Plant Transpiration

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License: Creative Commons Attribution Non-Commercial Share-Alike Plant Transpiration Braunton School
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22 Oct, 2007


An estimation exercise: how much water does a tree transpire?

Director's Notes:

Years ago, a TV series showed a memorable image of a tree, with a fire engine parked next to it gushing water upwards. This, they told us, was how much water the mighty oak was transpiring.

Memorable but, as it turns out, entirely wrong. The TV company had mistaken ‘litres’ for ‘millilitres’, so they were out by a factor of a thousand. Oops. Time to do the maths ourselves.

SciCast Notes:

I’ve mixed feelings about this film. On the one hand, it’s nicely made: there are close-ups that show me most of what I want to see, moving across to the tree is pretty smooth, the ‘Some time later…’ device to fast-forward is a witty way around that particular problem.

But on the other hand, I just don’t believe any of it. It’s remarkably convenient that there are such a round number of leaves on the tree, and that makes me suspicious. Suspicious enough to notice that the twig used in the measurements has several leaves on it — but this isn’t accounted for in the calculations.

So I think this is a big cheat and fib, and while I can enjoy the film and appreciate the technique, I’m not left with any clue as to how much water a tree might transpire in an hour.

A shame, because if I had reason to believe it, this would be a great film. A lesson to us all — make sure your films are believable. The easiest way of doing this is simple: don’t cheat!

— Jonathan