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.
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...
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.
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.