Where do mushroom forests thrive?

Question

Lichen are a symbiotic organism that involves algae or cyanobacteria living inside a fungus. The algae produce energy through photosynthesis, while the fungus protect the algae from the environment and provide access to water and nutrients. In many ways, this relationship is similar to that of corals and photosynthetic dinoflagellates.

I wish to create a forest of symbiotic mushrooms. The sun-facing surface of the mushrooms is permeated with a symbiotic photosyntesizer. Mushrooms, of course, can have a variety shapes and sizes with a diversity almost resembling a coral reef. Let us assume for the purposes of this question, that mushrooms' chitin based cell walls can be adapted to allow mushrooms to grow to several meters in height, at least. I will leave the maximum vertical height or horizontal cap spread of these mushrooms to the judgement of the answerer.

My question: In what climate or biome would mushroom forests have the most competitive advantage against other plants?

Secondary question: What sort of ecological relationships would these mushrooms develop with other plants?

This question is strictly flora/plant based, more fauna questions to come.

Answer 1

While it's hard to say exactly where fungi would out-compete plants, I have some good news for you. There is one major zone on Earth where both algae and fungi dominate, and plants do not:
Below the Antarctic Circle

Diversity Statistics

Antarctica currently contains 100 species of moss, 25-30 species of liverworts, and 2 species of flowering plants, with no trees or shrubs. Meanwhile, it has 250-400 species of lichens, which are included in its 1,100 species of fungi, and 700 species of algae. This is a huge difference.

Possible Explanation

Plants are heavily reliant on their vacuoles (water storage compartments within cells) to maintain structure and provide nourishment. In cold conditions, these freeze easily. This explains why the less structural mosses do better than upright plants in this environment, and why fungi do even better still. While fungi do have vacuoles, they are generally smaller in fungi species than in plant species. Fungi have stronger cell walls to compensate. In short, plants freeze easily. Fungi don't.

Realistic Size of Fungi in this Environment

This is your biggest problem. While fungi and symbiotes containing algae will out-compete plants in an extremely cold environment, they themselves may not be able to grow large easily. Your idea of stronger cell walls helps, but you may want to consider a method in which these fungi warm themselves (chemosynthesis, maybe?) in order to allow fast growth and metabolism.

Potential Ecological Relationships

These are not likely. Aside from the symbiotic processes between algae and fungi, or the interaction of fungi using mycelium under the soil, your fungi won't come in contact with many other organisms. Mosses could be plausible if they occupy or compete for the space that your mushrooms want.

Of course, remember that even though photosynthesis is occurring, your organisms will want nutrients from the soil to grow large. This means your fungi will likely decompose other organisms to acquire them.

Answer 2

The big advantage of a fungus-microorganism symbiotic relationship is that there are more possible photozynthesizing pathways available. Plants use chlorophyll to generate energy from the sun. It is clearly effective, but it is restricted in the wavelengths that it operates on.

Chlorophyll a is universally present plants, algae, cyanobacteria, and prochlorophyta. Chlorophyll b is also present in plants, and the absorption spectra of the two plants are shown in the figure above.

However, there are other chlorophyll types available. Chlorophyll d has a far-red absorption at 710nm, just outside optical range. It is present in red algae, which also has accesory pigments called Phycobiliproteins that have additional absorption peaks at 546, 566, 620, and 651. These are evolved for deep sea algae due to longer wave-length light penetrating farther in water than other parts of the visible spectrum. Another option is fucoxanthin, found in brown algae such as kelp with increased absorption in the 450-540nm range.

On land, with lots of blue wavelength scattering due to the composition of the atmosphere, the plant's absorption peaks are a pretty good deal. However, underneath a canopy of trees, as in a rainforest, the light spectrum is different. Reflected light from green tree leaves is green, in the ~550 range. Plants are not optimized to utilize this diffuse light in the understory. According to Denslow, 1987, photon incidence on the forest floor is only 1-2% of that from a clear sky, and plants do not utilize this light due to its spectral composition, instead relying on sun-specks that break through the leaves briefly for their growth. In this paper, gaps in a subtropical rainforest in Queensland caused by fallen trees have increased levels of blue-green and infrared radiation.

In conclusion, I propose that a fungus-algae symbiote could evolve to use a rigid, chitinized mushroom platform to absorb diffuse light in a rainforest understory. The rain and high humidity would allow the algae to thrive, and the leaf litter from the taller trees would provide abundant decaying material for the fungus to utilize. These persistent symbiotes would have a competitive advantage against understory plants due to light absorption better suited to the available spectra and secondary energy generation from decaying leaf litter, an energy source that is available during the entire daily cycle, not just during daylight.

Answer 3

Fungi ordinary thrive where there is dead biomass and moisture, but it is too dark for adequate photosynthesis, such as caves and the sub-canopy levels of a dense forest. Direct reliance on photosynthetic symbionts really throws off that equation, because it forces them to be second best competitors with purely photosynthetic plants.

The sort of place I would imagine would be best for a tall fungi forest would be in a delta of a continental sized river system (think New Orleans) in a location on a planet that is in the dark all or most of the time, but isn't far from the dark side-light side divide.

This would be warmer than more distant points and would get first dibs on biomass crossing over to the dark side. Temperature-wise too far on the sunny side would be too hot, too far on the dark side would be too cold, the breeze exchanges near the boundary would be just right.

This would provide ample moisture and biomass sludge flushed down the river system from the sunny side to feed them, but would suppress competition from photosynthetic plants.

A place like this would also be prone to London quality fog on a regular basis which together with the mushroom forest would really add to a memorable atmosphere and mood for the place.

Why would they get tall when they aren't reaching for the Sun? Perhaps the height would provide places for local fauna to take refuge during high tides at which time they would poop and/or die on the mushroom trees providing more nutrients. Height would also provide access to air for the fungi during high tides.

Answer 4

Maybe it doesn't actually have much to do with light as it does with nutrients.
Say the mushrooms are parasitic on other plants and aggressively predatory.
Their spores invade a forest, latch onto trees and begin sending their mycelia all over like vampiric fingers, spreading more mushrooms very fast, killing the trees off. After the tree dies the mushrooms begin feeding off their rotting remains and start growing taller. They don't have to get too tall since they don't need the sunlight, they just want to be high enough that the spores can get some distance.

The honey fungus is one type of parasitic mushroom which is able to infect and kill healthy trees. One of its subspecies is the Armillaria ostoyae, which is thought to be the largest organism in the world. Mainly growing underground, the mycelia of this patch covers over 2,400 acres in Oregon. It's estimated to be over 2,200 years old.

This parasitic super fungus, known as Armillaria Lamia, could also spread underground and not just through spores, attacking the trees at the roots and working their way up. Normal mushrooms do not photosynthesise, though there could be a secondary lichen that is symbiotic with the mushrooms.