A recent visit to the Phoenix area prompts this question. In the Sonoran desert there, Saguaros flourish - it’s easy to see hundreds (thousands?) at a time. They stand out in part because they are commonly 2 to 2.5 times as tall as anything else.
My question is: why? Growing tall implies a costly investment in root structure and strength, to support a “trunk” that can reach 40 or 50 feet and must handle strong winds (plenty of which will be experienced during a 150-year lifespan). And they almost never grow close to another Saguaro, so competition for sunlight (widely regarded as the reason deciduous trees grow tall) isn’t a factor.
It seems there has to be some non-obvious factor at work here.
Interesting question. I’ll be watching for an answer. Before someone who really knows what they’re talking about comes along:
I do know that the saguaro and most cactus species already form a robust root structure to maximize water collection, and a strong “trunk” to store truly massive quantities of water. A large saguaro can literally hold tons of water. When a cactus has sufficient structural strength to hold that much water, strong winds are relatively trivial.
Barrel cactuses are much more compact while storing similarly massive quantities of water, but they have much less surface area for photosynthesis. Other cactuses have branching structures with much more surface area but less volume available for water storage. I suspect each major species of cactus occupies some local maximum for the ratio between water storage and sunlight gathering.
The lack of sunlight is seldom an issue in the desert. How’s this for a hypothesis? The plant needs to be a certain size to hold all of the water it does. Growing tall affords less surface area to direct sunlight, than growing wide would.
My experience with succulents is closer to **campp **than to lazybratsche, FWIW. Shallow root systems seem to be the norm. I have never, however, grown or even been near a saguaro, so perhaps they have a different strategy than the agaves with which I’m familiar.
A good question and I will be watching as well. I will speculate that the round shape of the trunk helps divert winds, and since it has few branches to catch wind as well, it helps keep them stable. I think also, the ribs and needle structures help prevent evaporation. Perhaps the Saguaro evolved amongst other competing plants that have since gone extinct.
Tagging on the same question for an unrelated species - Joshua trees, which also stand out quite taller than anything living nearby.
The sagaro is one of the few trees in the desert that provides nesting for owls and woodpeckers, without something in the desert to provide nesting other systems might collapse. For instance without a perch for predatory birds rodents might become so numerous they wipe out there own food supplies.
Sure, but why should the saguaro care about that? What’s in it for it? Organisms evolve for their own benefit, not for the benefit of their ecological neighbors.
Unless those neighbors provide some benefit, like pollination. I don’t think owls pollinate, but some birds and bats do, and it may be that features that attract pollinators also attract other birds.
More speculatively, if rodents cause damage to the cacti (say, by making holes in the base), providing a home for predators might reduce the impact.
I could be wrong, but I think you can find an owl’s favourite perch by looking for the bundles of bones and fur that they throw up - they eat, for instance, a mouse, and whatever they can’t digest gets brought back up.
Growing tall may give these plants an advantage by attracting flying predators to use them as:
vantage points to spot prey.
or, perhaps, a safe place to rest after feeding.
If the predator expels waste while perched on the plant, it should fall close to trunk where the roots may be able to pick up any nutrients that leach out.
Hypothesis: The plants are spending energy on growing tall in exchange for the waste products of birds that are attracted to it. This may, after time, result in the plant being healthier and able to divert more energy into reproduction.
Note: That’s all a WAG. I know little about owls and less about cactusesess.
Are there other types of cacti that have a larger girth than a saguaro? Speculation… there may be a maximum girth for a cactus, and once you approach that, the only way to grow bigger is to grow taller.
I think big saguaros are about as fat as any cactus gets. Though it would be hard to say why there’s any important theoretical limit on horizontal growth.
But even if there is, one obvious possibility would be to grow tall enough to be above nearby plants, then spend resources growing “arms”. A few seem to do just this - but most get quite a bit taller, before developing a few arms.
I’m sure there must be something going on where the “just right” equilibrium is reached between maximizing surface area (for photosynthesis) and minimizing surface area (for water retention). Too little surface area, and the plant doesn’t get enough food (there aren’t any leaves). Too much surface area, and the plant can’t retain needed water during the dry times.
And that’s probably just to account for first order effects. Other effects like wind resistance, etc. are also going to come into play.
