Photosynthesis obviously depends on catching light and is therefore a process that takes place on the surface of a plant. How much of a surface is needed will depend on many things, such as how much energy is needed. As related before, C3 photosynthesis can only make use of up to 25% of the light falling on them (well, at noon in the tropics, that is). Photosynthesis becomes saturated, doing nothing with that extra light.
For now, let's assume the presence of leaves on a planet of choice as very similar to Earth's broad leaves; flat structures of, say, 10 cm across. They need light, and so face the sun. But not every leaf of every plant will receive full sunlight: the sun moves across the sky (well, as far as the plant is concerned it does), other plants may be in the way, and even its own leaves, placed higher up, will take light away from lower leaves. A typical Earth leaf transmits only 5% of the light striking it. The next leaf down in turn absorbs 95% of the -little- light striking it, leaving only 0.05 times 0.05 of the sun light, or 0.25% of the light striking it.* On Earth the critical level for photosynthesis to be of any use is at about 1% of full sunlight.
It is therefore reasonable to assume that plants would have only one or perhaps two layers of leaves, right? Additional leaves would not contribute anything, and yet trees typically have many more layers of trees. The answer to this riddle is found in the efficacy of photosynthesis and a fact that you might not have considered interesting in this respect: the size of the sun.
Click to enlarge; source: http://en.wikipedia.org/wiki/Umbra |
The same applies for objects closer by. All you have to do is to look at the shadow of your hand as you raise it from the ground on a sunny day. Leaves also cast an umbra, an area without direct sunlight, where it would be best not to place another leaf. The length of the umbra can be calculated as explained above, and for Earth the calculations that the umbra is about 108 times the width of a leaf. For a 10 cm leaf that would boil down to 1080 cm, or 10 meters. As we will see the distance is shorter in practice. Leaves may receive enough light in the penumbra to work well. Remember that on Earth photosynthesis is already saturated at 25% of full sunlight, so photosynthesis can work at full capacity even with a fair amount of shade.
I wrote a Matlab program to have a look at how the umbra and penumbra could look for some artificial leaves. The distance between the Earth and the sun is 149,597,870,700 meters and the diameter of the sun is 1,392,684,000 m., both according to Wikipedia. A leaf takes up half the area of a 10 by 1-0 cm square area. In the program, this meant that I could paint half the pixels in a square area black denoting the leaf. All the program does is to cast ray from a raster of points on the sun's disk to all points on the leaf area, and to see which rays are intercepted and which are not. I did that for three distances behind the leaf: 0.5, 1 and 5 meters.
Click to enlarge; copyright Gert van Dijk |
Click to enlarge; copyright Gert van Dijk |
Click to enlarge; copyright Gert van Dijk |
Click to enlarge; copyright Gert van Dijk |
Click to enlarge; source here |
So, what does all this mean for the design of trees on other worlds? Firstly, like on Earth, you can have multiple layers of leaves and still have enough light trickling down for lower leaves to be useful. The shape of leaves is also important. Apparently some Earth trees use this effect: the outer or upper leaves of olive trees are thinner than the leaves lower down, which makes sense in view of the experiments above.
An interesting consequence is that the distance between leaf layers would depend on the apparent diameter of the sun's diameter as seen from a planetary surface. Doubling the diameter would half the length of the umbra, so leaves could be closer together and still receive an adequate amount of light.
Should your alien trees have a few layers of leaves or multiple ones? Theoretical considerations on Earth suggest that fewer layers work better when the amount of light is low to start with: the absolute level decays very quickly with the number of layers. For alien worlds, 'low light' can probably be rephrased as a low capacity to make use of available light. That could be low light with good photosynthesis or good light with poor photosynthesis. The effect of changing the saturation point is more difficult to predict. On Earth, where photosynthesis saturates at only 20-25% of full light, shadows may still leave enough light. But if photosynthesis could use up to 75% of all light, the top layers might generate all the energy needed, so more layers would be superfluous. Then again, the plant might well have evolved to use all that energy, so perhaps lower layers would still be useful.
No doubt, additional demands, such as transport of metabolites and structural stiffness will complicate the picture. Nevertheless, on Furaha various plants carry their leaves in umbrella-like shapes, in which two or three layers of leaves form a nearly completely closed canopy, closing off the sky to potential competitors that might grow up beneath them.
* This example is taken from the excellent book 'The life of a leaf' by Steven Vogel
9 comments:
Amazing and very informative post!
=)
Just a little note for others, there apears to be a typo in the distance of sun and earth, it's supposed to be 150 million km, not 50 million km, so don't get confused by that. ;P
Oops.. Well spotted! Apparently the '1' was omitted while copying the number to the text. I have corrected it.
No problem! =)
Thanks for that Information, Sigmund Nastrazzurro
i working on Trees on tidal lock planet
with this data i can design the ecology better
the Tree can life in symbiosis with other plants under it.
like nitrogen binding plants.
in the tree trunk permanet shadow would life mushroom, feeding on death organic material.
Fascinating material, as usual. I especially like how your treatment of leaf shapes and attributes begins with a lesson in astronomy. :)
I'm wondering how things fare with fewer, larger photosynthetic surfaces. Many of the specimens on my drawing board are considerably more membranous; is that feasible, or are leaves the only plausible option?
Another consideration when designing leaves might be not just the size of the Sun, but the number of Suns. My own worldbuilding project involves a circumbinary planet, and thus the effective "radius" of the light source (two stars at a distance from one another in the sky) is larger than would be the case for a single Sun. I'll have to bear that in mind in future drawings.
Dromicosuchus: that can certainly affect the placing of the leaves. Do the stars have a similar radiation output? If not, your plants might be better at dealing with the light from one star than from the other, and that would then also affect leaf placement.
First, there's a typo: "and yet trees typically have many more layers of trees" - the second must be "leaves", right? Secondly, what I'm missing is a discussion of wind. Leaves are easily displaced by it, and may thus be able to temporarily escape the shadow of leaves above it.
The size and shape of leaves is also an adaptation to climate, weather, atmospheric composition, gas needs, water/solvent loss, and herbivores. The plant must limit its water loss to something survivable for its own combination of climate, solvent-use efficiency, atmospheric gasses, and biochemistry. This will set a maximum surface area of any surface that can lose water (espeically gas-exchange surfaces).
The leaf is better off not getting knocked off or torn apart by high winds, so in windier habitats compound leaves or very narrow ones may be more advantageous than big wide pancakes, depending on the plant's height. Frequent hail, windblown sand, etc. may influence this too.
The plant must have sufficient gas absorption surface area for its needs and the atmosphere around it: if this surface is on the leaves, it will set a minimum surface area per plant.
A plant that needs to shed precipitation will need leaf shapes or surface textures that facilitate this, dependent on what precipitation is made of, what phase it's typically in, and how much must be shed.
Herbivore pressure may also affect leaf and branch shape. For example, the twigs of native New Zealand plants are zigzag-shaped 'springs' as an adaptation against large birds, which pull twigs instead of biting them off. The spring-like twigs can withstand more pulling with less breakage.
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