Prototaxites was a massive, trunk-like organism (up to ~8m tall, ~1m wide) that dominated land ~420–370 million years ago, long before trees or complex plants existed. It looked like a tree, but chemical evidence suggests it didn’t photosynthesize. Internally it was made of interwoven microscopic tubes, unlike plant tissue. It’s often described as a giant fungus, but it doesn’t cleanly match modern fungi either, and some researchers think it may represent an entirely extinct branch of eukaryotic life. In other words, early “forests” may have been dominated by something we don’t have a modern analog for.
Plants grow tall to be able to gather light, instead of staying in a shadow.
Fungi and many other terrestrial organisms that reproduce like fungi (e.g. slime molds and myxobacteria) grow above the ground only in order to be able to launch their spores into the wind.
It does not seem possible to explain the size of Prototaxites by the need of launching spores in the wind.
The only plausible explanation is that it was tall in order to ensure access to light.
If it was not a plant, it might have had a symbiotic relationship with a phototrophic living being, which grew on the surface of Prototaxites, i.e. either a blue-green alga (Cyanobacteria) or a green alga, exactly like the present lichens. Prototaxites could have provided access to light, water and minerals, while the alga would have provided food.
Thought the same, but that implies both that it grew in very dense "forests" (mono-species because there were no competitors) and probably that it had leaves (because otherwise trunks don't occlude much light).
Although, counterexample: why do (some) cactuses grow tall? Claude provides these explanations that might apply:
Water collection and storage — Height means more volume for water storage. A large saguaro can hold thousands of liters of water in its stem, which is crucial for surviving long droughts.
Temperature regulation — Being taller gets the growing tip and flowers farther from the scorching ground surface, where temperatures can be extreme. The ground in deserts can reach 70°C (160°F), while air temperature a few meters up is significantly cooler.
According to the linked paper, the structure of the stem of Prototaxites contained several kinds of tubes, which might have formed some kind of simple vascular system, able to extract the water from the soil and circulate it through the body.
You are right however that the plants among which Prototaxites was growing had a much smaller height so the competition with them would not have been a strong reason for its height and for the competition between Prototaxites individuals there is no evidence that they would occlude much light.
Still, I am not aware of any better explanation for its height. At that time there were no flying insects. The terrestrial vertebrates and bigger arthropods were predatory. There were a few groups of non-predatory arthropods, i.e. millipedes, mites and springtails, some of which might have been able to feed on Prototaxites tissues, but such small arthropods are likely to have been able to easily climb its stem, so it seems unlikely that its height could have provided any protection for its reproductive parts.
Besides avoiding shadows, there is another explanation for the great height, but that is also applicable only to organisms able to capture solar light. As there is evidence in its isotopic composition that Prototaxites was not phototrophic, any explanation based on capturing light must involve a symbiotic alga. A great height could have helped with the ascent of water through the stem of Prototaxites, due to capillarity and evaporation at its top. However this explanation requires for the pumped water to be useful somewhere high in the stem, which would be the case if the water were given to a symbiotic alga, which would provide food in return.
"We therefore conclude that Prototaxites was not a fungus, and instead propose it is best assigned to a now entirely extinct ==> terrestrial <== lineage."
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