How would flora behave on a two continent planet?

Question

Note: This question is a follow up to this question, and hence forms part of a bigger complex of questions about geology and biology in this certain scenario.

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Meta: The planet consists of two enormous continents, each encompassing one of the two hemispheres (northern and southern). There are two small polar oceans (both mostly frozen over, encompassed by landmasses) that are relatively low in salt. The continents are divided by a huge, deep oceanic belt, along the equator of the planet.

The planet has bout 1.1 times the mass of Earth, and slightly more landmass than water (about 35% percent landmass to 65% waters).

Tectonics push plates from the equator to the poles, effectively forming mountain rings, parallel to the equator, on the continents.

The planet orbits at an Earth-like distance around and Sol-like sun with an Earth-like elliptical orbit.

The planet will have 3 satellites (aka. moons), the first is sized a trifle bigger than Earth's moon and is orbiting along the equatorial axis about the same distance as earth's moon, and the others are both about 1/3rd the size of the big moon and are at a 60° and 70° degree inclination relative to the plane of the biggest moon at less distance from the planet.

The planet will feature a Phlebotinum gas, providing about 400-500% the lifting capacity of helium/hydrogen (as described here).

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Regarding the stated facts, How would flora develop and behave on such a planet?.

To mention a few topics:

• How many species of a given plant (e.g. trees, wheat, grasses) would evolve?
• Would there evolve different species per climate zone or would the same species rather evolve specializations?
• How different could the flora on the northern hemisphere be from that of the southern?

Addendum: This question assumes no travel of people between both continents, nor does it assume any cultivation of plants, no plantations made by humans. Best assume NO humans at all for the sake of the question.

Answer 1

Given two polar masses totaling 35% of the planet's surface, it is useful to compute just how big those masses are. The formula for calculating area using a conical angle is $$\omega = 2 \pi (1 - \cos{\theta})$$ where $\omega$ is in steradians and $\theta$ is the included half-angle of the cone. Since the area of a hemisphere is $2\pi$, $\omega = 2.2$, and $$\theta = 50$$

So the land masses will start at approximately 40 degrees of latitude. This is very bad, since the Horse Latitudes https://en.wikipedia.org/wiki/Horse_latitudes extend from about 30 to 38 degrees of latitude, with little precipitation. This will only allow the area between 38 and 40 degrees to provide moisture for the subpolar air flow which would provide moisture to the land mass. That's not much of a source, since it's only a band about 200 miles wide or less. And it gets worse.

The ring of mountains caused by your specified tectonics will act to produce a rain shadow to the north of the ring (in the northern hemisphere). If the mountains form a complete ring on one continent, the result will be a continental desert north (or south) of the mountains. If there are breaks in the mountain rings, moisture will push toward the poles in those areas.

As a result, you can expect some precipitation along the coastal shores, with temperate rain forests along the equatorial side of the mountain rings. North or south of the mountains will be extreme deserts. The cold of the ice caps will serve to sequester moisture in the ice, making the northern (above 60 degrees of latitude) desert very dry. Gaps in the mountains will allow the development of temperate grasslands and forests in the areas immediately poleward of such gaps.

With the existence of a 5500 mile wide ocean running completely around the planet, the pattern of trade winds (and the Intertropical Convergence Zone) there should not be much communication between the two continents, so there should be very little common in the flora and fauna of the two areas. For comparison, the existence of the Central American Seaway, with a gap on the order of 1000 km, effectively separated North and South America, allowing quite disparate populations to evolve. All of this changed about 12 million years ago with the rise of the Isthmus of Panama, and produced the Great American Interchange https://en.wikipedia.org/wiki/Great_American_Interchange. The effects of an even larger oceanic gap should be even more extreme in preventing communication.

As a matter of fact, unless you can provide for a land bridge between the two back in the equivalent of the Cretaceous or later, there would be no obvious reason for both continents to evolve compatible angiosperms, so the two ecosystems might be wildly different. You might well get away with asserting mammal dominance on one continent, while dinosaurs roam on the other.

Answer 2

How many species of a given plant (e.g. trees, wheat, grasses) would evolve?

With your particular setup I would expect there to be large inland deserts as one traveled farther north and south from the equator, as it would be harder and harder for water to reach it. With this in mind and as the and the fact there are really only two continents, the number of species will be less. The spread of seeds could walk them selves around the globe in relatively short order. On top of that, many of our grains are specializations we've bred into them.

Would there evolve different species per climate zone or would the same species rather evolve specializations?

Yes each climate would evolve plants that are better suited to it. The big difference would be that most of the plants would have access to most of the 'same' type of climate completely around the globe per continent. Both continents would likely be quite a bit different though.

How different could the flora on the northern hemisphere be from that of the southern?

Possibly pretty extreme, possible mostly the same. More than likely the similarities would be along the coasts where coastal plants managed to survive a crossing to the other continent (seeds, storms etc). While Australia, New Zealand and Borneo have a lot of different flora and fauna the many other places, the all have trees, flowers grasses etc.

Answer 3

Diversity

If you have no way to transport plants between continents, the diversity will be exciting. This would be similar to a species separated during the divide of Pangea here on Earth. Common ancestors will be divided millions of years prior. Unlike Earth, there will be nothing to allow migration of species to other continents. As an story idea, if you want diversity you could have a continent with solely algae/lichen and bryophites (mosses), and ferns, while the other is populated with angiosperms (flowers). This is based on broad assumptions about the animal life on the planet.

Continent-specific

If your story wants wildly-different plant types on each continent, you must limit transportation between continents: birds, people, bits of flotsam, etc. can bring seeds across the oceans. As a story idea, you may limit almost all animals (except bugs & fish) to one continent, producing hardier, more diverse, and flowering plants. This is just an idea, because we're talking about a whole new world. Your other continent allows plants to freely compete against each other, producing enormous trees reaching for the sunlight.

Height

One limitation is that your planet has more mass than Earth. Suddenly you have 6.5692e+24 kg mass, which will be a limitation on how much water can be 'sucked' up a tree. Maybe your plants have figured out how to get around this, but if they're Earth-like, you're going to see a lot of low-lying plants. Perhaps someone can help with the impact on the atmosphere and geology as well.

Keep in mind this answer is assuming a lot: this is your world to build.

Answer 4

Well, first of all, will you have anything similar to Earthly plants at all? Let's suppose that's the case.

Other answers have pointed out how well isolated the two land masses would be from each other. How long has that been the case? Millions of years?

Look at how much plant life on Earth has changed since, say, the time of the Dinosaurs. Two things in particular I know about are flowering plants and grass.

Flowers co-evolved with insects. Look how successful that's been to biodiversity, such that most everything you know about is a flowering plant. If that initial innovation occurred on one of the isolated lands, and kicked off a co-evolutionary explosion of diversity, what about the other land? Well, it didn't happen there. An unrelated natural disaster provided a different event for adaptive radiation to fill the emptied niches, and the result is utterly and completely different.

Flowering plants eventually pulled another trick with fruits. In fact, it happens in many different tissues and in different ways. The point is to get animals to disperse the seeds. Maybe something with the same general result would happen in the other land, but without flowers to start from it would involve different tissues and resulting structures.

Not only do you have the usual problem of trying to invent something truely alien, you have to do it twice.

Answer 5

It all hinges on time, in my opinion. Have these continents been separated before plant life evolved? Has there been any contact since? Dry land plants evolved nearly 400 million years ago, and until around 150 million years ago, Pangea was still kicking around, when it (conveniently) broke in two massive continents, Gondwana and Laurasia. A lot of different plants had spread before the divide, and they evolved differently in the two continents since them. However, some of the plants that came later (much later in some cases) transposed these oceans and spread to the other lands. Ice ages, if I remember correctly, can dramatically change the shapes of dry lands, creating craazy connections between landmasses. Also, some plants can survive in salt water for a long time, and thus be transported by oceanic currents. The coconuts and some palms probably spread through the Caribbean all the way to the South Pacific this way.

About diversity, number of species etc, again it all hinges on time. As a rule of thumb though, the much greater chunk of species diversity is in the tropical regions, north and south. No one is really sure why though, some hypothesis say it is due to the increased energy input from the sun, others say it is the greater climatic stability (no freezing, etc)

Initially species would evolve specializations to each climate, but eventually each of these variations would become different species of their own, leading to very different lineages. It is very rare, though, to some plant families be restricted to specific climate zones. There are orchids everywhere, as well as grasses, daisies and such. Generally speaking though, early plant groups (conifers, ferns) will probably be more similar between the hemispheres than more recent ones (grasses, orchids). Again, it all depends on how much time has passed.