Grass on planets with very huge forests
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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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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