Why do leaf shapes vary so much?

22 February 2009

Question

If you look at trees, often closely-related trees, growing in exactly the same ground in exactly the same climate they have different-shaped leaves. Why? People will say, “it’s the airflow over it in different particular circumstances or the way that the water drips off it,” that’s the reason. But the trees are growing in exactly the same places alongside one-another. Why do they have different-shaped leaves?

Answer

We put this to Ed Tanner, Senior Lecturer in Plant Sciences at Cambridge University

I think the answer to the question about why closely related trees growing in exactly the same ground and the same climate have different-shaped leaves is actually that they don't. Because they're closely related they are very similar. For example, all oaks have broadly similar-shaped leaves because they share most of their genetic information. Perhaps a more interesting question is why distantly related trees growing in the same ground and in the same climate have different shaped leaves. The answer is it doesn't matter very much. As long as leaves are reasonably good at doing their job, which is fixing carbon dioxide in the atmosphere it doesn't matter whether they are wavy at the edges or not wavy at the edges. They have to absorb the light and once they've absorbed the light they would fix CO2. As long as they put their competitors in the shade any reasonably functioning leaf will do the job. It matters where your leaves are in relation to other trees. If you're an ash tree you've got to be above an ash tree or if you're a beech tree you've got to be above an ash tree. It doesn't much matter what your leaves are like.

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