Science Questions

Why do leaf shapes vary so much?

Sun, 22nd Feb 2009

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Sir David Attenborough asked:

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?



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.  A drop of water on a leaf.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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I would guess this could be the result of a combination of different trees requirements and the insect and other animal residents and visitors to which each tree plays host.

A tree requiring a great deal of water and/or water borne nutrients would require a bigger and broader leaf than one which has less such requirements. For example a fruit bearing tree might need more nutrients than a non-fruit bearing tree. Larger and broader leaves would allow a greater rate of evaporation, thus drawing more water and nutrients from the ground. The larger leaf area would also mean a greater rate of photosynthesis.

A tree may also benefit from the creatures it plays host to. Supplying the right size and/or nutritional value leaf could be the price the tree pays in return for the protection of the insects which live in it's canopy, or the animals which will spread it's seeds when the time comes. Don_1, Wed, 18th Feb 2009

The climate would be the same for neighbouring trees, but not constant over the lifetime of a tree.

Over the lifetime of a tree (decades) climate (e.g. rainfall) would not be constant.
During this lifecycle the changing conditions would sometimes favour one type of tree, sometimes the other. 

So different tree types existing side-by-side is a bit like nocturnal and diurnal animals existing in the same territory,
only the cyclic period for the trees is decades long, not daily.
RD, Wed, 18th Feb 2009

You will also find that much of this is down to genetic variability. The internal coding of a particular species that would be inherited by its offspring (its genotype) holds all the instructions for the next generation. During the interpretation of this information various anomalies may occur and provide the variability that drives the evolution of the species. This is the phenotype, or the actual physical manifestation of the genotype. There does not actually need to be any environmental input to drive these "within group/species" differences in leaf shape and size at all - it can all be explained by this genotype ==> phenotype process. dentstudent, Wed, 18th Feb 2009

If a genetic variant was better adapted than its predecessor and environmental conditions were constant, then the superior variant would outcompete and eventually eliminate its predecessor. There have to be cyclic conditions for coexistence. RD, Wed, 18th Feb 2009

RD - My point is only that this interpretation of genotype into phenotype can also explain a degree of variability within a species. Even if there was only one individual say, that produced many offspring (as is the case in many pioneer species of tree, for example birch), then there would be variation within that group due to phenotype, and hence also a degree of within population leaf size variability, for example. The environmental changes do not alter the phenotype interpretation, only (as you say) those individuals who may benefit because they are better adapted because of their phenotype. dentstudent, Thu, 19th Feb 2009

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