Saturday ~ March 03, 2010
I'm working on a biology review, and it's on leaf shape and leaf plasticity and am just trying to get my background information straight.
If the shape of leaves is an indication of the environment that the tree is in (e.g. adaptation for plants in a dry environment vs. those in a wet one) why is it that many plants that inhabit very much the same habitat have such different leaf shapes? (e.g. Sugar Maple and White Ash) Would this have anything to do with competition or in a sense niches?
Not only it depends on climate but on the chemical composition of the soils. Nevertheless plants can be adapted in any environ, although plants come from different environs. So in the same environ you can find different plants.
by climate:
warming and raining: great shape leaf
dry: little shape and hard leaf, sometimes spines.
wet soils: big leaf
dry soils: little, hard and spineous leaf. Sometimes meatous branches containing water.
Leaf shape is relatively unimportant evolutionarily. Yes, dry environments favor leaves with smaller surface areas. With very windy conditions (or plants growing in running water), optimum leaf shape may be affected by a need to reduce drag. However, in most conditions, it doesn't matter a whole lot how the leaf is shaped. With no selection pressure for leaf shape, plants are free to have all kinds of differently shaped leaves.
Excellent question! Note that box elder has a leaf shape very similar to white ash, and box elder is in the same genus as sugar maple, so if the white ash shape confers advantages over the sugar maple shape, you cannot explain the sugar maple shape by invoking a lack of plasticity in the sense of the amount of genetic change needed to get from A to B. You wisely chose two trees with nearly identical niches, as far as I can tell; they even have similar fruit dispersion mechanisms.
On the other hand, the leaf shapes of those two trees are quite similar when compared to the wide range of leaf shapes in plants overall. I think the other answerer has the right idea, and that between those two shapes there is not much selective pressure in their niche.
If you haven't included aquatic plants, they are worth a look. The very different conditions between aquatic and terrestrial environments has led to some very interesting morphological adaptations. Take a look at Hutchinson's Treatise on Limnology: Limnological Botany; I think it is volume three.
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