the trees, which could end up damaging the entire tree.
I will leaf you alone now, but make sure to listen to this episode of #LetsDiveIn at the top of the article.
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