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When there was no clearing done by humans, were all the areas providing "tree-friendly" conditions coated by vast expanses of forest, or were there any naturally occurring grassy plains as there are nowadays (that is resulting from wood cutting, like those in Britain - I reckon that until the logging activities throughout the middle ages, forests there were not scarce either)?

By what means could forest be naturally prevented from spreading wherever there are favorable conditions?

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    No. Some areas are not suited to forests (icefields, deserts, mature peat bogs. Matter of fact if you research, the modern ecological succession ends in peat bog not forest. That said, this is not a history question - this is a question about pre-history and indeed about pre-human ecology. – Mark C. Wallace Aug 28 '16 at 15:18
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    Read about steppe and savannah for starters. – Pieter Geerkens Aug 28 '16 at 16:57
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    "were all the areas providing "tree-friendly" conditions coated by vast expanses of forest" - isn't that rather circular logic? If "tree-friendly" is taken to mean "where trees thrive" then one would assume that those areas would be covered by forest. – KillingTime Aug 28 '16 at 20:19
  • When travelling around Tasmania a couple of years ago, I was surprised to find very large open grassland areas within the central forested parts around Cradle Mountain. Turns out these were natural moorlands - which I imagine would have been around for a long time before human settlement. – user13123 Aug 28 '16 at 23:47
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EUROPE DURING THE LAST 150,000 YEARS

This site, which includes many maps and references, describes how the vegetation cover of Europe has changed over the past 150,000 years. The map shown below shows Europe as it would be sans agriculture today.

Note that there are several types of forest, as well as several types of open land, such as tundra and steppe. Europe lacks large deserts.

enter image description here

This is the vegetation cover which would exist in the absence of agriculture, and which does exist at present in a highly fragmented form. Forest (green) predominates across most of the region, with deciduous forest across central and western Europe, and conifer forest (blue-green) towards the north and east. A steppe (grassland) belt exists in the south-east (yellow), with areas of forest-steppe (pink). Tundra (orange) exists in the far north where the climate is too cold year-round for trees to grow, and mediterranean vegetation - with hard-leaved evergreen shrubs and trees - predominates in parts of southern Europe (red).

  • Many thanks for the reply as well as the very informative link. – IronMaidenFanatic Aug 28 '16 at 18:53
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Grass is a plant that has evolved to help cause, and itself survive, fires. Most trees don't deal well with fire, particularly in places where there's a dry season. So even without humans, yes grass existed as a plant, and there are various biomes where it naturally takes over.

This might be better understood with this graph based on temperature and annual rainfall. enter image description here

Humans in fact (Hominina to be exact) are viewed by many as one of the two branches of primates to evolve specifically adapted to grasslands when the African Savannah first developed (the other being Baboons). The main problem grasslands pose primates is how to get around without trees to swing from. Baboons solved this by going almost fully quadrapedal. Our branch solved it by going bipedal.

So it would be more accurate to say we didn't make the grass, the grass made us.

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    Actually, trees are built to survive everything short of a crown fire - which won't occur in a forest that is allowed to have regular brush fires every decade or so. Grass is designed to burn off in a brush fire and move in rapidly to hold the soil in pace. In fact many softwood species (all the pines produce seeds cones that require hot temperatures to open and release their seeds. – Pieter Geerkens Aug 28 '16 at 16:55
  • Trees struggle in, for instance, cold temperatures (where tundra and muskeg occur naturally since trees can't survive) and in area such as Calgary subject to extreme winter chinooks. See also alpine meadows. – Pieter Geerkens Aug 28 '16 at 16:58
  • The sequoia and redwoods of California are hundreds of years old, in a climate very subject to large frequent fires. How is that compatible with "trees don't deal well with fire"? – Pieter Geerkens Aug 28 '16 at 17:00
  • Big trees deal with it a lot better than young trees. You might not think that's a big problem looking at a forest of mature trees, but if the fires are happening every single year, so no small trees ever get to become big.. – T.E.D. Aug 28 '16 at 17:03
  • ...and of course some trees have adapted better than others. Where I live is natural border between the two biomes in the USA, and our predominant tree is Blackjack Oak. These trees are unusually fire-resistant, and grow fast to nearly full height. But still, if there are enough fires, they can't make it. The aftermath of a fire in a blackjack stand is not pretty, but a few trees will regrow. – T.E.D. Aug 28 '16 at 17:11

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