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This is a reality check: I want a planet with huge forests and a relatively (to Earth) high level of CO2, but I also want there to be very large areas of grassland, (savannahs) where we naturally evolve a human-like, upright bipedal species (as opposed to species adapted to tree dwelling and navigation). I don't want to force these savannahs into particularly harsh climates; I just want to know if there are natural reasons they could develop and exclude most of the dense forest. Is this a plausible planet?

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    $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. I am not sure what exactly you are asking. What are "almost high amounts of co2"? How exactly is the composition of your atmosphere? We already have some huge forests on earth. How are your forests different from the already existing ones? Currently I don't see a difference to earth and therefore no reason why you should not be able to have a human-like species. Could you please edit your answer for clarification? Have fun! $\endgroup$ – Secespitus Jun 20 '17 at 5:43
  • $\begingroup$ I added reality-check tag - it fits when you're asking if something is possible. $\endgroup$ – Mołot Jun 20 '17 at 5:47
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    $\begingroup$ Note that most tree's and especially grasses are very quite 'young' on earth (40 - 200 mil. years). In contrast mammals are older (~300 mil. years) and dinosaurs were already on the decline when the first modern plants showed up on earth $\endgroup$ – Nicolai Jun 20 '17 at 6:18
  • $\begingroup$ Do you want your planet's landmass to be completely covered by forests? It might be a more interesting question if and how this is possible. Otherwise there shouldn't be a problem $\endgroup$ – Raditz_35 Jun 20 '17 at 9:28
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Grass evolved to deal with grazing animals. Whether grass, something similar, or something totally different evolves will depend on the selection pressures and random luck. I don’t think the concentration of carbon dioxide has anything to do with it.

Likewise, the presence of grass doesn’t have anything to do with whether something human-like will evolve.

More importantly, nothing about the “high CO₂ levels” as the only difference of note will preclude things turning out like you want. Did you know that there were times when Earth had more CO₂ ? The thing to watch is to make sure the overall temperature is OK. It may prevent coral and other seashells from forming. The levels may have been different at different times in your planet’s geologic history.

Of course, the presence of grass and savanna as an ecological niche will shape the evolution of animals and other species, as well. Grass evolved to deal with animals grazing, and in turn supported animals that graze.

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    $\begingroup$ At last! A truly sensible statement about evolution on this site: "Whether grass, something similar, or something totally different evolves will depend on the selection pressures and random luck." Evolution is driven by selection pressures and random chance. Plus one. $\endgroup$ – a4android Jun 20 '17 at 13:14
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    $\begingroup$ Grass might have had something to do with the upright bipedal nature of what became man, that has been part of evolutionary speculation in the past. In particular, our height, standing on two feet, gives us a longer field of view (for both avoiding predators and finding game) than if we were knuckle-walking apes or gorillas; that have their eyes at around 3 feet off the ground. They can stand with effort, but in the forest seldom need to; especially if they climb easily. Without trees, we may have evolved to upright bipedalism for constant better surveying, which also freed up our hands. $\endgroup$ – Amadeus Jun 24 '17 at 21:09
  • $\begingroup$ @Amadeus it now appears that bipedalism appeared while they were still living in trees. I looked up in Wikipedia to find the mix of biped and other features and that section concludes with «Modern apes and their fossil ancestors show skeletal adaptations to an upright posture used in tree-climbing, and upright, straight-legged walking has been proposed to have originally evolved as an adaptation to tree-dwelling.» $\endgroup$ – JDługosz Jun 24 '17 at 23:22
  • $\begingroup$ @Amadeus but I see your point about grass shaping evolution, and I added another para on the broadest concept. $\endgroup$ – JDługosz Jun 24 '17 at 23:26
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Grass is one of the first organism to conquer virgin soils, preceeded by lichens and followed by bushes and then trees.

Also having a larger amount of CO2 it doesn't mean all the places are favorable to trees growth. It may though vary the relative distribution of forests and grasslands.

Therefore I would say grass is still bounded to be present.

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1: Fire! from https://sites.google.com/site/grasslandbiome123/fires enter image description here

from https://www.nps.gov/glac/learn/nature/upload/Grassland-Fire-Brief.pdf

Wildfire has a reputation as a destructive force, but it often functions to maintain an ecosystem’s balance. Regular wildfires shape the makeup of vegetation by suppressing woody plants and favoring grasses. Because their growth structure is situated at or below the ground, and because fire moves quickly through grasslands, most grasses tolerate fire or even proliferate after a fire. Without fire, shrubs and trees would take over grasslands, effectively converting them to forest, and non-native vegetation would spread.

Lots of grasslands on many continents were maintained by fire: the main biomass of a perennial grass is underground, and fireproof. The biomass of a tree is above ground and burns. In a fire ecosystem grasses win, with the occasional fireproof tree here and there.

enter image description here from http://wyomingnaturalist.com/land_prairie.html A bur oak (fire resistant) lording it over the grass.

2. Elephants. enter image description here

In Africa elephants push down trees and eat them, preventing forests from taking over grasslands. Not as important as fire I do not think but still: credit where due.

I humbly submit that flame elephants might combine the best of both worlds when it comes to grassland promotion. Flame elephants would be good for lots of other things too.

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I will add that relatively constant intense grazing by large herds can also be responsible for creating vast grasslands; I recall an experiment done by students (must of been twenty years ago I heard about this) in which they fenced off an area about the size of an Olympic swimmming pool in a grassy expanse (with permission of the owner). It prevented cattle and goats from grazing that particular area. Within the year, they had volunteer (not intentionally planted) woody bushes and several young trees, just within their fenced area.

The hypothesis is that the seeds of trees will germinate in the field just fine, but the grazing animals mow them down before they get a chance to grow.

Now grass grows from the bottom; so cutting a leaf (blade of grass) does not kill the plant (although repeated cutting may starve it of what it gets from photosynthesis). So grass grows back after being cut by grazers. Trees and bushes, however, just die if the shoots are cut. Unlike grass, young shoots have very short and underdeveloped roots, and can be just pulled out of the ground; while mature grass can have deep roots, so just their leaves get ripped away.

By building their fence, the students protected the area from grazers and any lucky seeds making landfall in that area had time to grow.

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Savannah can handle less rain than is needed to support forests especially very seasonal rain, savanah can hand long dry periods better than trees. Grass can die off and recover every year, trees don't handle this as well in warmer climates. so dry warm climates will develop savannah as long as grass have evolved.

Also savannah are not without trees it is just that there is large distances between trees or patches of trees. Savanna are also often prone to fire which helps keep shrubs sparse. You really do not need to justify savannah on your planet, savannah will fill in anyplace the trees have stayed sparse and/or small because of the lack of rain or poor soil water absorption, again provided it has evolved.

A common form of savannah is grass or scrub cover the majority of the land and trees are tightly packed near any water sources like rivers, lakes, or lowlands where water will collect. These are called gallery forests. Notice how much grassland exists in the picture below. This would qualify as a savannah.

enter image description here

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Height can be your deciding factor: there are plenty of highlands without trees but with grass, e.g. this Bolivian altiplano:

enter image description here
(Source: Wikimedia)

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  • $\begingroup$ That is interesting, but why exactly does height help? How does it prevent tree growth, or promote grass growth? $\endgroup$ – Amadeus Jun 24 '17 at 21:12

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