John, I think the vertical ribs on them can do the accordian affect to allow for more water or protect themselves from drying out.
Being tall enough would have the advantage of being slightly cooler at the top - you’re farther from the ground that radiates heat back up, and you’re at a level where breezes get less turbulence from the terrain and low bushes. Since I have rarely seen saguaros growing on level ground (they seem to favor sort of hilly areas), this might be relevant.
Maybe because most (or all?) plants have specific areas, the “growing tip” or meristem, where new growth occurs? Even a spherical cactus has one “tip” where the new growth occurs, and they can only get so big.
Perhaps because it can.
How long will a household philodendron grow if unchecked, or kudzu?
Saguaros deal with this issue by means of waxy skin that seriously limits transpiration of water.
Maybe, but vast quantities can be found on nearly level ground around Phoenix.
I assume John Mace was referring to storage capacity, not water loss. Essentially in a desert water is limiting and sunlight is available in excess. Water is consumed in photosynthesis. So if the saguaro stores 300*X units of water during a brief rainy season, it has to have sufficient volume for that storage, and the optimal surface area is such that photosynthesis consumes only X units per day.
The question you are asking isn’t in any way unique to saguaro. It’s true of many savannah and desert trees.
In semi-arid and arid environments, moisture availability requires trees to have a root system that is anything up to 10 times more extensive than their canopy. As a result the trees are so far apart that light is rarely a limiting factor. Yet the arid regions of the world are dominated by tree wherever there is enough moisture for trees to survive. Saguaro aren’t unique in this regard.
As to why arid environment trees grow so tall, that’s a matter of some debate, with various experts championing various explanations, all with some good evidence. The truth is that all factors probably play a role. The more important ones, in what I think are order of importance are:
- It gets the plants above the competition. While arid zone trees won’t get shaded by other trees, they would get shaded by other shrubs. All you have to do is look at a pristine saguaro stand to realise how much competition there is for light at ground level even in a desert. While trees in arid regions can’t grow close together because of low soil water potential, shrubs and forbs suffer no such limitation, and shrublands and grasslands with canopy covers equivalent to normal forests are common. The tree lifeform gives massive competitive advantage for light against shrubs and grasses, even when there is no competition between trees.
This is particularly true for cacti, which have lost the ability to produce finely divided photosynthetic area. There’s a side-discussion above about the relative efficiencies of cylinders vs spheres for water retention, temperature control and so forth. But no matter what factors you consider, in the saguaro stand above, a cylindrical cactus with the same area as the suguaros would be buried under the shrubbery.
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It takes the pants away from fires. All deserts burn, and the low fuel loads mean the burns are mild compared to forests. For a long lived species that can’t root sucker and doesn’t produce resilient seeds, it’s essential to get above the fires if you want to have any chance at all of surviving. A cactus at ground level in the environment where the saguaro live would need to have a much shorter life span to hang on in the face of fire.
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It lets you capture rain. Most desert trees have adaptations to channel water down the foliage and trunks to the roots, and cacti are no different. Because rain often blows sideways, the taller the plant is, the more rain it can capture. For acacias, that trapped rain is equivalent to an extra ~25% rainfall annually. That’s a huge survival advantage. For a cactus, it would be much more. Someone else can do the calculation, but it’s quite clear that, even if we assume rain falling at 15degrees, a cylinder will capture many, many times more rain than a sphere with the same area.
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It lets plants capture light at dawn and dusk. Light itself may not be limiting in the desert, but for most of the day the light is too strong and the temperatures too high to enable efficient photosynthesis. The best period is when the sun is at a low angle and both temperature and light intensity have dropped off. Many arid zone trees have angled leaves for exactly this reason. By growing tall, a tree can present a large area to the sun even when it’s low. A spherical plant will only ever be able to present the same area regardless of angle. It’s surprising to many people, but the classic Christmas tree shape is just as popular on the tropics as at high latitudes, and for the same reason. High latitude trees need to be able to capture low angle light because the sun is at a low angle throughout summer. Arid zone trees need to capture low angle light because that’s when photosynthesis is most efficient. For cacti, which can’t produce branches, height is essential in capturing low angle light.
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It gets your reproductive bodies away from predators. If you look at the photo above, you’ll see that the flowers are clustered at the top of the stem. Aside from making them more easily accessible to bats and moths, that also keeps them away from deer.
Good post, Blake.
#3 sounds very plausible, in view of the grooved nature of the saguaro’s structure.
Under the assumption that photosynthesis requires a low sun angle, #4 also works well.
Why are Sagueros tall? Are you are telling me you don’t know the record of the Sagueros in the Desert Basketball League? They’d win every game if they weren’t so slow.
In the desert, bats use eco-location to identify flowers in bloom - at night! Thus, flowers at the top of a saguaro arm are more “visible” to pollinators. In short, height is driven by competition for pollinator activity.