I will be writing a short story about humans landing on an earth-like forest planet with a gravitational force of about 3 meters per second squared (In contrast, Earth has a gravitational acceleration of about 9.81 m/s^2). I need to find how the forest would adapt to the gravity - namely the height of the forest canopy (usual canopy height about 30-40 m) - and the types of flora that would dominate the forest.
The plants would be stringy, flimsy, not having to develop fiberous strength higher gravity requires.
Leaves would be larger and less numerous.
You might have fewer woody trees and bushes, ecology probably favoring dense fiber over actual wood.
Grasses and low-growing stuff probably wouldn't be much different.
Lower gravity means thinner biosphere. Between that and the tendency to thinner, flimsier growth, your canopy height might not actually change.
1$\begingroup$ Lower gravity means thinner atmosphere, too. Like, almost certainly not thick enough to support life. $\endgroup$ Aug 6, 2017 at 0:27
7$\begingroup$ @WhatRoughBeast No it doesn't. Titan has less than 1/6th Earth's gravity, and yet has a thicker atmosphere with higher surface pressure and more total mass than Earth's atmosphere. Venus has gravity similar to, but still lower than, Earth, and yet manages 90 times the surface pressure and even higher density. Lower gravity means thinner atmosphere given a fixed mass of air per unit surface area. But the mass of air per unit surface area is far from constant across different worlds. $\endgroup$ Aug 6, 2017 at 0:48
Trees exist because, once a species of plant starts going up from the ground until it reaches a certain average height, it will have an advantage over other species, because it will be able to overshadow them and thus catch more light and deprive its competition of it. Individuals of the same species which have the capacity to grow a bit taller than average will, ceteris paribus, be selected for. So there's an evolutionary pressure to grow as tall as possible. For trees on Earth the theoretical limit of vertical growth is between 122 and 130 m (Earth's actual tallest tree is 115.8 m tall). This is given by how much energy it takes to take water up from the roots to the tallest leaves and the pressure that the tissues can sustain.
In a planet with less gravity, trees could and would surely grow taller. They would also have to support their own weight, of course, so they would be as hard and solid as Earth trees.
Plant life in jungles and forests organizes itself into levels, from the undergrowth to the tallest trees, and that would probably happen in your planet as well. Maybe, since trees would grow so much taller, there could be more, and more diverse, ecological levels.
Back a few years ago, I've found the "epona project" featuring a planet with roughly half Earth's mass, meaning less gravity. This low gravity may create more challenges which must be considered
1- Mentioned in this project: Carbon cycle: A planet with low mass will have its geological activity subside much more quickly. Carbon-dioxide dissolves in the water and interacts with rock minerals, making carbonate rocks. Those rocks may be subject to geological forces (like magma heat) which releases it back to the atmosphere. When geological activity subsides, more carbon dioxide remains trapped in rocks. Plants need carbon dioxide for growth. Lower geological activity means less will be available to the plants. The assumption is that plants will evolve means for storing as much carbon dioxide as possible during the night and use it via photosynthesis during the day.
2- Atmospheric escape: (Not mentioned in this site), depends on the type of star the planet orbits, the planet's distance from the star, and the strength of the magnetic field. Less geological activity means a weaker magnetic field. Combined with lower gravity, the atmosphere will be stripped more quickly by solar wind. The plants must retain vital chemicals under a lower atmospheric pressure and that becomes more difficult as atmosphere wears-off gradually.
The planet will start its life as an earth-like planet with a lower gravity, but over time, a new strategy to deal with those challenges must evolve.
EDIT: Low planetary mass as you envisaged may be impractical and will lose atmosphere long before any life may have time to evolve. The only "way out" of this is to assume a much farther planet, like Titan - moon of Saturn - being so far that temperatures are much lower and the two negative effects are much reduced. In that case, water is no longer a vital solvent, and you must use more exotic things like Methane, Ethane and Ammonia...
$\begingroup$ Nice one! Thanks for bringing this one to my attention! $\endgroup$ Nov 15, 2017 at 20:01
Assuming the atmosphere has the same density, just on a slow rotating small planetoid.
I'm not sure but i believe plants would be a few % bigger than on earth (specialy trees), I believe I have read somewhere that the most limiting factor on the size of plants right now is the ability to flow water efficiently through its body.
Of course if you want to be 100% realistic, at 1/3rd of earth gravity the rotation would be slower as well, so a lot of ultraviolet rays will pass through what would be the atmosphere.
So I'd suggest the plants to be violet-ish color and feed mostly on this source of energy and most likely there won't be much if any oxygen.
$\begingroup$ Why would the rotation be slower on a smaller planet? Also, how can plants only feed on light? $\endgroup$– L.Dutch ♦Nov 15, 2017 at 10:25
$\begingroup$ slower rotation could generate less gravity acording to general theory of relativity (in the original question it's earth like ) so aldo the general effects would be lower on gravity than the mass effect but it would stil affect the end result. About feeding only on light it should be mostly xD , i'm no expert on micro biology but the basis of cells is(as far as i kno) "get energy then can do stuff" so an organism that absorbs light mostly to survive and gets some structural mass out of other elements wouldn't be so far feched for a scify ish theme $\endgroup$– KaotisNov 15, 2017 at 11:03
$\begingroup$ Are you aware of the conditions under which general relativity is meaningfully applicable? $\endgroup$– L.Dutch ♦Nov 15, 2017 at 11:03
$\begingroup$ Only the smal details (hence why i mentioned the slower rotation) if i'm mistaken sorry :) $\endgroup$– KaotisNov 15, 2017 at 11:05
In short, there would be no trees, and your humans would die upon exiting their spacecrafts. Or in other words, such a planet cannot exist.
The graviation that you wish to have is 20% lower than the gravity on Mars, which means that the atmospheric pressure is also comparably low or lower than on Mars.
Let's not even consider petty details such as lack of oxygen, just the mere atomospheric pressure.
There is no surface water on Mars, how come? That's because without sufficient pressure, it boils away. No water, no trees (or life for that matter).
Unless of course, the planet is a desolate, dark, frozen block of ice, in that case the water obviously wouldn't boil away, but there wouldn't be much leeway for trees (or the development of any significant amount of life in general) either.
$\begingroup$ Atmospheric pressure can be higher than Earth’s with a lower gravity, see Titan for an example. Mars has very little atmosphere because of it’s lack of a protective magnetosphere shielding it from solar wind, not because of lower mass. $\endgroup$ Nov 15, 2017 at 14:25