In our neck of the woods... Oak leaves

Many of the leaves have fallen now, but a few weeks ago, when Trish and I were sitting on our deck, I was staring at the leaves at the top of a nearby oak tree. Then I stared at the leaves at the bottom, then again I stared at the top. This is when I had an epiphany—in the category of Stan-discovers-something-all-true-botanists-probably-already-know.

What did I discover? The leaves at the top of the tree are skinnier (less surface area) than those at the bottom of the tree. The difference is striking. So, I took two photos of leaves on the same tree. This first photo is leaves at the top.

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And this second photos is leaves at the bottom of the same tree:

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See what I mean? What's up with that?

Well, although I was proud of myself for making this acute observation, this was already a well-known phenomenon. There are several reasons the top leaves of tall plants have less surface area than the bottom leaves.

The leaves at the top are in direct sunlight, whereas those at the bottom are shaded by the upper leaves. With smaller leaves at the top, more sunshine can get to the leaves at the bottom. And the larger leaves at the bottom can grab more of the light filtering through the top leaves. It's an equity issue, you see. The lower leaves can do their share of photosynthesis this way.

There's another reason too. The top leaves are exposed to more direct sun and more wind, so they evaporate away more water. By having less surface area, they lose less of the precious water the tree needs to survive. At the bottom of the tree, the leaves are shaded, there is less wind, and the air is more humid. So, those leaves can be larger without evaporating away too much water.

These kinds of adaptations always fascinate me, even if I'm late to the party.