Old-growth beech forest in Slovakia, often but mistakenly used as a model for all pristine European habitats
As mentioned above, deep, closed-canopy forest is often cited as the natural land cover in most of Europe, and is also generally the focus of many more modern attempts at habitat restoration. Unfortunately, this is not quite correct. Closed-canopy forests, also known as climax forests, are the peak of forest development. If undisturbed for a sufficient amount of time, vegetation will grow increasingly dense, eventually culminating in a forest with, as the name implies, an almost entirely tight and closed tree canopy. This can be seen in many places throughout the world, including a few in Europe, such as the above picture from Slovakia. The problem with the assumption that this would be the predominant form of vegetation throughout all of Europe is that, as noted above, for a climax forest to develop, it must be almost entirely undisturbed. Today, in remote areas like mountainsides and sparsely populated valleys, these conditions are quite easy to find, but this is due to two primary factors. The first, and most relevant for the majority of Europe, is that even in these seemingly pristine old-growth forests, large herbivores are often almost entirely absent, or at the very least highly diminished in numbers. Since the discovery of the concept of trophic cascades, it has become abundantly clear that animals exert a far greater influence on their habitats than previously thought. In Africa, much of the landscape is covered in open woodland. In many areas this is due to the dry climate, but research has shown that in other places, when elephants and other megafauna decline or disappear, the open woodlands quickly grow denser, and begin to turn to forest. In Europe, when one considers that, had it not been for human hunters, the continent would still be home to animals such as elephants, horses, rhinoceros, etc, it is reasonable to presume that many of the dense forest ecosystems we see today are actually a result of the same process observed in Africa. If the larger herbivores had never disappeared, they would have played a large role in suppressing the development of climax forest, as they do in Africa.
The second factor contributing to the development of closed canopy forests in areas like Slovakia is terrain. Most, but not all, of the dense primeval forests that exist in Europe today grow in mountainous areas. Part of this is because humans have cleared most of the lowlands for agriculture, but even in our absence, the mountains would still have had a thicker vegetation than the lowlands. The reason for this is actually much the same as the reason why humans have cleared the lowlands - large animals prefer the more easily accessible floodplains and river valleys, where food is more abundant and transport less difficult. If you are an elephant or even a horse, the lowlands are a much easier place to live than the mountains. Even where large animals do live in the mountains, they do so in lesser densities, and are often unable to reach many areas, thus enabling the development of relatively undisturbed forest. The rocky crags and winding canyons found in most mountainous ares are also ideal hunting territory for many predators, further reducing herbivore populations. This is not the case in most of the European lowlands, and as such, the mountains and slopes are most probably not good analogues.
Even though dense forest may not be the natural vegetation for most of Europe, mountainous upland regions like the Lake District should still be cloaked in forest
This is not to say that relatively dense forests did not exist in lowland Europe, a variety of factors could allow for certain areas to develop closed canopy forests. Wetlands, if dry enough to allow for tree growth, often serve as a barrier to large animals, and can allow for deep forests to develop even in regions with a heavy grazing pressure. Islands in areas such as Denmark and the Netherlands would not be able to support large populations of herbivores, and as such would also have been covered in dense forests. Certain areas may have been home to particularly large populations of predators, and these too would have been more densely vegetated than the surrounding regions. In general however, completely dense, thick forest would not have been the dominant vegetation type in most areas. This is not to say that most of Europe was covered in steppe or open grassland. Note that I have only been using the word "forest" up until now. It may have seemed slightly repetitive, and perhaps you wondered why I did not spice it up with other words such as "woods" or "woodland". Well, the reason for that is that these words are not interchangeable. Woodland merely refers to a habitat in which trees predominate, and can apply to many different landscapes. As an example, a forest is a particularly dense form of woodland, whilst a savanna is an open form of it.
The majority of Europe would have been covered in woodland, forming something of a gradient between closed forest and open grassland. The term wood-pasture is often used today to describe this terrain, but while certainly apt, it carries connotations of a cultural landscape, shaped by people. For the unmanaged, naturally occurring semi-open landscape, I prefer "megafaunal savanna". Essentially, while most savannas are a result of climate, where conditions are too dry for forest development but wet enough for tree growth, a megafaunal savanna is an open habitat maintained by the titular megafauna instead of any environmental factors. This also makes this type of habitat particularly vulnerable, as it is wholly dependent on the large animals inhabiting it. In Europe, when the megafauna was wiped out, the megafaunal savanna disappeared entirely. It is in some ways the temperate region's equivalent to the mammoth steppe, an unique ecosystem now completely gone.
Konik Horses in one of the more densely vegetated parts of Oostvaardersplassen. The environment seen here is probably very similar to parts of the megafaunal savanna
Semi-open woodland, typical of a wood-pasture environment
So what would this so-called savanna have looked like? Well, it depends on where you were. In the more hilly and rugged parts of the lowlands, there would be many nooks and crannies inaccessible to most large animals, allowing trees to grow unhindered. As a result, these regions would probably be more densely vegetated, though plenty of light would still reach the ground. On the flip-side, some regions would house particularly large numbers of herbivores for at least some parts of the year, and as a result, these would quite probably have been almost entirely open, forming grasslands similar to those seen in parts of Oostvaardersplassen. A fenced polder, Oostvaardersplassen is quite controversial among many conservationists, mainly because of the doubts surrounding its validity as an analogue for ancient Europe. It is, at least to me, undeniable that the density of herbivores in the reserve is probably higher than it would have been in most parts of Europe. But most does not mean all, and some regions would indeed have attracted especially large populations of animals, as can be seen in many parts of Africa, and for these areas, Oostvaardersplassen is possibly quite a sound analogue.
The majority of the megafaunal savanna would probably have been more densely wooded than Oostvaardersplassen however, more similar to the wood-pasture in the above picture. Relatively large stretches of open space would have separated more dense clusters of vegetation, with smaller stands of trees and bushes dotted throughout the fields. But how would trees have been abundant if the grazing pressure was so high? It has been observed in many wood-pastures that while grazers do indeed tend to halt natural regeneration, trees have a way around this. Thorny shrubs ward off grazing animals, and often times saplings will take root within these shrubs. Here they are able to grow in safety, eventually becoming larger than the bushes in which they originated, but by then they are large enough to endure the attacks of the grazers and browsers. This phenomenon can in fact be seen in the image above, as many of the bushes shown there are indeed of the thorny variety, and if one looks carefully, trees can be seen emerging from some of them. This same process also occurs on the African savanna, and was almost certainly the most common way in which new stands of trees originated in many parts of ancient Europe.
There is more to cover regarding the original land cover of Europe, namely the wetland regions, but that is a subject for another post. In summary, in a world without humans, and where the original European megafauna had not been hunted to extinction, the majority of the continent would not be covered in forest, nor would it be covered in open grasslands. Different areas would house different conditions, but in general, semi-open woodland would have covered most of the continent, with areas of increasingly dense or sparse vegetation. Under these conditions, biodiversity would have been incredibly high, with both cover and light dependent species coexisting in one environment, as the varied nature of this so-called megafaunal savanna allowed for animals of many different niches to share the same environment. I believe that it is this habitat which should be viewed as the ideal throughout most of Europe, as it not only best represents the original, non-altered landscape, but is also quite probably the most biodiverse one possible. Only by looking back into the past, and trying to discern just how things were before we came along, can we truly know what to strive towards. It offers necessary perspective, and a point of confidence and relative certainty as we move forward with habitat restoration and rewilding, all things we severely need if we want to succeed in getting nature out of its dire present state.