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Though my star is 1.71 LSol, my planet orbits at about 2.14 AU. Following a back-of-the-envelope equation for apparent brightness, this gives the surface of my planet approximately 30% of Sol's light energy. It would actually be substantially less; my planet is extremely geologically and volcanically active, so I would expect a high concentration of reflective volcanic particulates to be a semi-permanent fixture in the upper atmosphere. It also has a high obliquity of 31.1 degrees, and 93% of its surface is ocean; as such, hurricanes are very common and very powerful. My atmosphere is quite thick and rich in carbon dioxide: 16% CO2 in a 134.76 kPa atmosphere.

My question is:

How would plants evolve differently to more efficiently capture the light energy coming to the surface, yet survive the constant hurricanes (or at least reproduce quickly enough that the hurricanes would be a non-issue)? My working theory is that the plants could only survive with a much more balanced and diverse range of photosynthetic pigments; and/or plants would barely survive at all, and the flora of my world would mostly be made up of oceanic blue-green algae.

Keep in mind that increasing the surface area of the leaves would only work in very specific regions of my planet's land masses; these plants also need to worry about conserving temperature (a BOTE calculation puts my planet's average temperature around 10-15 degrees C) and water (the only areas sheltered enough from the constant hurricane and also warm enough to sustain large-leafed plant life are high-altitude deserts on my planet).

Here's what I have so far:

So I was thinking mostly coniferous needle-leafed plants with a blue-green photosynthetic pigment balance like phycobilin and chlorophyll in most "temperate" climates, and/or a carotinid-based plant phylum that evolved separately in an isolated region to the northwest. (Since 93% of its surface is ocean, and since it's so geologically active, my planet has plausible continents that have separate evolutionary timelines.)

Please let me know if you think I'm on the right track; I am NOT a biologist or botanist by any stretch of the imagination!

EDIT TO ADD:

I have been informed that my concerns about temperature regulation are largely unfounded, which is a good reminder that I need to thoroughly check my assumptions about what exactly would be different from Earth. Here is a very helpful reply I got from u/Cruzzfish on the r/worldbuilding subreddit!

"They really wouldn't be that different at all. Many plants already require lower than 30% of the sun's full brightness if they don't want to sunburn. Even full sun plants don't have too much of an issue operating at <10% of the sun's brightness for extremely extended periods of time.

Notable differences would be.

More salt tolerant wetland plants to take the storm surges

Less brightly colored ones since no need of sun protective pigments.

I don't know what you mean by conserving temperature. Plants don't need to worry about that sort of thing. But a planet with an average temperature of 15 Celsius is not cold at all. Earth average temperature is just over 13 Celsius..."

"...Seaside goldenrod is decently salt tolerant but I don't know how it reacts to floods. Generally there are a lot of seaside ____ plants that would be good to look for."

And from u/svarogteuse:

"They would evolve just like plants on the Gulf Coast/Caribbean; they drop small leaves freely but regrow them quickly.

About 2 years after Katrina I went to Bay St. Louis, the place it actually came ashore. Along the beach was a brand new road recently rebuilt. To the left the remains of a boardwalk and sand. To the right beautiful park land for about 6 blocks with huge live oak trees. The live oaks looked a little funny. Took a few days to figure it out. The trees had limbs bigger than my torso and twigs with leaves. Nothing in between. Every thing smaller had been ripped off by the hurricane.

Oh and the park land wasn't parkland. It had been full of houses and now the only thing left was foundations. Everything else had been carried off by the 25' storm surge. Storm surge the 200 year old oaks (but no smaller ones) withstood."

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  • $\begingroup$ I am digging that green check but maybe take it back for just a bit and see what else happens. Some awesome speculative biology happens on WB stack and people might be more willing to weigh in if they think they might win the green. They might! $\endgroup$ – Willk Feb 16 '18 at 14:51
  • $\begingroup$ Thank you for reminding me to be patient! I'll give it another little while and see who else wants to comment. Thanks again for your answer! $\endgroup$ – Rúnatál Davino Feb 16 '18 at 14:53
  • $\begingroup$ I'm not 100% sure if this statement is true : "as such, hurricanes are very common and very powerful. ". Hurricanes are warm cored systems and require a certain amount of heat to power them (sea surface temperatures need a 'minimum' of around 26.5 degrees in order to support a hurricane). You would be looking cold core, more Nor-Easter style storms sciencing.com/… however these form with a significant cold/dry air mass meets warm/humid air. Your low land percentage might make this large cold dry mass of air an impossibility. $\endgroup$ – Twelfth Feb 16 '18 at 20:02
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    $\begingroup$ You might have a different issue. "Snowball Earth" is the postulated condition of the world after it had cooled and the oceans condensed, since the young Sun only had @ 30% of today's luminosity. The entire planet is thought to have been completely covered in ice, with few warm spots, mainly volcanic vents providing areas of liquid and open water. $\endgroup$ – Thucydides Feb 16 '18 at 20:03
  • $\begingroup$ @Thucydides - I think you are correct there. I'd be far more inclined to believe that setup than to think this setup could support hurricanes of any form. $\endgroup$ – Twelfth Feb 16 '18 at 20:18
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Perhaps think in terms of shade tolerant plants. These tend to be low and broad leaved.

I'd avoid pine needles. The leaves should be broad - this is the 'kitchen' of the plant where the food is made. Broader leaves capture more sunlight. Pine needles evolved with plenty of sunlight and are optimized I believe to retain water (not a concern for you. )

I'd add in extra photosystems. Chloroplasts typically have two, and from these they energize electrons from water to an energy state high enough to fix CO2. You are asking about a low - light environment, and so you could add in another photosystem (or other device) to help funnel more photons to your electrons to energize them.

Another cool idea is to have the chloroplasts be more mobile so that they migrate within the leaves to the area with the most light. Cool video here. And, the leaves can be mobile too, orienting to light with phototaxy.

As far as the hurricane - this has been asked elsewhere on the site. You can use the advice you received on Reddit and also think in terms of advantages - Seed dispersal is enhanced by the winds and water, so your plants may evolve to have buoyant seed pods that split when waterlogged.

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Lichens.

http://www.pbase.com/philharmostes/image/49852777/original lichen field

Lichens are the "plants" which make do in the place other plants can't. The driest, coldest, darkest, harshest places. Lichen lays low so storms are less problematic. I suspect the varied pigments are actually protective, not photosynthetic - but when you have such little biomass and your energy requirements are low you can take your time gathering energy.


/Are there any changes you can think of, or any special parameters that would make a dense forest cover in at least one area of my world plausible/

https://en.wikipedia.org/wiki/Wollemia

Wollemia was only known through fossil records until the Australian species Wollemia nobilis was discovered in 1994 in a temperate rainforest wilderness area of the Wollemi National Park in New South Wales, in a remote series of narrow, steep-sided sandstone gorges 150 km north-west of Sydney.

My understanding is that this microhabitat ascted as a refuge for the Wollemia - they did not get burned up by fires or outcompeted by other trees. So to your world - maybe it used to be a nicer place, or the trees were brought by aliens and managed to hang on in this one spot - protected from weather by steep cliffs and maybe at altitude - more sun and less clouds.

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  • $\begingroup$ This is something I forgot to consider, so thank you very much! Since lichen don't provide much in terms of soil stabilization, I guess if I went this route more of my world would be desert? $\endgroup$ – Rúnatál Davino Feb 16 '18 at 13:22
  • $\begingroup$ @RúnatálAtlasThorn it would only be a desert if it didn't receive much water. Having mostly lichen as cover combined with strong winds and hurricanes would make your world a badlands type place. Lots of erosion by wind and rain, little topsoil, high drainage, and minimal vegetation. So not a nice place to live, and most laymen would call it a desert. $\endgroup$ – Dan Clarke Feb 16 '18 at 13:42
  • $\begingroup$ @DanClarke Thank you for the clarification! Are there any changes you can think of, or any special parameters that would make a dense forest cover in at least one area of my world plausible? $\endgroup$ – Rúnatál Davino Feb 16 '18 at 14:19
  • $\begingroup$ Comments from elsewhere: from Cruzzfish1 via /r/worldbuilding sent 10 minutes ago "They really wouldn't be that different at all. Many plants already require lower than 30% of the sun's full brightness if they don't want to sunburn. Even full sun plants don't have too much of an issue operating at <10% of the sun's brightness for extremely extended periods of time. Notable differences would be. More salt tolerant wetland plants to take the storm surges Less brightly colored ones since no need of sun protective pigments. I don't know what you mean by conserving temperature..." $\endgroup$ – Rúnatál Davino Feb 16 '18 at 14:28
  • $\begingroup$ @RúnatálAtlasThorn, I don't think you'd find just lichen, although with the heavy, multiple storms a year they'd be pretty common. figure lots of low lying fast growing bushes and plants, nothing too tall or thick. Able to bend in the wind without snapping and deep roots. That's about all I can say, not enough for a full answer. With the edit, Willk's answer is about as good of one as any I could make. $\endgroup$ – Dan Clarke Feb 16 '18 at 14:44
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I think you have a far more difficult issue at hand. The lack of luminosity may plunge your planet into a permanent ice age, and plants may be the last things to worry about.

It is thought that once the Earth and planets had stabilized after the late heavy bombardment period, Earth was in the grip of a permanent ice age sometimes known as "Snowball Earth"

enter image description here

Artistic reconstruction of Snowball Earth

The Sun had only about 30% of today's luminosity, so very little solar energy was striking the Earth and keeping it warm. Water froze into massive glaciers covering most of the planet, with a few warm spots around volcanic vents providing clear water and open ocean. The only life lily to thrive under those conditions would be extremophile bacteria surrounding the vents, and some bacteria or algae capable of living in the open water where some sunlight filtered through.

Life on Snowball Earth was extremely limited, and clinging to some pretty narrow niches for hundreds of millions to billions of years. It is thought the end of Snowball Earth was a series of volcanic eruptions which released enough carbon dioxide to cause a spike in temperatures (in ancient rock formations, there are rocks and boulders randomly dropped into sediments. The interpretation is these rocks were deposited when glaciers melted.

enter image description here

Sudden depositing of glacial rocks into sediment at the end of Snowball Earth

So unless you ramp up the luminosity of your sun, your planet will be a very marginal and unpleasant place to live.

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  • $\begingroup$ Snowball Earth was in large part due to the removal of CO2 from the atmosphere - via photosynthesis. Thus, plants (well, actually cyanobacteria.) . Snowball Earth resolved via volcanic action. The OP can add volcanoes to keep the atmospheric carbon high enough for a greenhouse. I'll grab a reference. pnas.org/content/102/32/11131.short And in other words, Earth was not a snowball prior to Snowball Earth, even though the sun was faint. $\endgroup$ – DPT Feb 16 '18 at 20:56
  • $\begingroup$ Yes, thank you for your input; I had considered a Snowball planet briefly but then realized that the geological activity of my planet would be high enough to compensate for a loss of atmospheric carbon. (I think.) $\endgroup$ – Rúnatál Davino Feb 16 '18 at 21:03

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