Due to a thick cloud layer(at approx the same heigh as Earth's) surrounding this planet, it's plants have evolved to stay above the clouds so that they can absorb sunlight from their nearby star. Is this possible and how would it in that case manifest? A few ideas I have are plants that can float above the clouds somehow, absorbing water from the clouds, and simply very tall trees sticking out of the ground, do these have any merit?
There are plenty of plants that survive just by absorbing nutrients carried by the atmosphere. A Bromelliad, Tillandzia, is routinely seen growing in roofs, high tension electric cables, nets and fences. It absorbs humidity of the air, and probably nutrients carried by rain water, storing them in their leafy rosettas. A lot of Orchids also acquire their mineral nutrients through their aerial root system from the air. There is also a type of vegetation called nebular forests, typically seen way high in the mountains, that acquire their water trough condensation from the clouds that form in the early morning, since they live above the clouds. A tree tall enough to reach above the clouds would probably spend way more energy transporting nutrients throughout its body, as has been mentioned before. The tallest trees, giant redwoods, reach at most 100m (around 330 feet), and they are really pushing the envelope there, absorbing water from the atmosphere through aerial roots to supplement their water intake. I think your best bet would be very small airborne plants, with air sacks to keep them buoyant way up high. The hydrogen idea is neat, but probably very impractical biologically. The biggest metabolic expenditure a plant makes is to acquire a reserve of reducing power in form of NADH coenzimes, which basically allows them to store solar power in chemical form, so I can't see them just wasting that much energy to keep them afloat. Maybe a different, less energetically expensive gas? Here is a different thought, maybe they can regulate the temperature of the air inside the airsacks, by any number of means (some plants evolved tiny silicate lenses to enhance their luminous input, that is a nice trick to regulate heating from light) to effectively control their density, allowing them to keep afloat.
Also, keep in mind that there are plenty of autotrophical organisms (that make their own food, basically) that don't rely on visible light. Some can harness infrared, as thermal energy, some can use chemical sources, like the sulphur acids liberated near deep oceanic volcanoes, etc. Maybe plants evolved to harness these sources of energy deep in the shadowy realms? Plenty of soil there, I imagine, and probably water too.
The clouds are very high, a plant cannot grow that high, the expense of energy required to move nutrients and water from its roots to such a height would be higher than the amount of energy it could absorb from the sun. It would also collapse under its own weight.
Plants floating above the clouds are equally impossible because (besides the floating), they'd need water and nutrients. The nutrients are in the earth below the plants and the rain only starts at the cloud level, which is below them.
Well let's bring in some science, according to this paper the maximum height of a plant is roughly the height of a redwood (120m). So this can only happen if conditions are different. The issue is not the strength of wood but rather the efford required to bring water up to the leafs. Part of the problem being that photosynthesis requires water and if the energy requirements of bringing water to the leafs (using osmosis)exceed the energy production of photosynthesis then the tree cannot grow.
This can be helped in several ways: higher humidity and air roots, lower gravity or simply lowering the height of the clouds
While I see no way for plants to evolve in this situation I see how you could have plants:
The heart of the plant is a large hydrogen gas bag, the hydrogen being produced by disassociation of water. It has one or more roots, mostly a very tough fiber many miles long and arranged in a loop. Unlike traditional roots these do not draw up nutrients. Rather, there are nodules along the root that collects water and nutrients. The plant slowly draws the fiber around the loop, the force needed is only that needed to lift the collected material as other than that the two sides of the loop are of the same weight.
Yes, this is energy intensive, the plant will obtain materials very slowly. It will have to be very good at not wasting anything drawn up from the roots. Cactuses would have a lot to learn from it and anything that can be obtained from the atmosphere will certainly have to be. Nitrogen can be had from the air, if it can keep it's moisture inside the biggest need from the roots will actually be hydrogen. (Unfortunately, probably drawn up as water due to the energy demands of separating it.)
I doubt if this scenario of plant life above the clouds is feasible for an earthlike planet. Apologies if this deviates from your earthlike environment starting point. It's just that there are other possibilities worth considering.
However, there might be autotrophic, photosynthesising aeolian or windborne organisms drifting above the cloud layers of a hot jupiter. Organisms of this type might be not quite plants as we know them, but closer to metazoan versions of volvox species. Creatures that have plant-like stages in their life histories or which may revert to plant-like phases due to environmental conditions. While they have animal-like phases at other times. Sort of plant-animals. The volvox reference is for comparison to something similar here on Earth.
Hot jupiters would have the winds to support soaring or drifting lifeforms. Plus they would be close enough to the primary stars for lots of sunlight.
The light and heat conditions on a hot jupiter should produce huge quantities of prebiotic molecules to kickstart the evolution of life there. Probably, more than enough for a rich biosphere. Life will colonise every available niche. This will include the atmosphere and winds above the clouds.
How about alien plants growing over the top of high mountains?
In Earth, there are mountain peaks high above clouds, but don't have rich fauna due to cold climates and lack of moisture. If your alien planet is warm and moist enough, (maybe that could explain the thick clouds,) variety of plants could grow on over-5000-meter peaks.
And floating all over the clouds could be their seedlings. Each individual peak must have been an isolated environment, unless these alien plants spread their seeds and spores on the wind. Because the planet is not like Earth, maybe not just seeds but youngster plants. Saplings won't survive midair well, but that's how seeding works: spread as many seeds as possible and some will settle somewhere and take root.
Going back to basics. An earthlike planet with dense cloud cover and yet there are plants above the cloud layer. They can be either floating or extremely tall trees. Trees on our Earth have grown to the limit that natural trees can. So we can rule out super-tall trees. Although alien biological engineers might bypass those limits with some unexpected mechanisms.
Let's assume a form of airborne algae has evolved on Cloudland (my name for your planet). It should be able to flourish up there. More sunlight, better photosynthesis. Gradually evolving over hundreds of millions to fill all available floating plant niches above the clouds. Conditions will be generally cold, icy and dry. They stick close to the clouds for moisture. Plants need nitrogen for growth. Perhaps there are acidic clouds to supply that. if there are acidic clouds, then there must be something to produce them. Volcanoes? Volcanoes usually eject sulphates and sulphuric acid. Not what we want. Of course! Lightning and lots of it. That will make nitric oxides. Add water and you have nitric acid. Might be better to have your plants absorbing nitric oxides direct, but able to handle nitric acid droplets.
There will need to be some mechanism to get the airborne algae above the clouds. A stormy atmosphere? Definitely as Cloudland's atmosphere needs lots and lots of lightning to ensure the floating plants get their nitrogen.
What still worries me is the possibility that floating plants need to have densities of aerogels, therefore capable of floating or drifting easily on the wind, and yet might need to be structurally very strong. Perhaps that was a thought I had about your very tall trees. Floating plants might resemble masses of thin fibres as they will need to maximize their surface area to catch moisture, nutrients and sunlight. Rather like thin green netting instead of a bush or tree. This would also reduce their mass per area making it easier for them to sail on the wind.
Oh man... You would not believe how long this took to think of. I spent all dinner discussing metal qualities. Anyways, here it is...
- THE METAL TREE* The giant of the forest reaches high into the clouds. Given our planets ridiculous amounts of non corroding/rusting titanium the trees have bark completely made of this metal. Like any other plant these giants need water. They are able to use it in the same fashion as redwoods.(look it up!) It reproduces like grass with shoots as no seed would have enough resources to support growth past the clouds. This tree is also able to support a diverse ecosystem in its leaves.
There, that was a lot of fun to think up. Good question. :)
I don't think a plant would be able to reach high enough to get sunlight from above the clouds. In order to grown and evolve, they had to still start well below the cloud layer. Plus, a plant that extended miles into the sky would have to have some kind of incredibly ridged support structure to hold itself up. Especially when it has to fight the planet's winds and storms.