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I think there are some obvious costs for trees to grow tall. Carbon and other nutrients costs, maintenance cost, energy cost (for growing, to bring water (and nutrients) up to the higher leaves, etc...). So what are the main reasons that cause some trees to grow so tall?, For what purposes is the adaptation of being tall?

Here are three hypotheses I can think of:

  • Competition for sunlight
  • Better dispersal
  • protection against predation
  • Some birds (maybe especially those that are potentially good seed dispersers) prefer to land on high trees (avoiding the cost of regaining altitude). Being high attract these birds which eat the fruits and disperse the seeds.

What are the others hypotheses/reasons?

I am currently in Bali, Indonesia looking at palm trees that seem to have a very low competition for sun light (the forest is not dense, the light is intense), there are no tall predators (but being tall might be a protection against climbing predators as well and there are indeed monkeys and other potential climbing predators) and looking at coconuts, papaya and banana I can hardly think that being tall helps to disperse further away (seeds dispersal by barochory). So, why (evolutionary reasons) are palm trees so high?

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3 Answers 3

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Beside the reasons you've given I would add environmental disturbance (e.g. flooding) as a evolutionary factor that could explain selection for taller individuals. Many palms also inhabit and thrive in disturbed habitats (Frangi & Lugo, 1998; Salm, 2005), which indicates that this could potentially (i.e. speculative) be an important factor for this taxon. Palms also have the combination of tall height and a relatively low "wind profile" (relative to many other groups of trees), which indicates that they are adapted to exposed/disturbed habitats.

This, along with the reasons you've already given, seems to cover the most obvious evolutionary reasons for selection on tree height. However, these will naturally trade-off against negative effects on e.g. fecundity from investing and maintaining biomass.

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Thank you, this kind of speculations was exactly what I was looking for! –  Remi.b Oct 23 '13 at 10:20

The height of tree species varies, of course, as each tree fills a given niche, much like animals do; you might as well ask why blue whales grow so big. The answer, of course, is that they seek out and successfully get the nutrients they require. It is advantageous for some trees to be huge and get the most sunlight possible and so some trees do.

In the specific case of palm trees, however, they aren't true trees but rather a grass, which means they are a monocot and (I'm a little shaky on plant physiology but...) that the outside isn't living but the inside is. Monocots grow straight up, in one main shoot, and palm tree follow the same pattern. Tons of sunlight, nothing to come mow or it down, and nowhere to go but up means they'll be big. True trees, on the other hand, have what is called heartwood in the center, which is technically dead tissue.

Another thing to think about is that trees aren't moving, so they only way they can improve their condition is by growth. An animal, on the other hand, can migrate to follow food; growth, particularly for mammals, also has a huge heat cost.

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Thanks for your answer. It doesn't provide the answer I was seeking though. I was looking for functional evolutionary reasons for a tree to be tall (like the three examples I provided). I know it is a reaction to natural selection, I know the concept of ecological niche. You're saying that tree can improve their condition by growth but my question is why? Or more specifically why growing tall? –  Remi.b Sep 13 '13 at 6:18
    
For the case of palm trees, saying that they cannot grow wide but only tall is some kind of answer although it still does not answer to why growing tall rather than not growing (and invest more into reproduction)? Hope my comment makes sense! –  Remi.b Sep 13 '13 at 6:18
    
@Remi.b I tried to say that in my first paragraph, and again in my last. It's straightforward, and you guessed it: The taller the tree, the more sunlight it can get. Other factors like soil and temperature can play a role as well. –  Amory Sep 13 '13 at 6:28
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@Remi.b just because a particular individual (tree) has no competitors does not imply that the species evolved free of competition. I would guess that it always comes down to getting more sunshine. For example, a polar bear born in a zoo in Brazil will still be furry and white even though those characteristics don't help that particular individual. –  terdon Sep 13 '13 at 14:12
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@Remi.b And I (we?) are wondering why that's not enough for you? Trees require sunlight to survive, and growing taller means more sunlight, so why shouldn't they? A taller tree would have a fitness advantage that would be selected for. I'm sure in some scenarios predation could play a role but not as strong as the desire for sunlight. –  Amory Sep 14 '13 at 6:31

Tsunami protection seems like a good candidate, seeing as only one can wipe out an entire population of low-growing trees. The tall one (palms) have a low drag profile almost on the entire height, making them well suited to withstanding a sudden surge of water. They would also have the bonus of shielding their not-yet-ripe fruit from being taken to sea.

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